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Quantum Diaries blogs about LHCb
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16 March 2017: The magic of the Ωc baryon.
Observation of five new narrow Ωc0 excited states.
As NASA announced a few days ago the discovery of the first system of seven Earth-size planets orbiting around a single star, so LHCb unveiled today the discovery of a new system of five particles, with the observation of five new narrow Ωc0 excited states.
LHCb physicists reconstructed a highly pure sample of charmed baryons Ξc+, with quark content csu, decaying into a proton p, a kaon K- and a pion π+. The p K-π+ invariant mass spectrum is shown in the left image below. The red peaking distribution shows the Ξc+ contribution above the background indicated by the blue dashed line.
Subsequently the Ξc+ candidates were combined with K- mesons present in the same event. The Ξc+ K- invariant mass distribution obtained in this way is shown in the right image above, revealing for the first time five narrow structures with an overwhelming statistical significance. These structures are interpreted as manifestations of excited states of the Ωc0 baryon. These excited states decay into a Ξc+ baryon and a K- meson via the strong interactions, in contrast to the weak decays responsible for the three particles used to form the Ξc+ mass peak.
The Ωc0 baryon is a higher mass partner of the Ω- baryon, a particle which played a very important role in the history of particle physics. In the 1950s many different particles were discovered. Initially thought to be elementary, the ever growing list of discoveries led physicists to doubt this assumption. Therefore efforts were made to find a classification scheme in analogy to the periodic table of chemical elements. The most successful such scheme was proposed by Gell-Mann. In this model mesons and spin 1⁄2 baryons are organized into octets (Eightfold Way) while spin 3⁄2 baryons form a decuplet, as displayed in the image on the left. When the scheme was proposed, the top-most particle in the image, the Ω-, was not yet discovered, but hypothesised to exist in order to complete the pattern. The regular structure of the decuplet enabled many properties of this new particle to be predicted, including its mass. The Ω- was subsequently discovered through a single famous bubble-chamber photograph obtained at the Brookhaven laboratory in 1964 (the corresponding line diagram is shown in the right picture). This picture validated the Eightfold Way, and led Gell-Mann to propose the quark model in 1964, which explains the structure of the octets and decuplet. In the quark model the Ω- is composed of three strange quarks (sss). The Ωc0 is like a Ω-, but contains a charm (c) quark in place of one of the strange quarks.
The ground state of the ssc system, the Ωc0 itself, has been known about for quite some time. The discoveries announced today are excited states of this system, analogous to the excited states of atoms. These states are named the Ωc(3000)0, Ωc(3050)0, Ωc(3066)0, Ωc(3090)0 and Ωc(3119)0, where the numbers indicate their masses in MeV as measured by LHCb. By discovering these particles LHCb has reinforced the exceptional role of the Ω baryon family in the history of particle physics.
11 March 2017: New results presented at the Rencontres de la Vallée d’Aoste.
The LHCb collaboration presented several new results at the 31st Rencontres de Physique de la Vallée d'Aoste taking place this week in La Thuile, Italy. A few selected items are listed below.
Processes where a B meson decays into a pair of oppositely charged leptons are powerful probes in the search for physics beyond the Standard Model. Following up the preliminary result that was presented recently at a CERN seminar on the first observation by a single experiment of the decay Bs0→ μ+μ-, with a statistical significance of 7.8 standard deviations, the final result was shown at La Thuile for the first time. This presentation provoked much interesting discussion. A search for the tauonic decays B→τ+τ-, where B can be either a B0 or a Bs0 meson, was also shown in La Thuile. The τ leptons are reconstructed in the analysis through the decay τ→π-π+π-ντ. Decays of B0 and Bs0 mesons into τ+τ- pairs have not yet been observed, but the LHCb analysis yields the first direct limit on the Bs0 branching fraction as well as the world’s best limit on the B0 branching fraction.
The first observation of the four-body charmless baryonic decays Bs0→ppK+K-, Bs0→ppK±π∓ and B0→ppπ+π- was reported as well as the evidence for the B0→ppK+K- decay, see images above. These decays are of great interest in the search for further manifestations of CP violation in baryonic B decays, see the first evidence for the violation of the CP symmetry in baryon decays.
All of these results have been obtained owing to the excellent performance of the LHC collider and LHCb detector. The physics opportunities that exist with an upgraded LHCb detector in the High Luminosity LHC era are even more impressive. A phase-1 upgrade of the experiment is currently being prepared and will be installed in the long-shutdown 2, in 2019-2020. The physics case for the so-called LHCb Phase-II upgrade, following the recent submission of an Expression of Interest by the LHCb collaboration, was also presented for the first time in public.
14 February 2017: First single experiment observation of the decay Bs0→ μ+μ-.
From discovery to precision measurement.
[ Branching fraction Bs0→ μ+μ- = (3.0±0.6+0.3-0.2)x10-9 ; B0→ μ+μ- < 3.4x10-10 ]
Today, in a seminar at CERN, the LHCb collaboration presented the first observation by a single experiment of the decay Bs0→μ+μ-, with a statistical significance of 7.8 standard deviations. The measured branching fraction (3.0±0.6+0.3-0.2)x10-9 is in agreement with the Standard Model (SM) prediction of (3.65±0.23)x10-9. It is the most precise measurement of this quantity to date.
The full 3 fb-1 of data collected during Run 1, and 1.4 fb-1 of data accumulated during Run 2 were used to obtain this result. A special event selection (BDT for experts) was used to classify data into bins with different ratios of Bs0→ μ+μ- decays and background contributions. The μ+μ- invariant mass spectrum for the bins with the smallest background contribution (BDT>0.5) is shown in the image. The contribution of the Bs0→ μ+μ- decay is clearly visible at the Bs0 mass and is indicated as the red peaking distribution. The green peaking distribution shows a possible contribution of the B0→ μ+μ- decay at lower mass, which is expected in the SM at a rate of about 30 times smaller. The size of this contribution is not found to be significant, and so an upper limit is set for the decay at a value of 3.4x10-10. The other contributions show the contribution of background processes. They provide very little contamination in the region of the Bs0→ μ+μ- signal.
The probability, or branching fraction, of the Bs0 meson to decay into two oppositely charged muons is very small in the SM and is well predicted. On the other hand, a large class of theories that extend the SM, such as, for example, supersymmetry, allows significant modifications to this branching fraction and therefore an observation of any significant deviation from the SM prediction would indicate a discovery of new effects. The decay of a Bs0 meson into a muon pair has therefore long been regarded as one of the most promising places to search for these new effects. This decay has been searched for more than 30 years by different experiments at different accelerators as shown in the image. The LHCb collaboration obtained the first evidence, with a significance of 3.5 standard deviations, in November 2012 and, together with the CMS collaboration, the first observation, with a significance of 6.2 standard deviations, in May 2015. Previous results already severely constrained the type of SM-extension models that are still allowed, as described, for example, in the 30 March 2012 news. The results announced today isolate even more precisely the parameter region in which these new models can exist, and therefore focuses future experimental searches and theoretical attention. All candidate models of physics beyond the Standard Model will have to demonstrate their compatibility with this important result.
Some new-physics models also allow the possibility of a different Bs0 “effective” lifetime from what is predicted in the SM. LHCb also reported today the first measurement of this quantity, and found it to be 2.04±0.44±0.05 ps in agreement with the SM prediction. The left image shows the characteristic exponential decay time distribution of Bs0→ μ+μ- decay events.
A typical Bs0→ μ+μ- decay candidate event recorded in 2016 is shown below. The two muon tracks from the Bs0 decay are seen as a pair of green tracks traversing the whole detector in the left image. The right image shows the zoom around the proton-proton collision point, the origin of many particle tracks. The two muon green tracks originate from the B0s decay point located 17 mm from the proton-proton collision. You can play with images of 3 new Bs0→ μ+μ- decay candidates recorded during the 2016 data taking period using the LHCb 3D event display. This web based event display will run on your computer or smartphone without need to load any specialized software.
LHCb looks forward to making even more precise measurements of the Bs0 branching ratio, continuing its search for the B0→μ+μ- decay, and improving the knowledge of the effective lifetime. Stay tuned for updates from Run 2 data.
Click the images for higher resolution. More details can be found in the LHCb presentation at CERN and soon in the LHCb publication. Read more in the CERN update for the public both in English and in French and in the Symmetry magazine. The preliminary numbers and plots presented at the seminar have been replaced with the final ones on March 11.
30 January 2017: Measurement of matter-antimatter differences in beauty baryon decays.
Towards an important milestone in particle physics.
The LHCb collaboration has published today in Nature Physics the first evidence for the violation of the CP symmetry in baryon decays with statistical significance of 3.3 standard deviations (σ). CP violation has been observed in K and B meson decays, but not yet in any baryon decay. If the measurement is confirmed with a statistical significance of 5σ using a larger data sample, it will be the first time that an asymmetry in the decay rate of baryon and an anti-baryon is observed. In the quark model of particle physics mesons are composed of a quark and antiquark pair while baryons (anti-baryons) are composed of three quarks (anti-quarks).
In order to obtain this result LHCb physicists made the first observation of Λb0 beauty baryon decays into four other particles pπ-π+π- and pπ-K+K-, as seen in the images above showing accumulation of events at the Λb0 mass. About 6000 and 1000 decays were found for the two decay modes, respectively. The beauty baryon Λb0 is composed of udb quarks and its mass is about 6 times the mass of a neutron (udd).
The two decays proceed mainly through these two “Feynman diagrams”. It is important to measure the size and nature of CP violation in these decays in order to determine whether they are consistent with the predictions of the Standard Model of particle physics or, if not, what extensions of the Standard Model would be required to explain them.
Once the signals have been established, the analysis turned to the study of matter-antimatter asymmetries. Before being observed in the LHCb detector, the particles can “resonate” to form intermediate particles. For example, in the Λb0 → pπ-π+π- decay prominent signals of Δ(1232)0 particle decaying to a proton and a π- meson, and rho(770)0 meson decaying to a pair of π+ and π- are found. The decay products of Δ and ρ particles define two planes with a relative angle Φ, as seen in the left image below. The measured asymmetries presented in the right image show local differences and, at some values of the angle Φ, they become as large as 20%. The statistical significance of these asymmetries differing from zero is 3.3σ, leading to the first evidence for CP violation in baryon decays. In the past, analogous large effects were also seen in three-body charmless B decays. The analysis of Λb0 → pπ-K+K- decays has lower sensitivity due to a smaller number of observed events, and has not revealed evidence for non-zero asymmetries so far.
The full 3 fb-1 run 1 data sample was used to obtain this result. The number of beauty particle decays recorded by LHCb in run 2 is already larger than that used in this analysis. Therefore in the near future we will know if the evidence reported here will grow towards a 5σ observation.
5 December 2016: End of 2016 data taking period.
The 2016 data taking period ended this morning. This was a very successful year, both because of the excellent performance of the accelerator and of the LHCb experiment itself.
The left and central images below show the growth in integrated luminosity during different years of LHC operation. Integrated luminosity is a measure of the number of proton-proton collisions produced by the accelerator. The increase of integrated luminosity with time in 2016 was similar to that in 2012; the period of data taking was, however, shorter. This excellent performance is mainly due to the higher efficiency of the accelerator this year. LHCb has recorded 1.67 fb-1 of proton-proton collisions this year, 5 times more than in 2015, a year in which LHC collider was setting-up its operation at the record energy of 13 TeV. The total integrated luminosity recorded during run 2 reached now the target of 2 fb-1 compared to 3 fb-1 collected in run 1.
The number of collected events per unit time for a given physics process, N, is proportional to the instantaneous luminosity, L, which is a measure of the brightness of the colliding beams, and to the characteristic property of interaction, cross-section, σ; N=σL. The cross-section for b- and b-quark production at 13 TeV proton-proton collision is about twice that of run 1 (see 5 August 2016 news, item (1)). Since the integrated luminosity is merely the instantaneous luminosity added up over the time the accelerator is operating, the 2 fb-1 Run 2 data sample contains a larger number of beauty particle decays than the 3 fb -1 Run 1 data sample. The increase of number of recorded charmed particles per unit of integrated luminosity was even higher, by a factor of 5, both due to the higher cross-section at 13 TeV and to improvements in the event selection (so-called “trigger”) during data taking. This is excellent news for future LHCb physics analyses. The right image above shows a typical proton-proton collision event.
The proton collisions with lead ions took place during last three weeks of data taking period. The left image above shows the increase of integrated luminosity as a fuction of time depending in which LHC ring protons and lead (Pb) ions are located; the first one (p or Pb) indicates which beam is pointed towards the one-sided LHCb detector. In total 31 nb-1 were recorded. The right image above shows a typical high multiplicity proton-lead-ion collision event, the left one below shows a zoom around the collision vertex and the right one below shows the event as seen from above. Particles identified as pions (orange), kaons (red), protons (magenta), electrons (blue) or muons (green) are shown in different colours. In addition to collected p-lead collisions, LHCb collected data from proton-helium interactions at the same time. This was achieving by injecting helium gas as a target directly in the region of the LHCb Vertex Locator (VELO), and recording those occasions when the circulating protons hit these near-stationary helium nuclei. Such a mode of operation is called "fixed target" data taking and at the LHC is unique to LHCb.
A traditional end of the year shutdown period starts now. It is used for maintenance and improvements of the LHC and its detectors. This time it will be longer due to major installation work carried-out in one the LHC detectors. Therefore the name of “Extended Year End Technical Stop”, EYETS, has been given to this shutdown period. LHCb plans for this period include some relatively minor maintenance work on the detector, together with more major interventions on the access lift and the crane in the underground cavern. It is planned that protons will start to circulate again in the LHC at the beginning of May and that the first proton-proton collisions for physics will take place in mid of June.
Click the images for higher resolution.
1 December 2016: New results presented at the CKM workshop.
The LHCb collaboration presented several new results at the 9th International Workshop on the CKM Unitarity Triangle (CKM2016) taking place this week at the Tata Institute of Fundamental Research in Mumbai, India. A few selected items are listed below.
(1) A surprising first observation of a decay of the charm beauty meson. LHCb searched for the decay of the Bc+ meson into charm D0 and strange K+ mesons and surprisingly found a signal. The Bc+ is the only meson consisting of two heavy quarks of different flavour, namely a b and a c quark. Unlike other B mesons, the b quark decay accounts only for a small fraction of Bc+ decays, with the c quark decay accounting for the majority of them. The image shows a clear enhancement around the Bc+ mass in the D0 and K meson invariant mass spectrum. The contribution of the Bc+→D0K+ decay, shown as the red Gaussian distribution, is observed with a statistical significance of 5.1σ. The result is surprising because the measured decay rate is high compared to theoretical predictions, and it was not yet thought that there were sufficient data collected to see a signal. It will be a nice challenge for theoretical physicists to understand the origin of this unexpected result. For experts: The so-called “tree-amplitude” decay Bc+→D0π+ is not yet observed. The Bc+→D0K+ decay proceeds predominantly through penguin and annihilation decay diagrams. This is the first observation of a Bc+ decay dependent on such processes.
(2) CP violation and γ angle measurements in Bs→DsK decays. The image shows the DsK invariant mass distribution for different charge combinations of Ds and K mesons. The Bs meson contribution is shown as the dashed red line while the other shaded areas represent different background components. A detailed analysis allowed LHCb physicists to extract a value of the Cabibbo-Kobayashi-Maskawa (CKM) angle γ (see introduction in the 5 October 2012 news). The Bs→DsK decay proceeds via so-called “tree-level” diagrams, in which contributions of new physics are not expected. Together with other “tree-level” γ angle measurements (see item (1) in the 13 March 2016 news) it provides a powerful consistency check of the Standard Model and establishes reference measurements which could then be compared with the γ angle determination from other processes in which signs of new physics effects could show up.
(3) First observation of CP violation in Bs
(4) Study of the γ angle from B-→D0K*- decays. The potential for the measurement of the γ angle and of various CP violation measurements in B-→D0K*- meson decays was presented. In this measurement the D0 meson decays into various pairs of charged π and K mesons. B-→D0K- decays have been extensively analysed in the past but the B-→D0K*- mode is studied at LHCb for the first time. The analysis has demonstrated that high purity is possible in this decay mode at LHCb owing to the absence of any significant physics backgrounds, as shown in the image. While in items (1), (2) and (3) the analyses used the full 3fb-1 LHC run 1 data sample, this analysis profited from an additional 1fb-1 collected during run 2 data taking period. The significant increase in signal yield per unit of integrated luminosity in run 2 (see 5 August 2016 news, item (1)) bodes well for improvements in the knowledge of γ in future analyses at LHCb.
See more details in the LHCb CKM workshop presentations linked from the section titles in italic and soon in the LHCb papers and conference notes. Click on the images for higher resolution.
28 September 2016: Charming time asymmetries.
The LHCb collaboration presented many new interesting results at the VIIIth International Workshop on Charm Physics (Charm 2016) which took place in Bologna. In particular, precise measurements on the difference between the lifetime of D0 mesons, composed of a cu quark pair, and their partner D0 mesons, with opposite quark content cu (when decaying either to a pair of pions or a pair of kaons) were reported, in order to search for CP-violating effects in the Charm sector.
The phenomenon of CP violation, that is related to the difference between properties of matter and antimatter, is still unobserved in the charm-quark sector. As charm mesons are composed of up-type quarks only, this uniqueness makes the study of their properties particularly relevant. Such properties might be sensitive to effects beyond those predicted by the Standard Model. According to the Standard Model, CP-violating asymmetries in the charm sector are expected to be very small, below the 10-3 level for this measurement. Remarkably, the LHCb experiment is now approaching a level of precision where such small effects could be observed.
Charm mesons are produced either directly in the proton-proton collisions or in the decays of heavier beauty particles. Only the first category was used in this analysis. The difference of the D0 and D0 meson decay rates into K+K- and π+π- pairs was measured as a function of their decay time t. Two distinct measurements were done, which make use of the same data sample but with different experimental approaches. For the K+K-, the asymmetry of the rates is shown in the images. The variation of these quantities depend on the so-called parameter AΓ. The numerical results for AΓ can be found in the LHCb presentation at the workshop as well as in the two conference notes (09) and (10). These are the most precise measurements of CP violation ever made in the Charm sector, and are consistent with no CP violation with a precision of a few parts in 104. Many checks have been done in order to verify the good accuracy of the measurements to such an impressive level of precision. As an example, results obtained with two different orientations of the LHCb magnetic field, "up" and "down", are compared in the left image above for the data taken in 2012. Note that LHCb presented also other interesting measurements of CP violation at Charm workshop, as well as at the ICHEP conference before, see item (2) of the 5 September 2016 news. Read also the CERN update for the public in English and French.
19 August 2016: Excellent performance of LHC and LHCb.
The LHC collider and the LHCb detector continue to work very well. LHCb has collected already 1 fb-1 (integrated luminosity) of data this year, three times more than in 2015. Given that 7 more weeks of proton-proton collisions remain in this year's schedule, it can be hoped that the final data set will be significantly larger. Taking into account that the beauty-particle production rate at the higher collision energy of run 2 is more than twice that of run 1 (see item (1) of ICHEP 2016 news), the total number of beauty-particle decays collected during run 2 is likely to be already higher than the total of run 1 by the end of this year. To reach this achievement LHCb has profited from the improvements to the data acquisition. In addition, the data accumulated in 2016 benefits from the revolutionary design of the new LHCb trigger.
The proton-proton collision period will end at November 1st and then will be followed by a LHC machine maintenance period (technical stop) and three weeks of proton collisions with lead ions. The image shows the integrated luminosity progress during the different years of data taking. Follow the progress of data taking by clicking at the links to live information at the top of this page, “LHC and LHCb Status Displays”, “LHCb Event Display”; the frequently updated image reports the LHCb delivered and recorded luminosity.
5 August 2016: New results presented at the traditional ICHEP conference.
The LHCb collaboration presented this week new results at the 38th International Conference on High Energy Physics, ICHEP, which is taking place place at Chicago. A few selected items are listed below.
(1) Measurement of the b-quark production cross-section in pp collisions at 7 and 13 TeV. The probability of b- and b-quark production (cross-section) in proton-proton collisions can be calculated in the framework of the theory of strong interactions, quantum chromodynamics (QCD). Sizeable uncertainties exist in the absolute predictions, but they are strongly reduced in the ratio R13/7 of calculations performed for 13 TeV and 7 TeV. The image shows a comparison of these calculations with the results of the LHCb measurement as a function of the variable, pseudo-rapidity η related to the b-hadron production angle θ measured with respect to the direction of incoming proton beams, η=-ln(tan(θ/2)). A striking difference between measurement and theory is observed at low η values corresponding to b-hadron production at larger θ angles. These results provide significant information for the understanding of QCD.
(2) Most precise CP violation measurement of a single decay of charmed particles. The measurements of D0 mesons decays into pairs of K+ and K- mesons was reported. This measurement is interesting since charmed D-meson decays are suitable for probing CP violation in the up-type quark sector. CP violation is related to the difference between the properties of matter and antimatter. It is measured here as the asymmetry parameter ACP, which differs from zero if and only if the probability of the D0 meson to decay into a K+K- pair differs from that of the D0 meson. Recent studies of CP violation in weak decays of D mesons show a good consistency with the hypothesis of CP symmetry so far, in agreement with the expectation of the Standard Model, which predicts very small violation in the charmed system. This is to be contrasted with K and B meson decays, where CP violation is well established, again in broad agreement with the predictions of the Standard Model. By combining the results presented at the conference with previously reported the most precise determination of CP violation in the D0 decays into K+ and K- as well as into π+ and π- meson pairs from a single experiment have been obtained. The image summarises the current situation. The LHCb results presented at Chicago are shown as a dashed green ellipse. The D0 and D0 decays were identified using a so-called “pion tagged” technique. The dashed blue ellipse shows previous results which used a “muon tagged” technique and the red one represents the combination of both measurements. The black ellipse shows a world combination made by the Heavy Flavor Averaging Group, HFAG, which includes the results of other experiments as well as older LHCb results. As apparent from the image there is no evidence for CP violation yet.
(3) Rarest fully hadronic B decay mode ever observed. The B0-meson decay into a K+K- meson pair is observed for the first time ever with a significance of more than 5 standard deviations. This decay was searched for in the past for a long time by other experiments. The left image below shows a contribution of this decay to the the K+K- invariant mass spectrum. It occurs at a rate of less than 1 time in 10 million B0 decays. This result will help to refine the QCD calculations of the dynamics governing the decays of heavy-flavoured hadrons. The understanding of this dynamics is a fundamental ingredient in the search for new particles and interactions beyond those included in the Standard Model.
(4) First attempt to probe anomalous photon polarisation in Bs to φγ decays. The b-quark to s-quark transition accompanied by a photon γ is considered as a very interesting process in which signs of new physics could show up. New physics models often predict a different value of photon polarisation than that predicted by the Standard Model. LHCb physicists already succeeded to observe a non-vanishing value of the γ polarisation for the first time in B+→K+π-π+ γ decays (see 28 February 2014 news for introduction and the measurement details) and have probed its nature in B0→K*e+e- decays. This week the collaboration reported the first measurement of the photon polarisation in Bs to φγ decays. The right image above shows the φγ invariant mass spectrum with a clear accumulation of events at the Bs mass. The lifetimes of the mesons in this sample have been analysed and from this study information has been extracted on the photon polarisation. The result is consistent with the Standard Model prediction within 2 standard deviations.
(5) Measurement of time-dependent CP violation in B0 to D+D- decays. The Standard Model angle β of the unitarity triangle is usually measured in the decays of the B0 and B0 mesons to J/ψ and Ks0 mesons, see 3 March 2015 news. The measurement of β in the decay of the B0 meson into D+D- mesons is relevant as it enables higher order Standard Model and new physics contributions to be probed and constrained. LHCb reported the measurement of the, so called, C and S observables. The results are consistent with the Standard Model expectation, which in the absence of these additional contributions, are S of around -0.75(-sin2β) and C = 0. The images below show the comparison of the LHCb results with previous measurements by the BaBar and Belle collaborations.
28 June 2016: Observation of four exotic-like particles.
Today the LHCb collaboration has submitted for publication two papers (one brief, and the other one containing full details) reporting the observation of four "exotic" particles decaying into a J/ψ and a φ meson, only one of which was well established before. These results were based on a result of a detailed analysis of charged B+ meson decays into J/ψ, φ and K+ mesons using the full run 1 data sample. The data could not be described by a model that contains only ordinary particles, i.e. composite hadrons built of either a quark and an anti-quark, or three quarks. Each of the four particles is observed with a significance exceeding five standard deviations. The sophisticated analysis of the angular distribution of B meson decay products observed with the LHCb detector (a so-called multidimensional full amplitude analysis) allowed LHCb to determine properties (quantum numbers) of the particles with high precision. The images below show the reconstruction of the contributions (in the J/ψφ invariant mass distribution) marked as peaking structures with different colours. The right image shows the spectrum for the selected events with the φK invariant mass to be above 1950 MeV. The properties of these structures are consistent with their interpretation as four-quark particles, which are considered as "exotic", (hence the “exotic-like” name in the title), although the details of the four quark ccss binding mechanism is still under discussion. The binding mechanism could involve tightly bound tetraquarks or strange charm charged meson pairs Ds Ds* bouncing off each other and rearranging their quark content to emerge as a J/ψφ system (called a “cusp” by experts; note that J/ψφ and DsDs* systems have the same quark content). The high statistics of the LHCb data set and the sophisticated techniques exploited in the analysis will help to shed further light into the production mechanisms of these particles. More information can be found in the first paper, with further details given in the second.
The interest in these four states is also that they are the only known exotic candidates which do not contain u and d quarks, which are the lightest quarks and those which human beings and the matter around us are made of. As such, they may be more tightly bound than other exotic particles.
The observation of the X(4274), X(4500) and X(4700) particles was announced for the first time today. Evidence for the X(4140) near-threshold J/ψφ mass peak at a level of 3.8 standard deviations was first announced by the CDF collaboration and later confirmed by the CMS and D0 collaborations. Searches for this particle by the Belle and BaBar collaborations gave negative results. The LHCb analysis yields a clear observation of the X(4140), and indicates a particle with similar mass but larger width to the earlier measurements from CDF, CMS and D0. It is important to emphasise that simple `bump-hunting' in the mass spectra is not sufficient to learn about the nature of such complicated hadronic structures. Rather, a multidimensional full amplitude analysis, as described in the two papers submitted today, is crucial for data interpretation, and has allowed LHCb to characterise fully the particles, and to determine their quantum numbers.
These results have been recently presented by LHCb physicists at the Meson 2016 workshop, and at the Rencontres de Blois, FPCP, LHCP and BEACH conferences. The LHCb collaboration has made several other important contributions to the investigation of exotic particles. In February 2013 the quantum numbers of X(3872) were determined. In April 2014 the collaboration published results of measurements which demonstrated that the Z(4430)+ particle is composed of four quarks (ccdu). In July 2015 the first observation of two pentaquark particles, i.e. hadrons composed of five quarks, was announced.
25 May 2016: The VELO team is the fastest.
Last week the LHCb Running Team, VELOcity, won the first place in its category (senior) during the traditional CERN Relay Race.
Contrary to statements of other competitors, based on the team's name, the LHCb team was not using bicycles to win the race. The VELO is the name of the LHCb VErtex LOcator, the precise silicon detector located around the proton-proton collision point. The detector has been “running” successfully since the LHC start up. Its winning formula of precision measurements and proximity to the beam line allows it to locate the point (vertex) precisely where the beauty particles decay, as seen in many images on this page. Today’s race has shown that not only the VELO detector is on track to chase down more physics, but the VELO team is the fastest going.
click the image to see better the winner's smiling faces
9 May 2016: The most precise measurement of the assl asymmetry.
[ assl = (0.39±0.26±0.20)% ]
Last week at the 16th International Conference on B-Physics at Frontier Machines, "Beauty 2016”, Marseille, France, the LHCb collaboration presented the updated result of a measurement of the semileptonic asymmetry, assl, related to a difference between a probability of a beauty meson, B0s, to oscillate into its antimatter partner, B0s, and a probability of the reverse process. (An introduction to beauty and charm oscillations can be found in the 7 November 2012 news item.) Any difference in this probability would be a manifestation of CP-violation, which is the difference between the properties of matter and anti-matter. The label "s" indicates decays of B0s mesons composed of anti-beauty b and s quarks, while "sl" (semileptonic) indicates that leptons, in this case muons, are present among decay products. The full run 1 data sample of 3 fb-1 was used to obtain this update of the 1 fb-1 2012 measurement. The LHCb result is the most precise measurement of assl to date and is consistent with the value predicted in the framework of the Standard Model. For this particular quantity the amount of CP-violation is expected to be tiny and hence the predicted value of assl in the Standard Model is very small. Therefore the possible contribution of as yet undiscovered effects, which help to drive the B0s - B0s oscillations, could lead to significant changes in assl. The precise LHCb result allows constraints to be placed on the properties of these possible new effects, and points the way for future theoretical and experimental studies.
The image shows the overview of the most precise measurements of assl and adsl. The adsl results were obtained from analogous measurements of B0-B0 oscillations. The new LHCb result is shown, as well as the LHCb adsl 2014 result. The horizontal and vertical bands indicate the naive averages of pure assl and adsl measurements obtained by different experiments. These averages are consistent with the small values predicted by the Standard Model and show no evidence for new physics effects.
The yellow ellipse shows a result from a measurement of the D0 experiment at the Tevatron which is related but not uniquely determined by a linear combination of assl and adsl. This result could indicate the presence of a new physics contribution, see the 7 July 2012 news for details. However, this measurement is not in good agreement with the results for the individual measurements of assl and adsl.
23 April 2016: Run 2 Year 2 physics commissioning with stable beams.
This night the LHC collider has reached the “Stable Beam” state for the first time this year following the traditional LHC winter shut-down and the collider restart commissioning procedure. In “Stable Beam” conditions the experiments can switch on safely their sensitive sub-detectors. Therefore LHCb physicists were able to activate the tracking detectors as well as the Vertex Locator VELO and to observe particle tracks for the first time this year. The left image shows the LHC main information screen. The right image shows a typical event fully reconstructed during data taking. Particles identified as pions, kaon, etc. are shown in different colours.
Thanks to the joint effort between the SMB, EN and TE CERN departments in collaboration with LHCb, this year data taking is controlled from the new LHCb Control Centre presented in the left image below. The right image is a photograph of a meeting yesterday morning between LHCb physicists in preparation for today's data taking.
Click the images for higher resolution.
The proton intensity in the first “Stable Beam” run was very small with only 3 proton bunches per beam. LHCb can nevertheless use the collected data for physics commissioning with low-intensity stable beams. It is planned that the intensity will be increased during the following runs this weekend. During the coming week the LHC commissioning will continue and the data taking will resume at the beginning of May with increased proton-proton collision intensity. During the first period of operation LHCb will profit from its revolutionary improvement of data acquisition and analysis developed a year ago. The calibration (precise determination of the relationship between the detector response and physical quantity being measured) and alignment process (determination of the relative geometrical locations of the different sub-detectors with respect to each other) now takes place automatically in the computer farm during data taking and the recorded data are immediately available for the physics analysis. Hence it will be possible to begin analysing the new data for physics measurements very rapidly.
Many exciting results have been reported on this web page. They are based on data collected during the three-year Run 1 and the first year of Run 2. In this, the second year of Run 2, it is expected that LHCb will collect substantially more data than in 2015. This larger sample will enable LHCb to obtain even more precise, interesting and, hopefully, surprising results, see video. As before follow LHCb data taking by watching live event display as well as live LHC and LHCb status pages.
11 April 2016: Theatre of Dreams: LHCb looks to future
80 physicists gathered in Manchester on April 6th and 7th to discuss the future of the LHCb experiment. The LHCb collaboration is currently constructing a significant upgrade to its experiment which will be installed in 2019/2020. The workshop in Manchester, entitled “Theatre of Dreams: Beyond the LHCb Phase I Upgrade”, explored the longer term future of the experiment in the second-half of the coming decade, and thereafter. The image below shows the Workshop participants at the University of Manchester museum.
In the mid-2020s the LHC will be upgraded for higher luminosity operation. At this time the ATLAS and CMS experiments plan to undertake major phase II upgrades of their experiment. These works will necessitate a long-shutdown of at least 2.5 years duration. The meeting discussed enhancements to the LHCb experiment, dubbed a Phase Ib upgrade, that could be installed at this time. Although relatively modest, these improvements could bring significant physics benefits to the experiment. These include an addition to the particle Identification system using an innovative time of flight system based on Cherenkov light; placing detector chambers along the sides of the LHCb dipole to extend the physics reach by reconstructing lower momentum particles; and replacing the inner region of the electromagnetic calorimeter with new technology, thus extending the experiment’s measurement programme with photons, neutral pions and electrons.
In the second half of the 2020s, the LHCb upgraded experiment that is currently under construction will reach the end of its planned programme. At this time a Phase II upgrade of the experiment could be foreseen. The goal would be to collect an integrated luminosity of at least 300 fb-1, with an instantaneous luminosity a factor ten above the upgrade that will operate in the 2020s. Promising high luminosity scenarios for LHCb from the LHC machine perspective were shown which would potentially allow this goal to be reached, and each of the elements of the LHCb experiment presented their first thoughts on how these goals might be achieved. The experimental physics programme, the theory perspectives of heavy flavour physics, and the anticipated reach of Belle II and the other LHC experiments were also considered.
Many promising ideas were presented at the workshop and these will be followed up in the forthcoming months to identify the requirements and the R&D programmes that will be needed to bring these concepts to reality.
The meeting was sponsored by STFC, Institute of Physics and Institute of Particle Physics Phenomenology and the University of Manchester.
13 March 2016: World's most precise measurements
and search for the X(5568) tetraquark candidate.
[ γ=(70.9+7.1-8.5)° ; Δmd=(505.0±2.1±1.0)ns-1 ; X(5568) not confirmed ]
The LHCb Collaboration has presented today at the Rencontres de Moriond EW new important results.
(1) CKM γ angle measurements. The parameters that describe the difference in behaviour between matter and antimatter, known as CP violation, are constrained in the so called CKM, or unitarity, triangle. The angles of this triangle are denoted α, β and γ, and among these, γ is the least precisely known. A detailed introduction to the measurement of γ can be found in the 5 October 2012 news and in this CERN Courier article. The γ value of (70.9+7.1-8.5)° presented today was obtained from the combination of many different LHCb measurements, notably including some new results unveiled today for the first time, and is the most precise determination of γ from a single experiment. One of the new analyses presented today uses decays of charged B mesons into charmed D mesons and pions π or kaons K. Then the D mesons decay in turn into various combinations of π’s and K’s. The image displays the different rates of positive and negative B mesons, clearly indicating different properties of matter and antimatter.
(2) Determination of the B0 oscillation frequency. A fascinating feature of quantum mechanics, in which the B0s, B0 and D0 particles turn into their antimatter partners, has been discussed already few times at this page, see 15 March 2011, 7 November 2012 and 3 March 2013 news. This feature is called oscillation or mixing. LHCb physicists presented today the most precise single measurement of the parameter which sets the B0 meson oscillation frequency to be Δmd=(505.0±2.1±1.0) ns-1. The full run1 data sample of semileptonic B0 decays with charged D or D* mesons was used in this analysis. The image shows the characteristic oscillation pattern of B0 mesons.
(3) Non-confirmation of the X(5568) tetraquark candidate. Two weeks ago the D0 Collaboration at Fermilab reported the observation of a narrow structure, X(5568), in the invariant mass of the Bs0 meson and a charged pion π, see Fig. 3 in the D0 publication, and interpreted it as a tetraquark candidate composed of four different quarks (b, s, u and d). An introduction to four-quark systems, or tetraquarks, can be found in the 9 April 2014 news.
The LHCb Collaboration reported today a result of a similar analysis using a sample of Bs0 mesons 20 times higher than that used by the D0 Collaboration. The Bs0π invariant mass spectrum is shown in the figure using the Bs0 mesons decaying into J/ψ and φ mesons or into Ds and π mesons. No structure is seen in the region around the mass of 5568 MeV (indicated by the arrow). Hence the LHCb analysis does not confirm the D0 result. Read more in the LHCb conference note, in the LHCb paper, in the LHCb CERN seminar and also in the LHCb presentation at the Rencontres de Moriond QCD.
11 December 2015: Awards of the LHCb Kaggle competition.
The world’s community of machine learning data scientists organizes “Kaggle” competitions to solve the most difficult and interesting challenges in different fields. This year, for the first time, a competition has been proposed by the LHCb experiment. The aim of the exercise was to establish the best way of looking for a phenomenon that is not (yet?) known to exist – the decay of a tau (τ) lepton into three muons μ’s, denoted τ → μμμ. This decay is forbidden in the Standard Model of particle physics and therefore its observation would indicate a discovery of "new physics", which is now the key goal of the LHC. The awards of this competition were announced today at the Applying (machine) Learning to Experimental Physics workshop organized in the framework of the Twenty-ninth Annual Conference on Neural Information Processing Systems.
Machine (computer) learning is widely used in particle physics in order to select a small number of interesting candidates found in individual collisions, called by physicists “signal events”, among often a large number of uninteresting events called “background”. Some machine learning algorithms are inspired by biological neural networks. They are used to estimate functions that can depend on a large number of input information supplied by physicists, which is different for the signal and background events. The model then “learns” from the simulated signal and background events what the best criteria to separate them are. An output function is then generated indicating if the experimentally measured events look more or less like the signal or background events. Currently the most popular machine learning algorithm used in particle physics is called a “Boosted Decision Tree (BDT)”, see for example, its use in the Bs→ μμ decay observation paper.
673 teams took part in the competition from all over the world. They worked with real data from the LHCb experiment mixed with the simulated data of τ → μμμ decays. The solutions presented by the winning teams were presented at the workshop. The LHCb collaboration have offered also two physics prizes for the most useful solutions for their analysis. The winners will be invited to the Heavy Flavour Data Mining workshop in Zurich in February 2016. The prizes were sponsored by Yandex and Intel.
The competition may allow LHCb physicists to learn some computing tricks from the machine learning community. This community, on the other hand, may possibly have improved their skills by having tackled this very challenging particle physics challenge.
2 December 2015: A mysterious ridge effect.
The LHCb collaboration has submitted today a paper reporting the study of correlations in particle production in proton-lead ion collisions at the LHC. The plots (see below) showing the angular distribution of these correlations exhibit features similar to a “ridge” in a mountain landscape. Therefore physicists name this kind of analysis a study of a “ridge effect”.
Two sets of data were taken: proton-lead and lead-proton, where in the second case the direction of the proton and lead ion beams were reversed. This allowed the LHCb detector, recording the particles only on one side of the interaction point, to make measurements in the case of the proton beam pointing towards the LHCb detector as well as in the opposite case of the lead beam pointing to it.
The idea of the analysis is simple. LHCb physicists plotted differences Δ between production angle of particle pairs both in angle θ along the direction of incoming proton or lead beams, and in angle φ around this direction. The difference in angle θ is plotted as the difference of a related variable, pseudo-rapidity η, where η=-ln(tan(θ/2)). In this way the plot bin sizes closer to the beam direction are expanded.
The Δη-Δφ distributions show three different features. The high peak structure near Δη=Δφ =0 is caused by a clustering of particles, forming a jet-like structure. This peak has a width of about Δη=1 and is truncated in the images in order to allow a better visibility of other features. Since momentum must be conserved, another structure is observed on the opposite side at Δφ=π. The distribution in Δη is, however, broad and is referred to as the “away-side ridge”.
The third structure, the “near-side ridge”, is the most interesting and its study is the main subject of today's paper. It is seen on both sides of the high peak at Δη=Δφ =0 in the images above. Only properties of 3% of events, characterized by the highest “activity”, are contributing to the shown distributions. A very precise Vertex Locator detector (VELO), surrounding the proton-lead interaction region, is used to measure the number of produced particles in the collision and to define the activity of the event. The two plots above show that the near-side ridge is more pronounced in the highest activity events (event class 0-3%) when the lead (Pb) beam is pointing towards LHCb detector (Pb+p) than in the opposite case.
The numbers of particles observed in the LHCb detector are not identical in these two cases. It is higher in the case of lead ions pointing towards the LHCb detector (Pb+p) than in the opposite case (p+Pb). Here LHCb physicists have introduced an original idea to the analysis and also made an important discovery. By comparing events with similar numbers of particles observed in the acceptance of LHCb detector (similar absolute activity), LHCb physicists found out that the sizes of the near-side ridge effect in the direction of the lead (Pb+p) and proton (p+Pb) beams are in fact compatible.
The ridge-like near side effects were studied in heavy ion collision experiments in order to investigate a possible manifestation of a quark-gluon plasma formation, see 10 May 2013 news for an introduction. The observation of the similar near-side ridge correlations in proton-proton collisions by the CMS collaboration and in proton-lead collisions by the LHC experiments came as surprise since the formation of quark-gluon plasma was a priori not expected in these collisions. The interpretation of today’s LHCb results obtained in the forward acceptance is clearly an interesting challenge for theorists.
LHCb, the world's first dedicated b-physics experiment at a hadron collider, has obtained not only excellent heavy flavour results, but in addition the quality of the LHCb detector and its unique forward geometry allows it to obtain also important results in other fields, like electroweak physics or heavy ions physics, as reported today. The LHCb contribution to the heavy-ion physics will increase significantly in the near future since presently, for the first time, the collaboration is taking also lead-lead collision data.
Read more in the LHCb publication.
5 November 2015: 20 birthday candles for LHCb.
In August 1995 a Letter of Intent was submitted for LHCb, the world's first dedicated b-physics experiment at a hadron collider. Today, the LHCb Collaboration has marked the 20th anniversary of this event with a special celebratory meeting.
Click the cartoon (Adrien Miqueu author) for higher resolution.
The beauty quark b and anti-quark b bound-state Υ was discovered in 1977 at Fermilab. For many years after, the study of beauty hadrons, composed of a b quark or its anti-quark b and other quarks, was dominated by experiments at e+e- colliders, which included DORIS, CESR, VEPP and LEP. So called "fixed-target" experiments at hadron accelerators, at which beauty particles were produced in collisions of accelerated protons with stationary objects were limited in their scope, as the rate of beauty particle production was small compared to production rate of other particles. At hadron colliders, on the other hand, the beauty particle production rate is much higher. The Fermilab collider experiments CDF and D0 at the Tevatron, although not designed specifically for studies of beauty particles, took advantage of this opportunity and started to obtain interesting results in the 1990s. The e+e- collider B factory BaBar and Belle experiments were approved in 1993 and started to take data in 2000.
It was clear that the high proton-proton collision energy at the LHC would give rise to a very high beauty particle production rate. But how to conduct high precision experiments in this very difficult environment? At the LHC workshop in Evian in 1992 three b-physics experiments were proposed:
- COBEX proposed to use a forward spectrometer in the proton-proton colliding mode, as the majority of beauty particles are produced around the direction of colliding protons.
- LHB proposed to use extraction of protons from the collider in a fixed target experiment.
- GAJET proposed to inject a gas into a collider tube; the gas molecules could then play the role of a fixed target.
In June 1994 the LHC Committee decided not to approve any of the three individual experiments, but requested that the interested parties form one new collaboration to propose a single new experiment based on the collider mode to exploit its large bb cross section with a convincing trigger strategy. Adopting the collider mode was the correct choice since not enough b hadrons would have been produced at the LHC in a fixed target setup, compared with the B-factory and Tevatron experiments, which had performed beyond original expectations.
The Letter of Intent for LHCb was submitted in 1995 and the experiment was approved in 1998. The design of the experiment was re-optimized in 2003, a process in which many important improvements were made. The tracking stations in the magnet were removed, reducing strongly secondary interactions of particles from the beauty particle decays, instead all the first tracking stations were made in silicon technology. The particle interactions in the collider beam tube were reduced by replacing the standard technology with one made of beryllium. Finally, improvements in technology allowed the whole LHCb detector to be read out to the computer farm at a 1MHz rate, thereby improving the beauty particle selection process.
But what could be the name of the experiment and its logo? Alternative name versions were used 20 years ago: “LHC-B”, “LHCB” or “LHCb”. The final name was fixed in 1997 with the cute choice for the LHCb logo. Here the “cb” is transformed in the mirror image into “CP” which is then “violated” by the red bar pointing to “CP violation” as the one of the most important themes of LHCb research.
The ideas imagined 20 years ago have been very successful and allowed excellent physics results to be obtained, as reported on this web page. Beauty hadron distributions can be measured with background levels as low as those obtained at e+e- colliders and they are collected at much higher rate. In addition the production of all beauty mesons and baryons is observed at LHCb contrary to B-factory experiments, which are limited to studing light beauty meson decays only.
Read more about the history of LHCb, its physics results and prospects for future in the LHCb20-fest presentations. By clicking on the detector photos at the LHCb20-fest web page you can learn more about the LHCb detector. By clicking on the physics result images at the same web page you can learn more about significant LHCb results.
14 October 2015: Revolutionary improvement of data acquisition and analysis.
The first integrated data-taking and analysis in a High Energy Physics experiment.
The procedure of data-taking and analysis at hadron colliders is performed in two steps. In the first one, called by physicists “online”, the data are recorded by the detector, read-out by fast electronics and computers, and finally a selected fraction of events is stored on disks and magnetic tapes. The stored events are then analyzed later in the so called “offline analysis”. An important part of the offline analysis is the determination of parameters which depend on the data-taking period, for example alignment (determination of the relative geometrical locations of the different sub-detectors with respect to each other) and calibration (precise determination of the relationship between the detector response and physical quantity being measured). The whole data sample needs then to be “reprocessed” by the computers with this new set of parameters. The whole process takes a long time and uses a large amount of human and computer resources.
In order to speed-up and simplify this procedure, the LHCb collaboration has made a revolutionary improvement to the data-taking and analysis process. The calibration and alignment process takes place now automatically online and the stored data are immediately available offline for physics analysis. The new procedure allowed LHCb to present the first LHC run 2 physics results at the EPS-HEP conference just a week after the data-taking period ended. In the following the new procedure is described in more detail.
Current technology does not allow all LHC proton-proton collision data to be stored and analysed. An event selection procedure is therefore necessary following the scientific goals of each experiment. This selection procedure can be made by fast electronics (“hardware trigger” in physicists language) or/and by computers (“software trigger”). At LHCb the fast electronics (hardware trigger) reduces the 30 million per second (30 MHz) LHC proton-proton collision rate (as visible by the LHCb detector) to 1 MHz using the characteristic properties of beauty and charm particle production and decays. The data are then read-out from the whole detector and are transferred at the 1 MHz rate to around 300 electronic cards as the one shown in the left image. The fast calculations performed in these cards allow the data volume to be greatly reduced by removing the content not containing information about the current event. The data from all these cards are then transferred to a predefined computer in the LHCb farm, shown in the right image, situated near the detector 100 m underground. The data transfer speed of the network between the cards and the computer farm is so high that it would be capable to carry the entire mobile phone traffic of a country such as Switzerland. The farm contains about 1800 computer boxes with a total of about 27 000 physical processors, nearly doubling the available processing power with respect to the LHC run 1 period.
The software trigger computer programs, called also “High Level Trigger (HLT)”, run in the computer farm. At the first stage, HLT1, the less interesting events are removed and the data flow rate is reduced to 150 kHz. The selected events are stored in a 5 PB disk “Buffer” (see image). An automatic procedure is then run which aligns around 1700 detector components and calculated about 2000 calibration constants, all within a few minutes. The alignment and calibration parameters are then used in the second stage of the software trigger, HLT2, processing the data stored in the Buffer with the same quality as would be the case in the offline analysis. The additional selection reduces the data flow rate to 12.5 kHz and the reduced data sample is stored for future analysis. The output data are directed into two samples. The larger volume “full” data sample allows the whole offline reconstruction and associated processing to be redone as required. The “turbo” data sample keeps only information necessary to perform physics analysis with the offline quality obtained during the HLT2 processing. This “turbo” sample was used to obtain results presented at the conference just a week after the period of data collection ended, as was mentioned above.
This new approach of making offline-quality information available to the trigger, and performing physics analysis directly on the trigger output data, represents a paradigm shift in data processing for particle physics experiments, and will have significant consequences for the future physics programme of LHCb.
26 September 2015: A measurement of a fundamental parameter of the Standard Model.
[ sin2θWeff = 0.23142±0.00073(stat)±0.00052(sys)±0.00056(theory) ]
The LHCb collaboration has submitted a paper based on run 1 data which reports the measurement of a fundamental parameter of the Standard Model (SM), the electroweak mixing angle, θW. This parameter quantifies the relative strengths of electromagnetism and the weak force. It is, therefore, an important experimental challenge to measure it.
The squared sine function of the electroweak mixing angle sin2θWeff was precisely measured by the experiments at the Large Electron-Positron Collider (LEP) in the 1990s. The electrons and positrons were collided at the energy of about 91 GeV at which the Z boson resonance was formed. The Z bosons than decayed into pairs of leptons (e+e-, μ+μ-, τ+,τ-) or pairs of quarks. By determining the electric charge and direction of these decaying particles a quantity called the "forward-backward asymmetry" was measured, and then used to calculate the value of sin2θWeff. The forward-backward asymmetry is related to how often the produced matter particle travels in a similar ("forward") direction as the incoming matter particle involved in the collision. An important measurement was made also at another electron-positron collider SLAC Linear Collider (SLC) with the detector SLD. The sin2θWeff parameter was determined from the analysis of a so called left-right asymmetry obtained by counting the difference in number of Z bosons produced with two opposite (left and right) spin orientation of colliding electrons using the longitudinally polarized SLC electron beam. The measurement requires no detailed final-state identification. It turned out that the most precise measurements of sin2θWeff, the LEP forward-backward b quark asymmetry (AFB(b)) and the SLD left-right asymmetry (ALR), differed by 3.2σ (see a figure from the LEP-SLD electroweak report in which all the details of measurements are explained). This 3.2σ LEP-SLD puzzle has been interpreted by some commentators as an effect driven by new physics. It is a very interesting challenge for measurements at other particle colliders to resolve this puzzle.
Measurements of sin2θWeff have continued at the Tevatron at Fermilab and LHC at CERN. The task is, however, more difficult than at an e+e- collider. The parton (quark and antiquark) distributions inside proton need to be known precisely. At the LHC the Z bosons used in this analysis are produced mainly in the collisions of (valence) quarks with high momentum and (sea) antiquarks with low momentum. The Z bosons then decay into a pair of electrons or muons. The LHCb geometrical acceptance is ideal for this measurement. The incoming quark direction, needed to define the sign of the asymmetry, can be identified correctly 90% of the time.
LHCb has measured the forward-backward asymmetry AFB in the angular distribution of muons in dimuon final states as a function of the dimuon mass both at 7 and 8 TeV centre-of-mass energies. An example of the angular asymmetry, for data taken at 8 TeV, is shown in the left image as measurement points compared to a (shaded) Standard Model prediction. The LHCb sin2θWeff result is in very good agreement with the other determinations from LEP, SLD, Tevatron and LHC, and is one of the most precise measurements obtained at a hadron collider. The precision of the measurement is not yet sufficient to shed light on the interpretation of the 3.2σ LEP-SLD puzzle as shown in the right image. However, it is very promising and shows that with the additional data, expected in LHC run 2 and beyond, a very interesting determination of this fundamental parameter should be possible.
27 July 2015: Is a b to a u quark transition modified by a new particle?
The LHCb collaboration published today in Nature Physics a paper based on run 1 data which reports the determination of the parameter |Vub| describing the transition of a b quark to a u quark. This measurement was made by studying a particular decay of the Λb0 baryon. Other measurements of |Vub| by previous experiments had returned two sets of inconsistent results, depending on which method was used to determine the parameter. Theorists had suggested that this discrepancy could be explained by the presence a new particle contributing to the decay process, which affected the result differently, depending on the measurement method. Today's result from LHCb removes the need for this new particle, while the puzzle of why the original sets of measurements do not agree persists.
The Λb0 baryon is like a proton, but containing a beauty (b) quark in place of one of the up (u) quarks such then when the b quark decays into an u quark it transforms the Λb0 into a proton. A W boson is emitted in this process and decays in turn into a muon μ and a neutrino νμ. The measurement of decays involving a neutrino is very challenging at a proton collider and it was quite a surprise that this measurement could be done. The probability of the Λb0→pμνμ decay depends on the SM parameter |Vub|. The image shows two simultaneous proton-proton collisions inside the LHCb detector shown by the pink ellipses. The Λb0 baryon is produced in the right hand side collision and travels a distance of about 1 cm until it decays into a proton (orange), a muon (pink) and an invisible neutrino.
The |Vub| is a parameter of the 3x3 Cabibbo-Kobayashi-Maskawa (CKM) matrix. In the SM the CKM matrix describes the decay of one quark to another by the emission of a W boson. While the SM does not predict the values of the parameters of the CKM matrix, the measurements of these parameters in different processes should be consistent with each other. If they are not, it is a sign of physics beyond the SM. The consistency is visualized with a help of so called “unitarity triangle”. The LHCb |Vub| result determines a length of the triangle side opposite to the angle β in the left image below, which together with the measurement of the angle γ gives a consistent SM description. On the other hand, the |Vub| result marked “Inclusive” does not, as will be discussed below.
The Λb0→pμνμ decay belongs to a class of so called “exclusive” measurements meaning that a specific (exclusive) decay is used. The Babar and Belle collaborations have in addition measured the decay rate in an “inclusive” way by summing over all possible B meson decays containing the b to u quark transition. Their results from exclusive and inclusive measurements showed a large difference. This could be explained by a new particle, in addition to the W boson, contributing to the quark transition. (For experts: if this new particle had a right-handed coupling, as opposed to the W boson that only interact with left-handed quarks, the inclusive and exclusive results could be made to agree.) In the right figure above, the crossing of the purple and the red band at about -0.2 on the εR x-axis shows that the new particle would have to have a strength of about 20% with respect to the W boson for the quark transition. The measurement from LHCb using a Λb0 decay gives a different dependence of the transition probability from a new particle as marked with the green band. This is due to the different spin of the Λb0 baryon compared to the B mesons used previously. That the crossing of the purple band with the green band is exactly at zero removes the need for a new particle. However, it still leaves the puzzle as to why the inclusive and exclusive measurements do not agree. Further intensive research, both at the experimental and theoretical level, will continue to try to understand this disagreement.
24 July 2015: First LHC run2 physics results.
Measurement of J/ψ production cross-sections in pp collisions at 13 TeV.
[ σ(prompt Jψ) = 15.35±0.03±0.85 μb; σ(Jψ-from-b-hadron) = 2.36±0.01±0.13 μb; σ( bb) = 518±2±53 μb ]
At the EPS-HEP 2015 conference in Vienna the LHCb collaboration has presented today the first measurement of the probability to produce a J/ψ meson in proton-proton collisions at 13TeV. Using this measurement they also determined the rate at which beauty quarks are produced at this new, higher energy.
The first task of physicists when operating an experiment at a higher energy is to measure the probabilities of well-known processes. These can be compared to theoretical predictions to establish a firm base upon which to build searches for new physics. The probabilities are related to quantities known as “cross-sections”, σ. How to measure these probabilities is explained in the "How bright is the LHC?" news.
The J/ψ was discovered on 11 November 1974. The importance of this discovery is highlighted by the fact that the subsequent, rapid changes in high-energy physics at the time have become collectively known as the "November Revolution". The spokespersons of the experiments who made this discovery, Richter and Ting, were rewarded for their shared discovery with the 1976 Nobel Prize in Physics. The J/ψ is composed of a charm quark c and an anti-charm quark c. At the LHC, collisions containing J/ψ meson decays are used as an excellent tool for detector calibration, as well as for the first cross-section measurements at new energy frontiers.
The production of J/ψ mesons can be described in two stages. In the first stage a cc quark pair is produced and in the second stage the cc pair forms a J/ψ meson. The first stage can be calculated with the theory of strong interactions, QCD. On the other hand, the second stage, after forty years of theoretical and experimental efforts, is still not fully understood. The J/ψ mesons that are produced in this way are called “prompt J/ψ”. J/ψ mesons can also be observed as a product of of the decays of beauty hadron (containing b quarks) and therefore this component is called “J/ψ-from-b-hadron”.
The two components are clearly visible in the left image which shows the J/ψ decay time distribution with respect to the pp collision time. The data are shown as black points with error bars, the solid red line shows the best data interpretation while the prompt J/ψ contribution is shown in cross-hatched blue. The J/ψ-from-b-hadron contribution in black falls exponentially with a time constant characteristic to the lifetime of beauty hadrons. The right image shows an example of the ratio distribution between the rate of J/ψ-from-b-hadron production at 13 TeV and 8 TeV. The cross-section of J/ψ-from-b-hadron decay is used to compute the total beauty quark pair bb cross-section. The expected rise of the beauty particle production rate of about a factor 2 with respect to run 1 at 8 TeV is confirmed by the data. This increase in rate will enable LHCb to obtain even more precise, interesting and, hopefully, surprising results in the LHC run 2 as explained by Barbara, Mika and Patrick.
14 July 2015: Observation of particles composed of five quarks,
pentaquark-charmonium states, seen in Λb0 → J/ψpK- decays.
[ m(Pc+(4450)) = 4449.8±1.7±2.5 MeV, Γ = 39±5±19 MeV ]
[ m(Pc+(4380)) = 4380±8±29 MeV, Γ = 205±18±86 MeV ]
The LHCb collaboration submitted today a paper based on run 1 data which reports the observation of pentaquark-charmonium states decaying into a J/ψ meson and a proton p. In the traditional quark model, the strongly interacting particles (hadrons) are formed either from quark-antiquark pairs (mesons) or three quarks (baryons). Particles which cannot be classified within this scheme are called exotic hadrons. In his fundamental 1964 paper, in which he proposed the quark model, Gell-Mann mentioned the possibility of adding a quark-antiquark pair to a minimal meson or baryon quark configuration. It has taken 50 years, however, for measurements to be performed that unambiguously demonstrate the existence of these exotics. In April 2014 the LHCb collaboration published results of measurements which demonstrated that the Z(4430)+ particle, first observed by the Belle collaboration, is composed of four quarks (ccdu). Today, the collaboration has announced the observation of a pentaquark, that is a hadron consisting of five quarks.
LHCb physicists have analyzed a sample of about 26 000 Λb0 → J/ψpK- decays with only 5% of background contamination. The Λb0 baryon is like a neutron, but containing a beauty quark in place of one of the down quarks. This decay can proceed by the diagram (a), which involves conventional hadrons and is dominated by Λ* resonances that decay in turn into a proton p and K- meson. It can also have exotic pentaquark contributions, shown in diagram (b), that result in resonant structures (called Pc+ in today's paper) at 4380 and 4450 MeV in the J/ψp invariant mass spectrum shown in the left image below. The Pc+ particles decaying into a J/ψ meson and a proton must have a minimal quark content ccuud, and are therefore called pentaquark-charmonium.
Claimed discoveries of pentaquark states by other experiments in the past have turned out to be spurious. LHCb physicists have therefore performed a thorough analysis to demonstrate that the signals observed in their data cannot be produced by conventional hadrons, and that they have the properties expected from an exotic resonance, that is a short-lived particle lying outside the traditional quark-model.
LHCb physicists first tried to describe the data with conventional hadrons from the traditional quark model, including 14 Λ* resonances. As this did not give a satisfactory description of the data, they tried to add one Pc+ state, and when that was not sufficient they added a second state. The first one has a mass of 4449.8±1.7±2.5 MeV and a width of 39±5±19 MeV, while the second is wider, with a mass of 4380±8±29 MeV and a width of 205±18±86 MeV. The statistical significance of each of these resonances is more than 9 standard deviations. The J/ψp invariant mass spectrum from the data (below, left) is shown as solid (black) diamonds with error bars, while the solid (red) points show the results of the best data interpretation taking into account quantum mechanical description of Pc+ states formation in the presence of conventional Λ* resonances. The blue open circles with the shaded histogram represent the contribution of the Pc+(4450) state. The purple filled circles connected by the histogram represent the Pc+(4380) state. This wider state is clearly seen in the insert for which a more restricted range of the Kp invariant mass spectrum (above 2 GeV) was required. Each Λ* component is also shown in different colors.
In the final step of the analysis the LHCb physicists set out to prove that both Pc+ structures really possess the properties of a resonant particle, that is a quantum state with well-defined quantum numbers, which is produced, lives for some time, and then decays. The images above prove to experts that this is indeed the case. The central one shows the so called Argand diagram indicating that the Pc+(4450) structure seen in the data (black points) represents really the resonant particle production and decay, since it approximately follows a circular path (red circle), as is expected for a resonant particle. The right one shows the so called Dalitz plot in which a distinct horizontal band near 19.5 GeV2 in the J/ψp invariant mass squared indicates the resonant Pc+(4450) and Pc+(4380) contributions. The analysis of angular distributions of the decay products leads to the most probable assignment of the two Pc+ quantum numbers JP to be 3/2- and 5/2+ although two other combinations are possible.
A typical Λb0 → J/ψpK- decay is shown above. The Λb is produced in the pp collision point, the origin of many particle tracks, and decays at a distance of 3.9 cm into a J/ψ meson, a K meson and a proton p. The J/ψ meson decays in turn into a μ+ and μ- pair. The kaon K and the proton p particles traverse the LHCb detector and are absorbed in the hadronic calorimeter. The two muons (μ) leave the calorimeter system and traverse the whole muon detector system. The event can also be seen in the top view as well as in the front view of the Λb decay point region.
A new field of research now opens up. Scientists will try to understand the “internal mechanism” of quark interactions inside pentaquarks. The two possibilities are illustrated in the figure. The color of the central part of each quark is related to the strong interaction color charge, while the external part shows its electric charge. The quarks could be tightly bound, or they could also be loosely bound in meson-baryon molecule, in which color-neutral meson and baryon feel a residual strong force similar to the one that binds nucleons together within nuclei. It is hoped that future measurements by LHCb will help to answer this question.
Click the images for higher resolution. Read more in the LHCb publication in arXive and in PRL, in the CERN Press Release in English and French, in the CERN Courier article, in the Nature news, in the APS Viewpoint, in the Symmetry magazine, in the Quantum Diaries blog and in different media articles. See also the Fermilab video explaining the LHCb observation and the Gell-Mann's comment.
Added in August 2016: see APS Synopsis, "Pentaquark Discovery Confirmed", and two papers showing that the evidence for pentaquarks in the discovery data is model independent and that the analysis of the Λb0 → J/ψpπ- decays confirmes observation of the two pentaquark particles.
3 June 2015: First Physics at 13 TeV.
LHC Run 2 started today. LHCb physicists have been eagerly awaiting this moment since 14 February 2013 when the last Run 1 collisions took place. The LHC has reached the “Stable Beams” state for the first time for two years. In these conditions the experiments could switch on safely their sensitive sub-detectors. This allowed LHCb physicists to activate the tracking detectors as well as the Vertex Locator VELO and to observe particle tracks for the first time. The right image shows a typical event fully reconstructed during data taking. Particles identified as pions, kaon, etc. are shown in different colours. The left image shows LHCb physicists in the control room.
LHCb has published many results based on data collected during the first three-year Run 1. But this was only the beginning. Collisions at 13 TeV will double the production rates of beauty hadrons enabling LHCb to obtain even more precise, interesting and, hopefully, surprising results.
Read more in the CERN Press Release.
25 May 2015: An intriguing anomaly.
Measurement of the decay B0 → D*+τ- ντ
[ Branching fraction ratio B0 → D*+τ- ντ/ B0 → D*+μ- νμ = 0.336±0.027±0.030 ]
Today, at the 13th Flavor Physics and CP violation conference in Nagoya (Japan), the LHCb collaboration presented the preliminary result from a measurement of the branching fraction ratio R(D*): B0 → D*+τ- ντ/ B0 → D*+μ- νμ. The result 0.336±0.027±0.030 is larger than the Standard Model (SM) expectation of 0.252±0.003 by 2.1 standard deviation (σ).
In the SM all charged leptons, such as taus (τ) or muons (μ), interact in an identical fashion (or, in physicists’ language, have the same "couplings"). This property is called "lepton universality". However, differences in mass between the leptons must be accounted for, and affect decays involving these particles. The τ lepton is much heavier than the μ lepton and therefore the SM prediction for the ratio R(D*) is substantially smaller than 1. This ratio is considered to be precisely calculable thanks to the cancellation of uncertainties associated with the B to D* meson transition.
Any measurement exhibiting a conclusive breakdown of lepton universality, after mass related effects are accounted for, would be a clear sign of new physics. The ratio R(D*) is particularly interesting since a large class of SM extensions contain new interactions that involve third generation of quarks and leptons, like here a b quark (from a B hadron) and τ- and ντ leptons. In particular, the presence of additional charged Higgs bosons, which are often required in these models, can have a large effect.
The image shows a comparison of different results for R(D*). Already a previous measurement from the BaBar collaboration was found to be 2.7σ above the SM prediction. Therefore, the particle physics community has been eagerly awaiting new results. The LHCb measurement confirms the behaviour seen by BaBar. A new measurement from the Belle collaboration, (marked "Hadronic tag") also presented at Nagoya, lies closer to the SM, but is also consistent with the BaBar and LHCb measurements. An older, less precise measurement by Belle (marked "Inclusive tag") is compatible with the same picture.
Taken together these results constitute an intriguing anomaly, and one that is sure to provoke much discussion. Future measurements from LHCb with the run-1 data set, and the data sample to be collected in run 2, will allow for the precision on R(D*) to be improved further. The R(D*) anomaly takes its place alongside the RK puzzle (3 June 2014 news) in hinting that the SM assumption of lepton universality may be incorrect.
The LHCb R(D*) measurement is the first measurement of this quantity at a hadron collider and also represents the first measurement of any decay of a B meson into a τ lepton at a hadron collider.
Click the image for higher resolution. Read more in the LHCb presentation at Nagoya, in the LHCb publication, in the Scientific American news, in the Nature news, in the Nature Research Highlights and in the Guardian blog.
21 May 2015: First 13 TeV collisions.
Today, proton beams collided at 13 TeV for the first time inside the LHCb detector. The collisions were part of the ongoing accelerator tests, and are not useful for physics studies. The collected data are, however, useful for refining the synchronization of the readout time of different parts of the calorimeters and muon detectors. The image displays an event taken today.
These collisions were the latest in a sequence of careful steps to prepare the LHC for physics operation at 13 TeV. Proton beams first circulated again after a two year long shutdown on the 5th April, and collided at the relatively low energy of 450 GeV on the 5th May. First collisions for physics studies, in so-called “Stable Beams” conditions, are expected in early June.
A new LHCb sub-detector, HeRSCheL, also collected today its first proton-proton collisions data. HeRSCheL, a system of forward shower counters, was installed during the two-year shutdown for distinguishing between processes where the interacting proton in the beam remains undetected passing down the beampipe with no activity recorded in HerScheL and processes in which a signal is observed. It consists of several stations, like the one shown in the video, located around the LHC beam pipe in the accelerator tunnel on both sides of the LHCb detector. It is interesting to note that some of these detectors are over 100m away from LHCb, but still see particles from the same collision.
Click the image for higher resolution. Read more in the CERN news.
13 May 2015: Observing nearly invisible.
Observation of the rare Bs0→μμ decay from the combined analysis of CMS and LHCb data
[ Branching fraction Bs0→μμ = (2.8+0.7-0.6)x10-9 ; B0→μμ = (3.9+1.6-1.4)x10-10 ]
The CMS and LHCb Collaborations have published today in Nature the first observation of the decay Bs0→μμ, with a statistical significance exceeding six standard deviations, and the best measurement of its branching fraction to date. Furthermore, a three standard deviation evidence for the decay B0→μμ is also obtained. Both measurements are statistically compatible with Standard Model (SM) predictions and allow stringent constraints to be placed on theories beyond the SM. This is one of the most important results obtained by the LHC experiments from the run 1 and tests the SM to the ninth decimal place. The excellent performance of the CMS and LHCb detectors and their data analyses was crucial in obtaining this result as well as the outstanding performance of the LHC itself. The two experiments found a total of about 100 Bs0 and B0 decays into two muons in a sample comprising 1012 beauty hadrons collected during 2011 and 2012.
The SM of particle physics describes the fundamental particles and their interactions via the strong, electromagnetic, and weak forces. It provides precise predictions for measurable quantities that can be tested experimentally. The SM predicts that the Bs0→μμ and B0→μμ decays are very rare, with about four of these decays occurring for every billion Bs0 mesons produced and one decay for every 10 billion B0 mesons. Prior to the start of operation of the LHC, no evidence for either decay mode had been found, despite around 30 years of searching at previous experiments. Upper limits on the branching fractions were an order of magnitude above the SM predictions.
The probabilities, or branching fractions, of the Bs0 and B0 mesons to decay into two oppositely charged muons are very small in the SM and are well predicted. On the other hand a large class of theories that extend the SM, like supersymmetry, allows significant modifications to these branching fractions and therefore an observation of any significant deviation from the SM prediction would indicate a discovery of new effects. The Bs0 and B0 meson decays into a muon pair have long been regarded among the most promising class of measurements where these new effects could show up. Previous LHCb results already severely constrained the type of SM-extension models that are still allowed, as described in the 30 March 2012 news. The results announced in today's publication isolate even more precisely the parameter region in which these new models can exist, and therefore focuses future experimental searches and theoretical attention. All candidate models of physics beyond the Standard Model will have to demonstrate their compatibility with this important result.
The two collaborations first released their individual results for Bs0 meson decay as described in 24 July 2013 news. While the results were in excellent agreement, both fell just below the five sigma statistical precision historically needed to claim an observation. The combined analysis improves the precision of the results and in the same time easily exceeds the 5 sigma requirement, reaching 6.2 sigma. This is the first time that a combined analysis of sets of data from more than one LHC experiment has been performed.
The Bs0 and Bs0 mesons are produced in the pp collision point and decay into a muon pair after a distance of the order of 1cm. The left image shows the Bs0 meson decay into two muons presented as green tracks traversing the whole detector. A few other Bs0→μμ decay images can be found at this web page (1, 2, 3). The right image shows one of the μ+μ- invariant mass spectra released today, in which an excess, attributed to the signal decays, can be clearly seen above the expected background shape.
The LHCb and CMS experiments will resume data taking in June with proton-proton collisions at a centre-of-mass energy of 13 TeV, which will approximately double the production rates for Bs0 and B0 mesons and lead to further improvements in the precision of these crucial measurements.
5 May 2015: First 2015 proton-proton collisions.
Today, for the first time in over two years, protons have collided inside the LHCb detector. Each proton beam had an energy of 450 GeV, which is the value they have when injected from the SPS to the LHC.
Although these collisions are not at the nominal energy of 13 TeV, and therefore not aimed for physics studies, the collected data are useful for precise synchronization of the readout time of different parts of the calorimeters and muon detectors with the time at which different particles originating from the proton-proton collision point traverse them.
The left image shows LHCb physicists in the LHCb control room. The right image displays an event taken today. Click the picture and play with the 3D view of a few recorded events.
In a follow up step, most probably tomorrow, LHCb physicists will inject neon gas into the LHC vacuum tube in order to measure the shape of the proton beam, by seeing where the proton-gas interactions occur. This beam-gas imaging method, used only at LHCb, allows the "luminosity" of the colliding beams to be determined, and is described in the news item of 7 October 2014. The luminosity is a vital component in determining how often different physical processes occur in proton-proton collisions. Measurements of these processes at the record proton-proton collision energy of 13 TeV are the among the first physics goals of LHCb at the restart of data taking in June.
5 April 2015: LHC proton beams are circulating again.
Today, for the first time in two years, both proton beams are circulating again in the LHC. The picture shows the LHC operators steering the two beams. The proton beam already traversed the LHCb detector one month ago and then continued through one quarter of the LHC circumference, see 7 March 2015 news.
The two month period of re-commissioning with beam starts now. The LHC magnets are already “trained” to accept the high current needed to produce the magnetic field necessary to hold the protons in their orbit with a record energy of 6.5 TeV inside the LHC ring. Therefore the prospects for reaching the proton-proton collision energy of 13 TeV at run 2 are very good.
The LHCb detector and its data acquisition system are ready to take data at this highest proton-proton collision energy. LHCb physicists still continue to analyse the run 1 data obtaining exciting results, many of which have been reported on this website. They are looking forward to the first 13 TeV collisions that are expected in June and are convinced that a bright and exciting future lies ahead.
20 March 2015: B0→K*μ+μ-: new analysis confirms old puzzle.
Angular analysis of the B0→K*μ+μ- decay.
Today at the 50th Rencontres de Moriond Electroweak, in La Thuile (Italy), the LHCb collaboration presented the result of a first full angular analysis of the B0→K*μμ decay using its full LHC run 1 data sample. A previously published analysis of the experiments 2011 data sample found a deviation with respect to a calculation based on the Standard Model, see 9 August 2013 news. The particle physics community has been eagerly awaiting the results from the full data sample ever since.
The analysis of the B0→K*μμ decay is considered as a very promising way to search for effects of yet undiscovered particles, see the 14 June 2013 news for an introduction. Unfortunately, the analysis of this decay is also complicated; the best sensitivity to the new particles comes from the study of the angular distribution of the muons and the kaon and pion from the K* meson decay. Physicists from the LHCb experiment have studied different angular observables as functions of the mass of the muon pair. It was one of these observables, "P5'", that showed a local deviation with respect to the Standard Model calculation (at a level of 3.7σ in the mass squared of the muon pair region q2 from 4.3 to 8.68 Gev2/c4). There is particular interest in these observables because their theoretical prediction is much less dependent on a good understanding of the hadronic physics involved in turning a B meson into a K* meson (so called form-factors). These observables are therefore ideal for searching for the effects of new particles in this decay.
The image shows the distribution of the P5’ observable as functions of the mass squared of the muon pair q2. The black points show the LHCb results presented for the first time today. The Standard Model predictions are presented as orange boxes. These were taken from calculations described in a recent theoretical paper. The behaviour seen in the 2011 data sample, shown as the blue points for comparison, is confirmed using the full data set. The measurements in the q2 region between 4 and 8 GeV2/c4 are both 2.9σ from the Standard Model calculation.
The results of the analysis of the B0→K*μμ decay will continue to attract the attention of the particle physics community in the coming years. In the next couple of years, the LHCb collaboration will improve the precision of their analysis with the help of data collected in run 2 of the LHC. It is also anticipated that the theoretical predictions (the orange regions in the image) will improve in precision. Theorists will be busy trying to make sense of this measurement, and seeking for possible associations with other unexpected effects found in similar decays, for example the RK anomaly (see 3 June 2014 news). Stay tuned for future developments on this page.
Read more in the LHCb presentation at La Thuile, in the LHCb conference note, in the CERN news, in the CERN Courier article, in theoretical physicists' blogs here and here, in the theorist's comment in the CERN Courier and in the LHCb paper.
7 March 2015: The proton beam has traversed the LHCb detector.
The LHCb collaboration is ready to take 13 TeV proton-proton collision data.
This weekend the LHC proton beam has traversed the LHCb detector for the first time in over two years. At the end of November 2014 the proton beam already arrived at a stopper placed in the accelerator known as the “TED”, located at the end of the SPS-LHC transfer line about 300m from the LHCb detector, see 23 November 2014 news.
This weekend the TED exercise was repeated and the LHCb Collaboration recorded again muons produced by protons absorbed in the TED. The TED was then opened and the proton beam entered the LHC ring and was first absorbed in another stopper called the “TDI” located inside the LHC ring about 50m from the LHCb detector. A very large number of particles produced during this absorption process traversed the LHCb detector. Later on the TDI was opened, the proton beam traversed the LHCb detector inside its beam pipe, then travelled through one quarter of the LHC circumference and finally arrived to the LHC beam dump area. The left image shows places, called “hits” by physicists, at which muons from the proton beam interaction at the TED traversed different LHCb sub-detectors. Click the picture and play with the 3D view of these events.
Both proton beams are expected to make the full turn of the LHC collider by the end of March and the first proton-proton collisions at the nominal Run 2 energy of 13 TeV are expected by the end of May.
The LHCb collaboration is ready to take high energy proton-proton collision data. The two year Long Shutdown (LS1) period offered an opportunity for prolonged access, and hence an extensive programme of consolidation and maintenance work. In summer 2014 a detailed field measurement of the LHCb dipole magnet was performed followed by the re-installation of the beam pipe, see the left image, through which proton beams will circulate in both directions. One section of the beryllium beam pipe was replaced. The new beam pipe support structure is now much lighter and therefore unwanted interactions with it of particles measured inside the LHCb detector are strongly reduced.
3 March 2015: Matter-antimatter trigonometry with LHCb.
Precise measurement of the unitarity triangle angle β.
Today at Les Rencontres de Physique de la Vallée d'Aoste, La Thuile, Italy, the LHCb collaboration presented an important result in our quest to understand the nature and origin of CP violation, which is a difference in behaviour between matter and antimatter. The result, derived from a careful analysis of the full run 1 data sample, is a measurement of the angle β of the ‘unitarity triangle’. This triangle is a geometrical representation of CP violating and associated parameters in the Standard Model. One side is defined to have unit length, the other two sides and three angles can be measured independently in different decays of beauty hadrons. It is the task of experimental physicists to measure these properties and see if they provide a consistent description of the triangle. Any discrepancy would point to signs of New Physics beyond the Standard Model. LHCb has already performed the world’s most precise measurements of γ, one of the other triangle angles, see 11 September 2014 news, and the mixing frequency of Bs mesons, see 7 November 2012 news, which is an essential ingredient for the determination of the side opposite to the angle γ.
The unitarity triangle is shown in the above left figure, with each experimental input represented by a coloured region. The result for β is given by the hatched blue diagonal band, marked as sin2βLHCb. The measurement is made from studying the decays of about 41500 B0 and B0 mesons to J/ψ and Ks0 mesons. The B0 mesons can decay to J/ψKs0 in two ways. They can decay directly to J/ψKs0 or they can oscillate, see 7 November 2012 news, into their antimatter partners B0 which in turn decay also to J/ψKs0. (This possibility of two paths in the decay process is a B physics analogue of the classical quantum mechanics two-slit experiment, which is described in this pedagogical video). The interference between the amplitudes for the two decay paths results in a time-dependent asymmetry between the decay time distributions of B0 and B0 mesons, as seen in the right image above. The oscillation amplitude measures sin2β, and gives the magnitude of CP violation present in the process. A value of sin2β=0 would indicate no CP violation. LHCb physicists announced today the value of sin2β=0.731±0.035±0.020, which is consistent with the geometrical expectation provided by the measurements of the other parameters of the unitarity triangle, and hence with the predictions of the Standard Model itself.
This result also confirms earlier measurements of the same quantity performed by the e+e- collider B factory experiments BaBar and Belle. These experiments were constructed specifically to measure sin2β. Their results were vital in confirming the broad validity of the Standard Model description of CP violation and led to the award of the 2008 Nobel Prize of Physics to the Japanese theorists M.Kobayashi and T.Maskawa, who had been central in developing this description. The solid blue diagonal band shows the new world combination of the B factory measurements with the new LHCb result.
Although the LHCb result for the angle β is not yet as precise as the combined result coming from the average of the B-factory measurements, it is similar in precision to the J/ψKs0 analyses of the individual experiments. It has been long-known that this measurement is a priori more difficult to perform in a hadron collider, such as the LHC, but the new result presented at La Thuile is an emphatic statement that LHCb can significantly contribute to our knowledge of this fundamental parameter. A still more precise result will be achievable with the data to be collected in run 2, which will allow for more stringent tests of the Standard Model.
23 November 2014: The proton beam knocks at the LHC door.
The LHCb collaboration took proton interaction data this weekend.
The proton beam knocked at the LHC's very solid door this weekend and found it still closed, but nonetheless managed to provide the LHCb collaboration with very interesting data. The CERN accelerator system (see video) is now fully operational, except for the LHC collider itself. This past weekend, CERN accelerator system operators tested the two transfer lines between the SPS and LHC. One of these lines ends with a so-called beam stopper known as the "TED", located at the end of the line about 300m from the LHCb detector. The TED is currently closed, and so absorbed the proton beam before it could enter the LHC. However many muons were produced during the absorption process, and these muons passed through the TED and traversed the LHCb detector.
This “beam dump” experiment therefore created an excellent opportunity for LHCb physicists and engineers to commission the LHCb detector and data acquisition system. The collected data are also useful for detector studies and alignment purposes (i.e. determining the relative geometrical locations of the different sub-detectors with respect to each other).
The image shows the shift leader, run coordinator, spokesperson and sub-detector experts in front of the LHCb data acquisition computer screens.
LHCb took its last collision data on 14th February 2013. The two year Long Shutdown 1 (LS1) period that followed has been used for an extensive program of consolidation and maintenance (see 24 January 2014 “underground” news). Collisions are expected to resume again in Spring 2015.
Click the images for higher resolution and read about the LHC side of the story here.
20 November 2014: LHCb at the Open Data portal.
The LHC experiments have today released physics data at the OpenData.cern.ch portal.
The LHCb collaboration has contributed with its International Masterclasses exercise, in which the lifetime of the charm particle called the D0 meson may be measured using real proton-proton collision data recorded by the LHCb experiment during the 2011 data taking period. Details of the LHCb exercise can be found at the LHCb masterclasses web pages.
19 November 2014: First observation of two new baryonic strange beauty particles.
The LHCb collaboration submitted today a paper reporting the discovery of two new particles. The particles, known as the Ξb'- and Ξb*-, were predicted to exist by the quark model but had never been seen before.
Like the protons that the LHC accelerates, the new particles are baryons made from three quarks bound together by the strong force. The types of quarks are different, however: the new Ξb particles both contain one beauty (b), one strange (s), and one down (d) quark. Thanks to the heavyweight b quarks, they are more than six times as massive as the proton. But the particles are more than just the sum of their parts: their mass also depends on how they are configured. Each of the quarks has an attribute called "spin". In the Ξb'- state, the spins of the two lighter quarks point in the opposite direction to the b quark, whereas in the Ξb*- state they are aligned. This difference makes the Ξb*- a little heavier.
The two new particles are observed through their decay into the ground state Ξb0 and a π-. The image shows the distribution of δm, which is defined as the invariant mass of of the Ξb0π- pair minus the sum of the π- mass and the measured Ξb0 mass. This definition means that the lightest possible mass for the Ξb0π- pair, known as the threshold, is at δm=0. The two peaks are clear observation of the Ξb'- (left) and Ξb*- (right) baryons above the hatched red histogram representing the expected background. Both particles have extremely short lifetimes and do not fly any measurable distance before they decay. But we can tell that the Ξb*- is the more unstable of the two, since the peak is wider and the basic rules of quantum mechanics relate the average lifetime of a particle to the width of its mass peak (after taking into account experimental resolution). This is consistent with the pattern of masses: the Ξb'- mass is just slightly heavier than the threshold, so it is allowed to decay into a Ξb0 and a π- but only barely. The insert shows a zoom in the Ξb'- δm region.
The masses, widths, production rates of these particles and more details on the analysis can be found in the LHCb publication. The result shows the extraordinary precision that LHCb is capable of: the mass difference between the Ξb'- and the Ξb0 is measured with an uncertainty of 0.02 MeV/c2, less than four one-millionths of the total Ξb0 mass. By observing these particles and measuring their properties with such accuracy, LHCb physicists make a stringent test of models of nonperturbative Quantum Chromodynamics (QCD). Theorists will be able to use these measurements as an anchor-point for future predictions.
15 October 2014: φs: CP violation in the Bs system — looking for a chink in the armour of the Standard Model.
Final run 1 result: φs = - 0.010 ± 0.039 rad
Today, at the workshop “Implications of LHCb measurements and future prospects” at CERN, LHCb physicists have presented their final run 1 results of their analysis of the phase φs (the Bs CP-violating phase, for experts), see 27 August 2012, 5 March 2012 and 3 March 2013 news for introduction. The measurement of φs is one of the most important goals of LHCb experiment.
The value of φs is precisely predicted in the Standard Model and sets the scale for the difference between properties of matter and antimatter for Bs mesons, known to physicists as CP violation. The predicted value is small and therefore the effects of New Physics could change its value significantly.
In the CP violation that drives φs the role of Bs oscillations (3 March 2013 news) is very important. Here the Standard Model predicts very small effects, thereby allowing New Physics to manifest itself. This is to be contrasted with other manifestations of CP violation in the Bs system, unaffected by oscillations, where the Standard Model expects large signatures. Such a signature was already observed by LHCb, see 24 April 2013 news.
The 96 000 B0s → J/ψ K+K- decays collected during 2011 and 2012 data taking periods were used in the analysis. The left image shows the J/ψ K+K- invariant mass spectrum. The very clean enhancement at the B0s mass is clearly seen.
The right image shows that the K+K- invariant mass spectrum is dominated by the presence of the φ meson resonance at a mass of 1020 MeV. The φ mesons, and similarly the J/ψ and ϒ mesons, are sometimes called heavy photons since they have the same quantum numbers as the photon. In their analysis LHCb physicists took into account different polarization states of the φ meson in analogy to different photon polarization states. For the first time, the phase of φs is measured independently for each polarization state of the K+K- system. No significant difference is observed between the different polarisation states.
The result φs = - 0.058 ± 0.049 ± 0.006 rad is the most precise measurement of φs to date. Recently the LHCb Collaboration has reported the final run 1 measurement of the φs using B0s → J/ψ π+π- decays, φππs = + 0.070 ± 0.068 ± 0.008 rad, consistent with the φs measurement reported above. The results from the two analyses have been combined giving the final value of φs = - 0.010 ± 0.039 rad. This is the most precise value of φs and is consistent with the Standard Model prediction of φs = - 0.0363 ± 0.0013 rad. The parameter region in which New Physics could still hide is now even smaller than before.
The LHCb today's result has been included in the HFAG world combination. The φs results of different experiments are shown in the image together with the ΔΓs variable (see 5 March 2012 news for introduction). The green ellipse in the center represents the LHCb result and the grey ellipse shows the world average. The result of the first φs measurement in the Bs → Ds+Ds- decays φs = + 0.02 ± 0.17 ± 0.02 rad is also included in the LHCb combination shown in the image.
click the images for higher resolution
10 October 2014: Celebrating 200th publication.
The LHCb Collaboration has celebrated this week its 200th publication! The paper “Test of lepton universality using B+ → K+l+l- decays“ appeared in Physical Review Letters on October 6th. The American Physical Society, publisher of the Journal, has selected this article to be presented in “spotlighting exceptional research” news.
7 October 2014: How bright is the LHC?
LHCb has the most precise answer.
The LHCb Collaboration has just published the results of the luminosity calibration with a precision of 1.12%. This is the most precise luminosity measurement achieved so far at a bunched-beam hadron collider (see below).
At the beginning of operation of a new particle collider, physicists start with measurements of probabilities of different physical processes and compare results with theoretical predictions. Differences may indicate signs for new physics or needs for an improved understanding at higher energies of already known processes. The probabilities are measured as the so called “cross-sections”, σ, in units of area. The number of events N measured in an experiment depends on the value of cross-section σ (given by the nature of the physical interaction) and on a collider parameter luminosity L, N=σL . The luminosity L depends on the number of particles in each collider beam per unit of time and on the size of overlap of both beams at the collision point.
The formula N=σL indicates that the luminosity can be obtained from a measurement of a process for which the cross-section can be calculated with high precision. In this way a precision of few x 10-4 was achieved at the e+ e- collider LEP from the measurement of e+ e- small angle scattering process whose rate was calculated precisely using QED theory. Cross-section predictions with similar or better precision are not available for proton-proton collisions. Therefore, purely experimental methods of luminosity measurements are used at the LHC. The number of particles in each colliding beam per unit of time (so called beam currents) are measured using LHC equipment. Two methods are used to measure the size of overlap of both beams at the collision point.
(1) The van der Meer scans (VDM) method, invented in 1968 by Simon Van der Meer, 1984 Nobel Prize winner, to measure luminosity in the Intersecting Storage Rings (ISR) at CERN, the world's first hadron collider. The idea was to measure the beams' overlap by scanning them across each other and monitoring the interaction rate. This method has been used by all four LHC experiments.
(2) The Beam-Gas Imaging (BGI) method was proposed by LHCb physicist Massimiliano Ferro-Luzzi in 2005. It takes advantage of the excellent precision of the LHCb Vertex Locator (VELO) placed around the proton-proton collision point. The BGI method is based on reconstructing vertices of interactions between beam particles and residual gas nuclei in the beam vacuum in order to measure the angles, positions and shapes of the individual beams without displacing them. This allows one to actually see the trace of the beams. To this date only the LHCb Collaboration is capable of using the BGI method.
The image shows the reconstructed positions of proton-beam-gas collision points. It is clearly seen that the proton beams are crossing at an angle. This is intentional since the protons in LHC are not continuously distributed around the ring but are concentrated in 1380 small regions called bunches (so the LHC is a bunched-beam collider). Colliding with an angle and not head-on assures that there are no unwanted collisions between bunches outside the region defined by the experiment. The luminosity was calibrated during special measurement periods and then relative changes were tracked by changes of counting rate of different sub-detectors. The vacuum pressure in the LHC was so low, that in order to increase the proton-beam-gas collision rate, LHCb physicists injected neon gas into the LHC vacuum tube during luminosity calibration periods.
Using the beam-gas data, LHCb physicists were able to reveal that a small fraction of the beam charge is spread outside the expected (i.e. "nominal") bunch locations. Since only collisions of protons located in the nominal bunches are included in physics measurements, it was very important to measure which fraction of the total beam current value measured with the LHC equipment participated in the collisions (i.e. contributed to the luminosity). Only LHCb could measure this fraction with sufficient precision and, therefore, the results of LHCb measurements of the charge fraction outside nominal bunch locations, called the "ghost" charge, were also used by the three other LHC experiments.
For proton-proton interactions at 8 TeV a relative precision of the luminosity calibration of 1.47% was obtained using van der Meer scans and 1.43% using beam-gas imaging, resulting in a combined precision of 1.12%. This represents the most precise luminosity measurement achieved so far at a bunched-beam hadron collider.
The LHCb BGI method was so successful that LHC engineers decided to use it also for the beam size measurements necessary for monitoring LHC operation. Dedicated equipment is now being built and will soon be installed in a specially modified region of the LHC ring. The equipment includes also a gas injection system.
The LHCb Collaboration is very well armed to measure precisely cross-sections of different processes at 13 TeV proton-proton collisions after the LHC restart in spring 2015.
29 September 2014: LHChamber Music.
Today, CERN, the European Organization for Nuclear Research, is blowing out 60 candles at an event attended by official delegations from 35 countries. Founded in 1954, CERN is today the largest particle physics laboratory in the world and a prime example of international collaboration, bringing together scientists of almost 100 nationalities.
To mark the occasion, music-minded physicists have transformed scientific data from the four underground detectors around CERN's Large Hadron Collider into a piece titled LHChamber Music composed by physicist-musician Domenico Vicinanza.
The video, presented during the official ceremony, can be viewed by clicking the arrow above.
The left image shows LHCb physicist Paula Collins, in front of the detector, playing music inspired by the data from the first observation of a heavy flavored spin-3 particle (15 July 2014 news).
The LHCb Collaboration has already presented another way of experimental data "sonification" in the 26 August 2013 news "Matter-antimatter quantum music".
11 September 2014: Combination of measurements of the CKM angle γ.
Today at the International Workshop on the CKM Unitarity Triangle, CKM2014, Vienna, Austria, the LHCb collaboration has presented a combination of measurements of the CKM angle γ (in tree decays, see below). For the first time a single experiment has achieved a precision of better than 10 degrees, which is better than the combination of the results of the B-factory experiments, BaBar and Belle. The parameters that describe the difference in behaviour between matter and antimatter, known as CP violation, are constrained in the so called CKM, or unitarity, triangle. The angles of this triangle are denoted α, β and γ, and of these it is γ that is the least precisely known. A detailed introduction to the CKM angle γ measurement can be found in the 5 October 2012 news and in the CERN Courier article. The measurement of the angle γ in different processes is one of the most important goals of LHCb experiment. The idea is to measure precisely the angle γ in processes in which a contribution from new physics is possible and in processes in which it is not. Comparison between the results of these two categories of measurement is therefore a powerful method to probe for the effects of new physics.
The value of the angle γ = (72.9+9.2-9.9)° presented today was obtained using B(s)→D(s)K(*) decays in the analysis, in which B or Bs meson decays into D or Ds mesons were observed in the full 3 fb-1 2011 and 2012 data set. The image shows a confidence level (CL) curve that indicates which values of γ describe best the LHCb data. The 68.3% horizontal line shows how the γ angle uncertainty was determined. Signs of new physics are not expected to show up in these decays (the so called tree-level measurements, for experts) and therefore they will set a base for comparison with the measurements where observation of new physics effects is possible.
The LHCb result will be improved still further before the start of new data taking period in spring 2015 using the already available data, since there are still important analyses to be completed. In addition LHCb has a large set of γ-sensitive observables in B→Dπ decays not discussed in this news. However their sensitivity to γ is suppressed compared to the B→DK-like decays.
9 September 2014: Measurement of the semileptonic asymmetry, adsl.
Today at the International Workshop on the CKM Unitarity Triangle, CKM2014, Vienna, Austria, the LHCb collaboration has presented the results of a measurement of the semileptonic asymmetry, adsl, related to a difference between a probability of a beauty meson, B0, to oscillate into its antimatter partner, B0, and a probability of the reverse process (an introduction to beauty and charm oscillations can be found in the 7 November 2012 news item). Any difference in this probability would be a manifestation of what is called CP-violation. The label "d" indicates decays of B0 mesons composed of anti-beauty b and d quarks while "sl" (semileptonic) indicates that leptons, in this case muons, are present among decay products. When a B0 meson decays semileptonically, the charge of the lepton determines whether it was a decay of a matter B0 or anti-matter B0 meson. On the other hand a presence of a “wrong-sign” lepton in a decay of a B0 meson indicates that a transition to a B0 meson took place before decay. Therefore these "wrong-sign" B0 and B0 decays were used in the analysis. The measurement of the adsl is very interesting since its value is predicted to be very small by the Standard Model and therefore any significant deviation from zero could indicate a (so called virtual) contribution of not yet discovered particles in B0 - B0 oscillations.
LHCb physicists presented today a new preliminary value of adsl = (-0.02 ± 0.19 ± 0.30)% using the full 3 fb-1 2011 and 2012 data sample. The LHCb Collaboration has published recently a corresponding value for the strange beauty mesons B0s of assl = (-0.06 ± 0.50 ± 0.36)% using 1 fb-1 of data taken in 2011. Both results indicate no CP-violation to be present within the sensitivity of the measurements and hence are consistent with the very small values predicted by the Standard Model.
The LHCb results are shown in the image together with the Standard Model prediction.
The yellow ellipse shows a result from a measurement of the D0 experiment at the Tevatron which is sensitive to both adsl and assl. It lies a significant distance away from the prediction of the Standard Model and hence has excited interest as pointing to a break down in the theory, see 7 July 2012 news for introduction.
The LHCb results are consistent with the Standard Model but do not exclude the D0 result. LHCb will soon update the measurement of assl, which may clarify the situation.
Read more in the LHCb presentation in Vienna and soon in the forthcoming LHCb publication.
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15 July 2014: First observation of a heavy flavored spin-3 particle
Today at the 15th International Conference on B-Physics at Frontier Machines at the University of Edinburgh, Beauty 2014, the LHCb collaboration has presented the results of a study of strange beauty meson B0s decay into an anti-charm meson D0, a K- meson and a π+ meson (B0s → D0K-π+). Previous results indicated the existence of a strange-charm D*sJ(2860)- particle in the D0K- invariant mass spectrum, and the study of the B0s → D0K-π+ decay allows one to study this structure and measure its properties. Today’s LHCb observation shows with 10σ significance that, in fact, this excess seen in the D0K- mass spectrum is composed of two particles with different spins, spin-1 and spin-3. This is the first observation of a heavy flavored spin-3 particle, and the first time that any spin-3 particle has been seen to be produced in B decays.
The B0s → D0K-π+ decay is clearly identified as seen in the left image above. The right image above shows the enhancement at the 2.85 GeV/c2 mass in the D0K- invariant mass spectrum divided into two components as measured by LHCb physicists. The wider one corresponds to the spin-1 particle contribution and the narrow one represents the spin-3 contribution. The non-peaking distributions show contributions from other resonances that peak far from the 2.85 Gev/c2 D0K- mass region.
The left image shows to experts that the data D0K- angular distribution (black points) is very well described by the presence of both spin-1 and spin-3 particles (solid blue curve). The models with only either a spin-1 (red curve) or a spin-3 (green one) particle are not supported by data.
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The D*sJ(2860)- particles are composed of an anti-charm quark c and a strange quark s. The quark-anti-quark pair is bound by strong interactions and can form different quantum states with different values of spin and angular momentum in analogy to the different quantum states of ordinary atoms. The presence of the spin-3 contribution gives a clear signature that both particles are members of the so called 1D family having two units of angular momentum between the quark and the antiquark. This discovery demonstrates that the spectroscopy of the 1D families of heavy flavoured mesons can be studied experimentally. Further insights can be expected with similar analysis of B decays at LHCb and the LHCb upgrade.
5 July 2014: First observation of Z boson production in proton-lead collisions
Today at the 37th International Conference on High Energy Physics in Valencia, the LHCb collaboration has presented the first observation of Z boson production in proton-lead collisions at the LHC at a centre-of-mass energy per proton-nucleon pair of 5 TeV.
The main goal of lead-lead collision measurements at the LHC is to study the possible formation of a quark-gluon plasma state of matter in which quarks and gluons are freely moving instead of being bound inside protons and neutrons. According to the Standard Model of Cosmology this state of matter existed in the universe until 10-5 seconds after the Big Bang. Although LHCb has not collected any data from lead-lead collisions, it has recorded data from proton-lead collisions. Studies performed on this data set make an important contribution to the interpretation of the possible signals of quark-gluon plasma production obtained in lead-lead collisions. LHCb has already shown interesting proton-lead results of this nature, as explained in the 10 May 2013 news.
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The first observation of Z production in proton-lead data presented today in Valencia confirms the importance of LHCb results in this field of research. The image shows the μ+μ- invariant mass distribution. The Z boson peak is clearly visible and the background is negligible. The Z boson was discovered at CERN (together with the W+ and W- bosons) in 1983 by the UA1 and UA2 collaborations and is the carrier of weak interactions.
The LHCb Z boson measurements are valuable for helping to determine the fraction of nucleon momentum carried by colliding partons (quarks and gluons) during the collisions of lead nuclei. The distribution of this fraction, usually termed "x", for partons in nucleons bound inside the atomic nucleus is different from the one for free nucleons. The Z bosons at LHCb are produced by the collisions of partons with small and large values of x allowing information to be learned about the behaviour of partons inside the nuclei both at very low and very large values of x. The capability to explore this kinematical range is an important and unique feature of LHCb experiment.
LHCb is planning to collect proton-lead data for a longer period during the new LHC run, which will allow more precise measurements of Z boson production, and other complementary studies, to be performed.
3 July 2014: Unpredicted production mechanism of ϒ(3S) bottomium state.
Today at the 37th International Conference on High Energy Physics in Valencia, the LHCb collaboration has presented important results on the production of beauty-anti-beauty quark pair bound states called Upsilon mesons, ϒ, in proton-proton collisions at the Large Hadron Collider LHC. The LHCb results include a precise measurement of the mass of one of these states, and more excitingly reveal the striking observation that about 50% of Υ(3S) mesons observed in LHC collisions are in fact not produced directly, but they originate from the radiative decay of χb(3P) mesons.
The beauty-anti-beauty bound state ϒ was discovered in 1977 at Fermilab near Chicago. Just like ordinary atoms bound by the electromagnetic force, the beauty and anti-beauty quarks, bound by the strong force, form different quantum states with different angular momenta and different spin orientations as discussed in the 6 September 2010 news. This atom-like system is called beauty quarkonium (or bottomium).
The different S quantum states (ϒ(1S), ϒ(2S), ϒ(3S)) have beauty quark and anti-quark spins aligned and no angular momentum L giving the total spin J=S=1. On the other hand the P states (χb(1P), χb(2P), χb(3P)) have angular momentum L=1. The χb(3P) state was recently observed by the Atlas and D0 collaborations. Today, the LHCb collaboration has presented the most precise measurement of its mass so far to be performed, and obtained the result 10511.3 ± 1.7 ± 2.4 MeV/c2 (χb1(3P) mass for experts). The dotted red arrow labelled with "γ" indicates a possible decay path of the χb(3P): it may decay into a photon γ and an ϒ(3S) state. Similar transitions are possible to the ϒ(2S) and ϒ(1S) states.
The signal peaks coming from the S states decaying into muon pairs are shown in the left image below. The different states, ϒ(1S) , ϒ(2S) and , ϒ(3S) are very well separated thanks to the excellent resolution of the LHCb detector. LHCb physicists have then calculated the invariant mass of different ϒ states with different photons γ in order to find out if the ϒ(1S), ϒ(2S), ϒ(3S) states originate from the decay of different χb states, or not. This analysis showed that about 50% of the observed ϒ(1S) and ϒ(2S) states indeed come from the radiative decay of the χb particles, as many theorists had predicted. It was, however, generally assumed that the ϒ(3S) mesons were produced directly. Today's LHCb results show, however, that, in fact, also about 50% of the observed ϒ(3S) mesons are not produced directly but they originate from the χb(3P) radiative decay as shown by the presence of the χb(3P) peak in the right image below.
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Theoretical physicists will need to take into account the new LHCb results in their calculation of bottomium states production in proton-proton collisions at the LHC. The differences in the rates of production of different ϒ states in proton-proton and lead-lead collisions at the LHC have been interpreted as possible evidence for the formation of quark-gluon plasma in lead-lead collision. This interpretation will now have to be reassessed, taking account of the new LHCb results.
1 July 2014: Guy Wilkinson and Monica Pepe Altarelli – new management for the LHCb Collaboration
Guy Wilkinson from the University of Oxford begins today his 3-year tenure as LHCb spokesperson. He replaces Pierluigi Campana from the Istituto Nazionale di Fisica Nucleare in Frascati. Monica Pepe Altarelli from CERN will play the role of deputy spokesperson during the same period by replacing Roger Forty and Burkhard Schmidt.
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Guy and Monica will face the huge challenges of run 2 and the following long technical shutdown during which LHCb will undergo a major upgrade. In the meantime, the discovery of new physics could be a dream within reach.
Pierluigi, Roger and Burkhard – thank you for your excellent work.
Guy and Monica – good luck.
17 June 2014: No diploma without LHCb Vertex Locator
The baccalauréat, often known in France colloquially as le bac, is an academic qualification which French and international students take at the end of the lycée (High School) (secondary education). It is the main diploma required to pursue university studies.
Today French students had to pass the examination in physics and chemistry. In the first exercise the French students were calculating collisions at the LHC. Not only was the importance of Higgs boson discovery discussed but also the Beauty meson production inside the LHCb VErtex LOcator (VELO). The VELO detector is composed of two halves placed on both sides of proton beams around the proton-proton collision point. A VELO half is shown in the image.
If you understand French you can try to pass the French baccalauréat exam here.
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3 June 2014: An interesting result presented at the LHCP conference.
The LHCb Collaboration has just presented at the Large Hadron Collider Physics (LHCP) Conference in New York an interesting result: the ratio RK of the probability that a B+ meson decays to a K+μ+μ- or a K+e+e- was measured to be different from one with a 2.6σ significance. The RK is the ratio of two rare processes that occur twice in 10 million events (2x10-7) representing b→s processes highly sensitive to the presence of virtual particles that only exist in extensions to the Standard Model.
In the Standard Model of particle physics this ratio is expected to be very close to one thanks to the so called lepton universality: leptons (like electrons e and muons μ here) behave in the same way (have the same couplings to the gauge bosons, for experts). The small differences in decay rates, tested so far with high accuracy, related mainly to the differences of their masses, are well understood. In the case of two decays discussed here the difference is expected to be at the per-mille level.
The ratio RK is measured in function of the μ+μ- and e+e- invariant mass squared q2. The LHCb result of RK = 0.745+0.090-0.074 ±0.036 measured in the q2 range between 1 and 6 GeV2 is shown in the image as the black point with error bars together with the results from other experiments. It is interesting to note that the result of BaBar Collaboration at low q2 shown by the red left point favors also a value below one. The blue point shows that the result of the RK measurement by the Belle Collaboration is consistent with one in the whole q2 range up to 22 GeV2. The Belle result does not, however, contradict a possible q2 dependence of RK which may be indicated by the BaBar results. As seen in the image the LHCb measurement is the most precise measurement of RK to date.
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Studies of RK could detect the existence of additional Higgs bosons, new scalar or new pseudo-scalar particles that violate lepton universality, or “heavy Z”, Z’, which was suggested as a possible explanation of deviations observed in the P’5 distribution in the analysis of B0→K*0μ+μ- decays, see 9 August 2013 news.
The pp collision data corresponding to 3 fb-1 of integrated luminosity collected by the LHCb experiment at centre-of-mass energies of 7 and 8 TeV were used to obtain this result. It will be interesting to follow up with future LHCb measurements of RK at the 13 TeV pp collisions after the LHC restart planned for April 2015.
Read more in the LHCb presentation at the LHCP conference here, at the CERN public page here, in the Symmetry article, in the LHCb publication here, in the APS Physics Viewpoint and in the theorist's comment in CERN Courier.
24 April 2014: Studying properties of exotic particles.
The LHCb Collaboration has published today a paper which shows that the f0(500) and the f0(980) mesons, long thought to be four quark states (tetraquarks) made out of the light udud quarks (f0(500)) or susu quarks (f0(980)), are NOT consistent with being tetraquarks. The four quark states cannot be classified within the traditional quark model in which the strongly interacting particles (hadrons) are formed either from quark-antiquark pairs (mesons) or three quarks (baryons). They are therefore called exotic particles.
The Collaboration has studied the B0→J/ψπ+π− decays. The left image below shows that the π+π− invariant mass distribution is well understood as a contribution of different resonances and background distribution. The broad contribution of f0(500) is clearly seen in the data. Note that the narrow K0s peak structure is seen in the same mass region. There is no, however, evidence for the f0(980) contribution. The f0(980) production is much smaller than that predicted for tetraquarks and is ruled out at the 8σ level. The tetraquark model predicts a much smaller difference in the production rates of the two f0 mesons. The right image below shows the corresponding π+π− invariant mass distribution for the B0s→J/ψπ+π− decays. The contribution of f0(980) is clearly visible. The absence of f0(980) in B0 decays and its presence in B0s decays as well as the presence of f0(500) in B0 decays and its absence in B0s decays is exactly what is expected if these states are normal qq states. Read more in the LHCb publication here.
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The precision of the LHCb detector and high production rate at the LHC allowed the LHCb Collaboration to made an important contribution to the understanding of exotic state candidates with heavy charm quarks inside, and to draw conclusions which of them are really exotic and which are not. Recently the Collaboration observed the Z(4430) particle and established its quantum numbers which make it the first confirmed unambiguous tetraquark (cucd), see 9 April 2014 news located just below at this page. This observation has implications to astrophysics and was followed up by an impressive media coverage.
The story of the X(3872) particle is more complicated. It was discovered about 10 years ago by the Belle Collaboration and since then was also studied by other experiments. The mass of 3872 MeV is located in a region where the pairs of charm (D) and anti-charm D mesons can be formed. In this mass region many charm quark-antiquark (cc) states are also present bound by a strong force in an atom-like system called charmonium. In addition the two different forms of exotic particles can be formed in this mass region: 1. About 10 fm large loosely bound DD* molecules in which D and D mesons are bound by a strong force in analogy to the deuterium nucleus composed of a proton and a neutron, and 2. About 1 fm compact four quark qqqq elementary particles, tetraqarks.
The LHCb has unambiguously determined the quantum numbers of X(3872) to be 1++, see 26 February 2013 news . This are the quantum numbers of an as yet unobserved charmonium state called χc1(23P1). However, the charmonium spectrum is very well understood and the mass of the X(3872) makes this assignment very unlikely. The LHCb measurement of the ratio of branching fractions of X(3872) decay into ψ(2S)γ and J/ψγ does not support a pure DD* molecule interpretation of the X(3872), see 24 March 2014 news, and favours a hypothesis for an admixture of cc charmonium state and DD* molecule. However, the relativelly high production cross-section of the X(3872) at the LHC observed by the LHCb Collaboration, see 27 October 2010 news, favours even more an exotic tetraquark compact particle interpretation. More studies of X(3872) production and decay properties are needed to clarify its internal structure.
9 April 2014: Unambiguous observation of an exotic particle which cannot be classified within the traditional quark model.
The LHCb Collaboration has published yesterday results of precise measurements of properties of the Z(4430)- particle which allow to determine unambiguously its exotic nature. In the traditional quark model, the strongly interacting particles (hadrons) are formed either from quark-antiquark pairs (mesons) or three quarks (baryons). Particle physicists were searching since 50 years for the particles, called exotic hadrons, which could not be classified within this scheme. Many candidates have been proposed but up to now there has not been unambiguous proof of their existence.
The first evidence for the Z(4430) particle has been presented in 2008 by the Belle Collaboration as narrow peak in the ψ’π- mass distribution in the B → ψ’Kπ- decays. In the latest Belle publication the observation of the Z(4430) particle is confirmed with a significance of 5.2σ and a 3.4σ evidence is presented that the quantum numbers JP = 1+ are favored over the other spin assignments. There are many so called charmonium cc neutral states in this mass region. The fact that the Z(4430) is a charged particle does not allow to classify it as a charmonium state making this particle an excellent exotic candidate. The BaBar collaboration could explain the Z(4430) enhancement in their data by a possible feature of experimental analysis (so called reflections, for experts), not contradicting in the same time the Belle evidence.
The LHCb Collaboration has reported yesterday an analysis of about 25 200 B0 → ψ’Kπ-, ψ’ → μ+μ- decays observed in 3 fb−1 of pp-collision data collected at √s = 7 and 8 TeV. The LHCb data sample exceeds by an order of magnitude that of Belle and BaBar together. The significance of the Z(4430)- signal is overwhelming, at least 13.9σ, confirming the existence of this state. The Z(4430)- quantum numbers are determined to be JP = 1+ by ruling out 0-, 1-, 2+ and 2- assignments at more than 9.7σ, confirming the evidence from Belle. The LHCb analysis establishes the, so called, resonant nature of the observed structure in the data, and in this way proving unambiguously that the Z(4430) is really a particle.
The minimal quark content of the Z(4430) state is ccdu. It is therefore a four quark state or a two-quark plus two-antiquark state.
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The black points at the left image above show the ψ’π- invariant mass squared distribution of the data. The blue histogram shows the Z(4430) contribution. The right image shows the so called Argand diagram proving to the experts that the Z(4430) structure seen in the data (black points) represents really the resonant particle production and decay, since it follows approximately a circular path (red circle).
Read more in the LHCb publication here, in the CERN Quantum Diaries in English and French, in the CERN Courier article and also in the "Quarks bonding differently at LHCb" blog. See special video at CERN CDS or YouTube and the Z(4430) for saxophone quartet.
"A model-independent confirmation of the Z(4430)- state" was submitted for publication in October 2015. A possibility of explaning the Z(4430) enhancement by the so called reflection of Kπ resonances, proposed by the BaBar collaboration, is excluded with a significance exceeding 8σ. Experts are invited to see the Fig. 9 and 12a and read the details in the paper.
Added in October 2016: Scientists will try to understand the “internal mechanism” of quark interactions inside tetraquarks. The two possibilities are illustrated in the figure. The color of the central part of each quark is related to the strong interaction color charge, while the external part shows its electric charge. The quarks could be tightly bound, or they could also be loosely bound in meson-meson molecule, in which color-neutral mesons feel a residual strong force similar to the one that binds nucleons together within nuclei. It is hoped that future measurements by LHCb will help to answer this question.
24 March 2014: New LHCb results at the Rencontres de Moriond.
The run 1 data taking period ended one year ago. The LHCb experiment collected 1fb-1 of data from the pp collisions at 7 TeV in 2011 and additional 2fb-1 at 8 TeV collisions in 2012. The results of analysis of beauty and charm particles decays were already presented at many conferences and reported in the news below. At this year Moriond conference more precise results of different analysis were presented using larger data sample and/or including other decay channels.
The decay of the beauty meson B into an excited K meson K* and a μ+ and μ- pair is considered as an important channel for new physics search, see 9 August 2013, 13 March 2012 and 22 July 2011 news for introduction. Different distributions and branching fractions have been studied for these B meson decays and compared with the Standard Model predictions. The differences in the results of measurements of neutral B meson decays into K*0μ+μ- and charged B+ meson decays into K*+μ+μ- is called an "isospin asymmetry". The Standard Model calculations predict this isospin asymmetry to be small what, in fact, was confirmed by the LHCb analysis of 2011 (1fb-1) data. On the other hand, when the physicists made similar analysis by replacing the excited kaon K* by its ground state K an evidence was obtained for a possible isospin asymmetry (see 25 May 2012 news). The analysis of full 3fb-1 data sample presented at the Rencontres de Moriond gave results consistent with the small asymmetry predicted by the Standard Model in both (K* and K) cases. However, even if the difference between results of measurements of neutral and charged B meson decays is small there is a tendency for differential branching fractions to have lower values than the theoretical predictions as seen in the images below.
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Exotic states, particles which are not composed of quark and anti-quark pairs (mesons) or three quarks (baryons), have been searched for since nearly 50 years. One of the most famous candidates for such an exotic state is called the X(3872). The LHCb Collaboration has unambiguously determined its quantum numbers JPC to be 1++ (see 26 February 2013 news for introduction). Possible exotic explanations of the X(3872) nature include a DD* molecules or multi quark anti-quark system such as a diquark-diantiquark tetraquark or charmonium-molecule mixture. Classical interpretation of X(3872) as a pure cc charmonium state is not excluded but this assignment is very unlikely (see again 26 February 2013 news).
The LHCb Collaboration has presented at the Rencontres de Moriond a result of measurement of ratio of branching fractions of X(3872) decay into ψ(2S)γ and J/ψγ, Rψγ. This ratio is predicted to be different for different natures of X(3872) as seen in the image. The LHCb result 2.46 ± 0.64 ± 0.29 is more precise, but compatible with other experiments. It does not support a pure DD* molecule interpretation.
28 February 2014: First observation of photon polarisation in b→sγ transition.
The LHCb Collaboration has submitted today for publication a paper reporting the first observation of photon polarisation in b→sγ transition. The full 3 fb-1 Run 1 data sample was used to obtain this result. The Collaboration has presented already the first evidence for the photon polarization in this process at the summer 2013 conferences using about 2/3 of the whole data sample, see the news of 19 July 2013 for an introduction.
Photon polarization is the quantum mechanical description of the classical polarized sinusoidal plane electromagnetic wave. Individual photon can have either right or left circular polarization or a superposition of both, read more here.
The beauty particles decay mainly into charm particles, less frequently into strange particles. About once in every 3000 decays into strange particles a photon is emitted. At the underlying quark level a beauty b quark turns into a strange quark s by emitting a photon γ. This famous b→sγ transition is considered as a very interesting process in which signs of new physics could show up. The first evidence for this process was obtained by the CLEO Collaboration in 1993 and since then it was intensively studied in many experiments. This decays occurs only rarely since it requires a quantum fluctuation where a pair of heavy particles (a top quark and a W boson) appear and then rapidly vanish. The interaction between these particles is such that the emitted photon is expected to be almost 100% (left-handed) polarized. However, since the “virtual” top and W particles are not seen in the detector, they could equally well be replaced by other even heavier particles that are predicted in various theories that go beyond the Standard Model. Such theories have been proposed to address important unresolved questions in particle physics, such as the origin of the imbalance between matter and antimatter seen in the Universe. These models generally predict different values for the photon polarisation, and therefore it is seen as one of the most important measurements that can be made with the latest generation of experiments.
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Researchers from the LHCb experiment have now succeeded to observe a non-vanishing value of the polarisation for the first time with a significance of 5.2σ. The analysis is based on nearly 14000 B+→K+π- π+ γ decays, for which the distribution of the γ angle with respect to the normal to the plane defined by the kaon and two pion system is studied in four intervals of the K+π- π+ mass which are shown in the left image. The two curves are fits to the data points, allowing photon polarisation (solid blue curve) or setting it to zero (dashed red curve). The right image shows a typical B+→K+π- π+ γ event.
This investigation is conceptually similar to the historical Wu experiment that discovered parity violation by measuring the asymmetry of the direction of a particle emitted in a weak decay.
28 January 2014: How long can beauty and charm live together?
[ τ( Bc+) = 509 ± 8 ± 12 fs ]
In the Standard Model of Particle Physics the strongly interacting particles (hadrons) are composed of three quarks (baryons) or quark-antiquark pairs (mesons). There are six types of quarks, three light: up (u), down (d) and strange (s) and three heavy: charm (c), beauty (b) and top (t). The top quark decays so fast that it cannot form bound particle states with other quarks. The heaviest quark which can form baryons or mesons with other quarks is the beauty quark, about five times heavier than proton. The particle physics theory predicts that the lifetime of particles made of the beauty quark and light quarks should be almost identical. The earlier experimental results were not supporting this prediction. Recently the LHCb Collaboration has shown, by measuring precisely the Λb lifetime, that indeed the theoretical predictions are correct, see 15 July 2013 news for details.
But what about the lifetime of particles composed of two heavy quarks? The LHCb Collaboration submitted today for publication a result of lifetime measurement of the Bc+ meson, formed of a b and a c quark. The lifetime of the Bc+ meson is measured using semileptonic decays having a J/ψ meson and a muon in the final state. The J/ψ mesons decay in turn into a pair of muons. The measured lifetime is 509 ± 8 ± 12 fs with an uncertainty less than half of that of the combination of results of previous measurements. This lifetime is much shorter than the lifetime of particles composed of a beauty quark and lighter quarks and closer to the lifetime of charmed particles formed by a charm quark and lighter quarks. This precise result is also very interesting for theorists studying decays of this kind of particles. The calculations are challenging since the effects of both strong and weak interactions need to be taken into account.
The image on the left shows the lifetime distribution of the Bc+ candidates, with the fitted components indicated. The analysis is performed on a data sample of pp collisions at a centre-of-mass energy of 8 TeV, collected during 2012 and corresponding to an integrated luminosity of 2 fb-1. Further improvements are expected from the LHCb experiment using Bc+ → J/ψπ+ decays, where systematic uncertainties are expected to be largely uncorrelated with those affecting the present determination.
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24 January 2014: “Underground” news.
The last run 1 collisions took place at LHCb on February 2013. The proton beams should traverse again LHCb in early 2015. The period in between is called Long Shutdown 1 (LS1). What is happening at LHCb, 100m underground, during this period?
”LHCb operated with great success throughout LHC run 1 and has not been subject to any major intervention since its assembly in 2008. The current long shutdown offers a first opportunity for prolonged access, and hence an extensive programme of consolidation and maintenance work has been scheduled. This programme involves all general and detector related services, equipment and safety systems” writes Rolf Lindner, the LHCb Technical Coordinator. Read more details here.
The left image shows the installation of the 30 tons shielding for the LHCb muon detector where 2100 blocks were piled up in a confined space. The right one shows the LHCb dipole consolidation.
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The LHCb underground cavern is also a very popular scientific “tourist” place. The total number of visitors in 2013 was 4524 for 392 visits guided by members of LHCb Collaboration. In addition about 2100 persons visited the LHCb detector and nearby LHC tunnel during CERN Open Days in September 2013. The CERN Visit Service showed the LHCb surface exhibition to 220 groups with 7935 visitors last year.
16 December 2013: LHCb VErtex LOcator VELO.
At LHCb the protons collide inside the VErtex LOcator detector (VELO). The VELO is composed of two halves, each consisting of 21 pairs of back-to-back silicon sensors, whose job is to precisely measure the position at which the charged particles pass (see left image below). You can watch the sophisticated construction process at the video here and can read more details about this detector here. Its sensitive detector elements are held out of harm's way while the beams are being injected and stabilized, but once safer, the silicon elements are moved mechanically in towards the beam to hunt for beauty and charm particles.
The LHCb VELO detector plays an essential role in locating precisely the pp collision point as well as the location of beauty and charm particle decays, see, for example, 12 March 2013 news. This detector is so important that the LHCb Collaboration decided to build a second identical Vertex Locator in order to replace the one located around beam should this ever be needed. Since the primary VELO detector is still working perfectly the replacement version is now displayed at the LHCb surface exhibition as seen in the right image below.
The LHCb Collaboration is working towards a major upgrade of the LHCb experiment for the restart of data-taking in 2019. Most of the subdetectors and electronics will be replaced so that the experiment can read out collision events at the full rate of 40 MHz. The upgrade will also allow LHCb to run at higher luminosity and eventually accumulate an order of magnitude more data than was foreseen with the current set-up.
The VELO performance will also be strongly improved. Pixel technology will be used to buid this third version of the detector. The new detector will contain 40 million pixels, each measuring 55 μm square. The pixels will form 26 planes arranged perpendicularly to the LHC beams over a length of 1 m (see image). The sensors will come so close to the interaction region that the LHC beams will have to thread their way through an aperture of only 3.5 mm radius.
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29 October 2013: New charm results.
[ x'2 = (5.5 ± 4.9)x10-5 ; y' = (4.8 ± 1.0)x10-3 ]
[ AΓ(KK) = (-0.35 ± 0.62 ± 0.12)x10-3 AΓ(ππ) = (0.33 ± 1.06 ± 0.14)x10-3 ]
The LHCb Collaboration has reported recently new important results on charm physics.
(1) Ten months ago, the LHCb Collaboration presented the first observation of the D0-D0 oscillations in which the D0 matter mesons turn into their antimatter partners. Contrary to the B0-B0 and B0s-B0s oscillations in which the mesons turn into their antimatter partners many times during their lifetime, the D0-D0 oscillations are very slow, over one hundred times the average lifetime (see 7 November 2012 news for introduction). LHCb has now updated this result using the full 2011 and 2012 data set of 3 fb-1. The new result is 2.5 times more precise. The values parameterizing the oscillations, the so-called mixing parameters y' and x'2, are shown above.
By now, CP violation, differences in the behaviour of matter and antimatter, has been observed in all oscillating neutral-meson (K0, B0, B0s) systems apart from the charm system. First evidence for charm CP violation (see 14 November 2011 news) has not been unambiguously confirmed to date (see 12 March 2013 news). The D0 mesons are the only mesons containing up-type quarks which undergo matter anti-matter oscillations (called also mixing) and therefore provide unique access to effects from physics beyond the Standard Model.
As part of the new analysis, LHCb has investigated whether there is a CP violating contribution to the oscillations, in contrast to the Standard Model expectation. This is done by investigating whether the oscillation parameters for mesons produced as D0 and D0 differ. Studying the D0 and D0 decays separately shows no evidence for CP violation and provides the most stringent bounds on the parameters (AD and |q/p| for experts) describing this violation from a single experiment.
(2) LHCb physicists measured the asymmetry AΓ of the inverse of effective lifetimes in decays of D0 and D0 mesons to the K- K+ and π-π+ final states. The measured values of the parameter AΓ shown above represent the world’s best measurements of this quantity, and are the first searches for CP violation in charm oscillations with sensitivity better than 10-3. They do not indicate CP violation, and show no difference in AΓ between the two final states.
The results of other experiments combined by the Heavy Flavor Averaging Group indicated a hint for possible non-zero values of the CP violation parameters (|q/p| and φ for experts). Both LHCb results presented above do not support this indication as seen in the image. The size of the contour with the new LHCb results is about a factor of two smaller in each of |q/p| and φ. They provide very stringent limits on the underlying parameters, thus constraining the room for physics beyond the Standard Model.
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8 October: 2013 Nobel prize in Physics.
The Nobel Prize committee of the Royal Swedish Academy of Sciences has decided to award the Nobel Prize in Physics for 2013 to François Englert and Peter Higgs for "the theoretical discovery of a mechanism that contributes to our understanding of the origin of mass of subatomic particles, and which recently was confirmed through the discovery of the predicted fundamental particle, by the ATLAS and CMS experiments at CERN's Large Hadron Collider".
Congratulations to François Englert and Peter Higgs.
The image shows LHCb spokesperson Pierluigi Campana showing the LHCb detector to Peter Higgs on December 18th, 2012.
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28-29 September 2013: CERN Open Day at LHCb.
see CERN Open Day web page here for details.
2 September 2013: Excellent performance of LHCb Ring Imaging Cherenkov (RICH) detectors.
The physics results presented on this web page were obtained thanks to the excellent LHC collider performance, to the excellent LHCb data acquisition and analysis, and certainly also to the excellent quality of LHCb detector. As an example the exceptional precision of the particle identification achieved by one of the two RICH detectors is shown in the left image below. RICH detectors work by measuring the emission of Cherenkov radiation. This phenomenon occurs when a charged particle passes through a certain medium faster than light does. As it travels, a cone of light is emitted, which the RICH detectors reflect onto an array of sensors using mirrors. The shape of the cone of light depends on the particle’s velocity, enabling the detector to determine its speed. Scientists can then combine this information with a record of its trajectory (collected using the tracking system and a magnetic field) to calculate its mass, charge, and therefore its identity.
But why the speed of light is lower in the medium? - see an explanation here.
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The LHCb Collaboration has recently published a paper in the European Physics Journal C describing the performance of the LHCb RICH detectors. The Journal has honoured the excellent quality of LHCb detector by placing the RICH performance image on its cover page as seen in the right image above.
Read more about the LHCb detector at this web site here.
26 August 2013: Matter-antimatter quantum music.
A fascinating feature of quantum mechanics, in which the B0s, B0 and D0 particles turn into their antimatter partners and back, has been discussed already few times at this page, see 3 March 2013 and 7 November 2012 news. This feature is called oscillations or mixing. The B0s mesons oscillate with by far the highest frequency of about 3 million million times per second (3*1012), on average about 9 times during their lifetime. The B0 mesons oscillate about 37 times slower with a frequency of about 80 thousand million times per second (8*1010). A musical tone is defined by its frequency. LHCb physicists tried to hear this beautiful (involving beauty quarks) matter-antimatter quantum music.
The left video image above shows the last stage of the event filtering process. Two accumulations of events are clearly visible allowing to select the B0 and B0s particles. As the blue box moves through the image we are able to hear the background noise, then the loud tone of B0-B0 oscillations, the background noise again and then the tone of the B0s-B0s oscillations. The higher frequency B0s-B0s oscillations are experimentally more difficult to observe and therefore their tone is weaker. The very high-pitched quantum oscillation frequencies were reduced by millions of times in order to fit into the range that can be heard by humans. Additional explanations can be found in the right hand side video above.
9 August 2013: LHCb results hint at new physics?
The LHCb Collaboration has just published the results of a new analysis of the B0→K*0μ+μ- decay, with K*0→K+π-. These results were presented three weeks ago at the European Physical Society Conference on High Energy Physics, EPSHEP, Stockholm, Sweden, and triggered very interesting discussions. The analysis of the B0→K*μμ decay is considered as a very promising channel to search for new physics effects, see the 14 June 2013 news for an introduction. A contribution from new physics particles could modify the angular distributions of the decay products. LHCb physicists have studied different variables related to these angular distributions as functions of the μ+μ- invariant mass squared. In previously published results, no significant deviation from the Standard Model prediction has been found, see the 13 March 2012 news. In order to increase sensitivity to new physics effects LHCb physicists started to analyse additional observables (the so called Pi' observables) which are considered theoretically clean. This means that they are less sensitive than other observables to some theoretical parameters that are not precisely known (form-factors for experts). Four such observables, labelled P4', P5', P6' and P8', have been studied.
The image shows the distribution of the P5' observable as a function of the μ+μ- invariant mass squared q2. The black data points are compared with the Standard Model prediction. A 3.7σ deviation of data above the prediction is observed for the third bin corresponding to q2 between 4.3 and 8.68 GeV2/c4. Taking into account that this deviation is observed in one out of 24 bins investigated in this work (the so-called look-elsewhere effect), the significance of the deviation becomes 2.8σ.
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These new results are of great interest to theorists, who are combining results from several measurements to search for effects of physics beyond the Standard Model. According to Joaquim Matias from Universitat Autonoma de Barcelona and colleagues the deviation in P5' and small discrepancies in the other angular observables for this decay, follow a pattern. In a recent paper the authors claim that a global analysis of the LHCb data, together with previous measurements, show a deviation of 4.5σ with respect to Standard Model expectations, which can be explained with the same mechanism (reduced Wilson coefficient C9 for experts). This demands further investigation, in particular to re-evaluate all the sources of theoretical uncertainty, and to understand the effects of correlations between the experimental measurements. A deep interplay between experimental and theoretical analyses will be essential to confirm or refute the pattern of new physics suggested by the B0→K*μ+μ- anomaly.
The results presented so far are based on 1fb-1 of data recorded from pp collisions at 7 TeV in 2011. Particle physicists are impatiently waiting for result of analysis of additional 2fb-1 of data taken at 8 TeV in 2012.
24 July 2013: The B0s→μμ decay is observed.
The CMS and the LHCb Collaborations have announced today at the 2013 European Physical Society Conference on High Energy Physics, EPSHEP, Stockholm, Sweden, that the B0s→μμ decay is observed. The LHCb Collaboration presented already at this conference the measurement of the B0s→μμ branching fraction of (2.9+1.1-1.0)x10-9 with a significance of 4.0σ (see the 19 July 2013 news). The CMS Collaboration presented the same day a similar result giving a branching fraction of (3.0+1.0-0.9)x10-9 with a significance of 4.3σ (see CMS public page article). The results of both experiments are compatible and, therefore, a decision was taken to combine them.
The CMS and the LHCb Collaborations have obtained a combined preliminary value of the B0s→μμ branching fraction of (2.9±0.7)x10-9. Although a thorough evaluation of the combined significance has not been performed, it is clear that the B0s→μμ decay is observed (with a significance above 5σ). The result is in agreement with the Standard Model prediction of (3.56±0.30)x10-9. The image shows the CMS and LHCb results and their combination together with the results of the CDF Collaboration as well as the D0 and ATLAS Collaborations 95% CL limits which are not included in the combination.
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The search for the B0s→μμ decay was considered as one of the most stringent tests of the Standard Model. Now it is found at a rate consistent to within 25% with that calculated within the Standard Model. This provides a fine-grained filter for the new physics models. All models of physics beyond the Standard Model will have to test their compatibility with this important result.
19 July 2013: First evidence of photon polarisation in b→sγ transition.
The LHCb Collaboration has just presented at the 2013 European Physical Society Conference on High Energy Physics, EPSHEP, Stockholm, Sweden, a first significant non-zero measurement of an observable proportional to the photon polarisation in b→sγ transition. The transition of a b-quark to an s-quark by emission of a photon (γ) is considered a very important process to investigate possible manifestation of new physics. This decay process is forbidden in the first approximation in the Standard Model (SM) of particle physics and moreover in the second-order processes that govern the process in the SM the emitted photon is expected to be strongly polarised. Therefore it is very sensitive to new physics effects arising from the exchange of new heavy particles in electroweak penguin diagrams (see 14 June 2013 news). Indeed, several models of new physics predict that the emitted photon should be less polarised than in the SM. Up to now different experiments have measured the decay rate of this process, ruling out significant deviations of the rate from the SM prediction and strongly reducing the allowed parameter space of new physics models. The photon polarisation was, however, never previously observed.
Free quarks are not observed in nature. Therefore physicists measure the b→sγ transition in decays of particles containing a b quark, like B mesons, into strange particles containing a s quark, like K mesons. The LHCb physicists have used the process B+→Kresγ where excited K meson states, Kres, decay in turn into three particles, K+, π- and π+. The red distribution in the image shows the contribution of more than 8000 signal events reconstructed and selected in the 2012 data sample, corresponding to an integrated luminosity of about 2 fb-1 collected in pp collisions at 8 TeV. The other distributions show different background contributions. An angular analysis of the B+ decay products has allowed to obtain first evidence, with 4.6σ significance, for photon polarization in the b→sγ transition with respect to the no-polarization scenario. Further theoretical analysis is, however, needed to obtain a numerical value for this polarization.
Read more in the LHCb presentation in Stockholm and in the LHCb conference note here.
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19 July 2013: Measurement of the B0s→μμ branching fraction and search for B0→μμ decays at the LHCb experiment.
The LHCb Collaboration has just presented at the 2013 European Physical Society Conference on High Energy Physics, EPSHEP, Stockholm, Sweden, improved measurements for the rare decays B0s→μμ and B0→μμ. Last year at the Hadron Collider Particle Symposium in Kyoto, the Collaboration presented the first evidence, with 3.5σ significance, for the B0s→μμ decay using the total 1.0 fb-1 of data taken in 2011 and 1.1 fb-1 of the data accumulated in 2012, see 12 November 2012 news. The full LHCb data sample of 3.0 fb-1 was used to obtain today's result. The analysis strategy is very similar to that reported in November 2012 with an improved event selection algorithm (BDT). The significance of the result has been improved to 4.0σ making the evidence even stronger. A branching fraction of (2.9+1.1-1.0)x10-9 is obtained. The result is in agreement with the Standard Model prediction of (3.56±0.29)x10-9. It puts very strong constraints on the parameters of different models of new physics and squeezes even more than previous results the parameters of supersymmetric extensions of the Standard Model (SUSY) - see 30 March 2012 news for introduction.
The μ+μ- invariant mass spectrum for the BDT selection algorithm bins with the smallest background contribution is shown in the left image. The solid blue line shows that the data distribution presented as black dots is well understood and can be separated into different components presented with the help of different colour lines. The dashed red narrow distribution shows the B0s →μμ contribution around the B0s mass of 5372 MeV/c2. The green dashed line shows a possible B0 contribution. The B0 decay yield is not significant yet and an improved limit on the B0→μμ branching fraction of 7.4x10-10 at 95% CL is obtained.
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A typical B0s →μμ decay candidate event is shown below. The two muon tracks from B0s decay are seen as a pair of purple tracks traversing the whole detector in the left image below. The right image shows the zoom around the proton-proton collision point, origin of many particle tracks. The two muon purple tracks originate from the B0s decay point located 50 mm from the proton-proton collision.
18 July 2013: Observation of unexpected resonant structure in B→Kμμ decays.
The LHCb Collaboration has just presented at the 2013 European Physical Society Conference on High Energy Physics, EPSHEP, Stockholm, Sweden, a first observation of unexpected resonant structure in B+→K+μ+μ- decays. Precise study of this decay could uncover a possible contribution from new physics. This contribution could, however, hide behind the dominant B+→K+μ+μ- decay modes which proceed through the decay of the B+ to a cc resonance (charmonium) and a K+ meson, followed by the decay of the resonance to a μ+μ- pair. To probe for physics beyond the Standard Model it is necessary to remove regions of μ+μ- mass dominated by the resonances. Up to now only the J/ψ and ψ(2S) resonances were taken into account because contributions from resonances with masses above 3900 MeV, where the kaon has a low recoil against the dimuon pair, were thought to be negligible.
The image shows the μ+μ- mass distribution in the low recoil region. What was expected is a smoothly falling distribution, dominated by the non-resonant decay. However, two peaks are clearly visible, one at the low edge corresponding to the decay ψ(3770)→μ+μ- and a wide peak at a higher mass. The mean and width of the wider peak are 4191+9-8 MeV/c2 and 65+22-16 MeV/c2, which are compatible with the so-called ψ(4160) resonance (the name ψ(4160) is misleading, first measurements of the mass of this state gave values lower than it is now known to be). First observations of both the decay B+→ψ(4160)K+ and the subsequent decay ψ(4160)→μ+μ- are reported with statistical significance exceeding six standard deviations.
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This observation is made possible due to quantum mechanical interference between the resonance and non-resonant signal. The resonance and the interference make up 20% of the yield in the low recoil region. This contribution is much larger than expected and in the future, with the large data sets available at LHCb, will need to be taken into acount when searching for new physics in rare decays such as B+→K+μ+μ-.
Read more in the LHCb presentation in Stockholm and in the LHCb paper here.
15 July 2013: A mystery of the beauty baryon lifetime resolved.
[ τ(Λb)/ τ(B0) = 0.976±0.012±0.006 ]
[ τ((Λb) = 1.482±0.018±0.012 ps ]
The LHCb Collaboration has just published an important precise measurement of the Λb beauty baryon lifetime. The Λb is a particle composed of the up (u), down (d) and beauty (b) quarks, so it can be understood as being like a neutron (composed of udd quarks) in which one of the d quarks has been replaced by the beauty (b) quark. Therefore the Λb baryon is about 6 times heavier than the neutron. The lifetime of the Λb baryon was first measured by experiments which took data at the electron-positron collider LEP in the 1990s. The results of the measurements were puzzling. It was a real nightmare for theoretical physicists. In fact, the calculations they were using, the Heavy Quark Expansion HQE, predicted that the Λb lifetime should be very similar to that of the B0 meson, but the LEP experiments found the Λb lifetime to be about 20% shorter than the B0 lifetime.
The LHCb Collaboration has recently discovered a new decay mode
Λb → J/ψpK-, J/ψ→μμ. The image shows the signal yield of more than 15,000 Λb decays in 1.0 fb-1 of LHCb data. This decay allows the precise measurement of the Λb decay point from the intersection of four charged tracks. The B0 decay point was also measured precisely using four charged tracks of the B0 decay into J/ψK*0, K*0→K+π-. In this way the LHCb collaboration made the most precise measurement of the Λb to B0 lifetime ratio to be 0.976±0.012±0.006, close to 1 and in agreement with the original HQE prediction. The mystery of the Λb lifetime is now resolved. Using previous determinations of the B0 meson lifetime, the Λb lifetime was found to be 1.482±0.018±0.012 ps.
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14 June 2013: LHCb: a place to find penguins.
How "penguins" can help LHCb to chase for new physics? - read explanations in the June issue of CERN Courier and then follow links to the LHCb physics results announced in the 13 March 2012 and 25 May 2012 news at this page.
The name of penguin appeared in particle physics in 1977 following a bet lost by CERN theoretical physicist John Ellis, see also explanations in the CERN Courier article. The original penguin diagram for b quark to s quark decay, where a gluon produces an ss pair, is shown in the image at the right hand side.
LHCb is investigating decays where a μ+μ- pair is produced from a photon or from a Z boson as seen in the diagram on the left named as an "electroweak penguin"; the diagram on the right is a "box" diagram.
See also the hangout video,
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6 June 2013: LHCb PhD student wins the Swiss FameLab competition.
Donal Hill, LHCb PhD student, was nominated as the best Swiss FameLabber 2013 during the Swiss FameLab final in Zurich. Nine candidates selected in a previous heat in Geneva were evaluated by a judging panel and by the audience. Donal will represent Switzerland at the Cheltenham Science Festival in the UK in June 2013.
FameLab is an exciting competition for young researchers. It encourages scientists to inspire and excite public imagination with a vision of the 21st century of science.
Congratulations to Donal!
17 May 2013: LHCb physicist rewarded by the European Physical Society.
The High Energy Physics Division of the European Physical Society announced today the winners of its 2013 prizes, which will be awarded at the Europhysics Conference on High-Energy Physics (EPS-HEP 2013), Stockholm (Sweden) 18-24 July 2013. The 2013 Young Experimental Physicist Prize, for outstanding work by one or more young physicists in the field of Particle Physics and/or Particle Astrophysics, was awarded to Diego Martinez Santos “for his outstanding contributions to the trigger and commissioning of the LHCb experiment, and the analyses leading to first evidence for the rare decay B0s →μμ”.
Congratulations to Diego!
The European Physical Society recognized in this way the quality and the excellence of the LHCb experiment, and of the whole collaboration who built and is operating it.
Read more on the analysis of B0s →μμ data in the 12 November 2012 news.
10 May 2013: First and important results from the proton with lead ion collision 2013 run.
The LHCb Collaboration has just presented at the Workshop on proton-nucleus collisions at the LHC, Trento, Italy, the first results from the analysis of proton with lead ion collision run data taken in January-February 2013. Already these first results made an important contribution to the understanding of heavy ion collisions.
In the Standard Model of Cosmology quarks and gluons were freely moving in a state called a quark-gluon plasma until < 10-5 seconds after the Big Bang. As the Universe cooled, they became confined inside protons and neutrons. The theory of quark-gluon interactions, the strong force interaction theory, QCD, predicts that the state of quark-gluon plasma can also exists in high temperature matter created by high energy collisions between large atomic nuclei, called by physicists heavy ion collisions. But how to prove that the quark-gluon plasma is really formed? A reduced rate of J/ψ particle production in heavy ion collisions was considered as a "smoking gun" argument in favour of quark-gluon plasma formation by physicists analysing results of measurements performed in the CERN Super Proton Synchrotron (SPS) after 1986 and more recently in the Brookhaven RHIC collider. Profound analysis has shown, however, that reality is more complicated. In some models, for example, the J/ψ particle could also be regenerated in nuclear matter, partons (quark, gluons) could be saturated and/or lose energy, etc. in normal (so called cold) nuclear matter.
Data recording collisions of protons with lead ions were collected in the LHC experiments in January-February this year. In such collisions, formation of a quark-gluon plasma is not expected, and therefore measurements based on these data allow the study of interactions in cold nuclear matter. The analysis of J/ψ production was of particular interest.
The LHCb results are shown in the image at the left hand side. A reduced value of the nuclear attenuation factor RpA, the ratio of the J/ψ production in the proton with lead ion (pA) collisions to that in proton-proton collisions as a function of the rapidity y is clearly seen. The rapidity variable is related to the J/ψ production angle with respect to the incoming proton direction. The experimentally measured points (triangles with error bars) in the image show that the largest suppression is in the forward direction.
The colored distributions show theoretical predictions of RpA calculated by François Arleo from LAPTH, Annecy and Stéphane Peigné from Subatech, Nantes, taking into account the J/ψ energy loss (E. loss) in cold nuclear matter with and without the parton saturation effects. The LHCb results are in agreement with these predictions.
Two sets of data were taken: pA and Ap, where in the second case the direction of the proton and lead ion beams were reversed. This allowed the LHCb detector, recording the particles only on one side of the interaction point, to make measurements in both forward and backward directions with respect to the proton beam (positive and negative rapidity).
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24 April 2013: First observation of CP violation in the decays of B0s mesons.
[ ACP(B0s→K-π+) = +0.27 ± 0.04 ± 0.01 ]
[ ACP(B0→K+π-) = -0.080 ± 0.007 ± 0.003 ]
The LHCb Collaboration has just submitted for publication a paper which sets an important milestone in the history of particle physics. A difference between properties of matter and antimatter, named CP violation by particle physicists, was discovered in 1964 in the decays of neutral K mesons and was rewarded with the 1980 Nobel Prize in Physics for James Cronin and Val Fitch. M. Kobayashi and T. Maskawa proposed in 1973 a mechanism which could incorporate CP violation within the Standard Model with not less than 6 quarks. In 2001, CP violation was observed in the decay of so-called Beauty Particles, the B0 mesons composed an anti-quark b and a quark d. The Standard model mechanism of CP violation was confirmed and therefore Kobayashi and Maskawa were rewarded with the 2008 Nobel Prize in Physics. In March 2012 the LHCb Collaboration reported an observation of CP violation in charged B± meson decays into DK±. Today, the LHCb Collaboration has announced an observation of CP violation in the decays of Strange Beauty particles, the B0s mesons composed of a beauty antiquark b bound with a strange quark s.
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The four plots on the left hand side above show the Kπ invariant mass distribution divided into different components as shown by the legend in the top-right figure. The different charge combination of K and π indicates if the decaying B0 or B0s particle is a matter or an antimatter particle. The two upper plots show that the decay rates of B0 mesons are different, as was already well established by previous measurements. The zoom in the lower two plots shows that the difference is also visible around the B0s meson mass, as indicated by the two green Gaussian distributions. Mathematically this difference is described by the asymmetry ACP(B0s→K-π+) = +0.27 ± 0.04 ± 0.01, which differs from zero with significance exceeding five Gaussian standard deviations σ. Therefore this result represents the first observation of CP violation in the decays of B0s mesons. The corresponding asymmetry for B0 meson decays presented in the two upper images, ACP(B0→K+π-) = -0.080 ± 0.007 ± 0.003, is the most precise measurement of this quantity to date.
The full 1.0 fb-1 data sample collected in 2011 was used to obtain these results, the precision will be further improved using the total dataset available which has more than tripled thanks to the excellent 2012 data taking period.
20 March 2013: LHCb 100th publication.
The LHCb Collaboration has submitted its 100th publication! It is signed by 621 authors from 63 different universities and laboratories from 17 countries. The paper "Search for direct CP violation in D0→h-h+ modes using semileptonic B decays" has been presented at the Rencontres de Moriond QCD, La Thuile, Italy, and is described in the 12 March 2013 news as the second independent analysis. You can celebrate this important moment in the life of our Collaboration with the help of a poster available in the LHCb secretariat or printing it directly from a file. The poster shows an event, described in the 12 March 2013 news, and used in the analysis.
The LHCb papers have made very important contributions to particle physics as described in other items on this page. However, even more important contributions are expected in the near future. In fact, most results presented in LHCb papers to date used the full 1.0 fb-1 data sample collected in 2011. The total dataset available for future analysis has more than tripled thanks to the excellent 2012 data taking period. The only published paper using the 2012 data is the analysis presented in the "First evidence for the B0s →μμ decay" paper, see 12 November 2012 news, in which already half of 2012 data sample was used.
The highlights of recent LHCb results showing the presentations at different conferences, conference contributions and papers can be found here.
12 March 2013: Improved search for CP violation in charm decays.
[ ΔACP = (−0.34 ± 0.15 ± 0.10 )%, pion tagged ]
[ ΔACP = (+0.49 ± 0.30 ± 0.14)%, muon tagged ]
The LHCb Collaboration presented today at the Rencontres de Moriond QCD, La Thuile, Italy, results of an improved search for the difference between properties of matter and antimatter, CP violation, in charm decays, see 14 November 2011 news for introduction. The difference (Δ) of CP asymmetry (ACP) between the decay rates of D (matter) and D (antimatter) mesons into K+K– pairs and into π+π- pairs was measured. The results presented today profited from three improvements to the previous analysis: the full 1.0 fb-1 data sample collected in 2011 was used instead of 0.6 fb-1, the analysis technique was improved and also in addition another independent method was used to select matter D and antimatter D particle decays.
In the Standard Model CP violation was expected to be very small in the charm sector, whereas new physics effects could generate enhancements. Therefore the 14 November 2011 announcement by the LHCb Collaboration of 3.5σ evidence of CP violation in charm sector, ΔACP = (-0.82 ± 0.21 ± 0.11)%, triggered intensive theoretical activity with conclusions that some special Standard Model effects could generate CP violation effects even as big as about 1%. This interesting LHCb result was later confirmed by the CDF and Belle collaborations. The new improved LHCb result presented today, ΔACP = (−0.34 ± 0.15 ± 0.10 )%, is more precise thanks to the larger data sample and several improvements resulting in better background suppression by a factor of 2.5. The central value is, however, closer to zero than in the previous measurement, which it supersedes.
In the measurement presented above the D (matter) and D (antimatter) mesons were selected using the D* meson decays, D*+(-) → π+(-)D(D), which means that the presence of π+ in the decay identified matter D meson production while π- accompanied antimatter D production. LHCb physicists presented today also results of a second independent analysis in which the D and D mesons were selected using so called semileptonic beauty B decays, for example B+(-) → μ+(-)νD(D). In the second analysis, the positive charge of μ+ identified the D meson, while the negative one, μ-, the D production. The image at the left hand side shows a selected event. A zoom around the pp interaction point shows a B+ meson decay point located at the distance of 17 mm from the pp collision point and the D meson decay place still 9 mm further away. The second analysis also measures a value that is consistent with zero: ΔACP = (+0.49 ± 0.30 ± 0.14)%. A combination of the two LHCb results gives ΔACP = (-0.15 ± 0.16)%.
click the images for higher resolution
The image at the left hand side shows a comparison of different measurements of ΔACP. The previous LHCb result is shown as the shaded grey point. A naive world average is shown as the yellow band. As shown in the image, the two new LHCb results are consistent with each other and with other results at the 2σ level, but do not confirm the previous evidence of CP violation in the charm sector.
Theoretical work has shown that several well-motivated models could induce large CP violation effects in the charm sector. These new results constrain the parameter space of such models. Further update of this and related measurements will be needed to discover if – and at what level – nature distinguishes between charm and anticharm.
More details can be found in the LHCb presentation in La Thuile, in the LHCb paper and conference contribution. Read also the CERN Courier article. See update with the full Run 1 data sample in the 2014 paper and in the 2016 seminar, in the 2016 paper and CERN Courier article.
3 March 2013: Precise search for new physics.
[ Δms = 17.768 ± 0.023 ± 0.006 ps-1 ]
[ φs = 0.01 ± 0.07 ± 0.01 rad ]
ΔΓs = 0.106 ± 0.011 ± 0.007 ps-1 ]
The LHCb Collaboration presented today at the Rencontres de Moriond EW, La Thuile, Italy, three important results of their more and more precise search for new physics. The 1 fb-1 data sample collected in 2011 was used to obtain these results.
(1) A fascinating feature of quantum mechanics, in which the B0s, B0 and D0 particles turn into their antimatter partners, has been discussed already at this page, see 15 March 2011 and 7 November 2012 news. This feature is called oscillations (mixing). The B0s mesons oscillate with by far the highest frequency of about 3 million million times per second (3*1012), on average about 9 times during their lifetime.
The B0s meson decays into D-sπ+ were used in this analysis with D-s decays into five different channels. The image at the left hand side illustrates the B0s-B0s oscillations in a spectacular way, showing how the matter turns into antimatter and back over many oscillation periods. The frequency of oscillation is defined by the Δms parameter. The value of this parameter as measured by the LHCb collaboration is shown above.
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(2 - presented also at the Rencontres de Physique de la Vallée d'Aoste) Knowledge of the value of the parameter φs is very important for physicists, since it set the scale of the difference between properties of matter and antimatter, called CP violation by experts, in the B0s sector. The B0s decays into a J/ψφ and a J/ψππ were already studied, see 5 March 2012 and 27 August 2011 news for introduction. LHCb physicists have improved and finalized these measurements. One important improvement is in "flavour tagging", which determines whether the initial state was produced as a B0s or B0s meson. Better tagging gives better sensitivity in the final result. The values of the φs parameter together with the difference between the width of a heavy and light mass B meson system (see 5 March 2012 news) are shown above. The left image below shows the small allowed region for these two parameters.
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(3) Finally, LHCb physicists have opened a door for important future measurements by presenting the results of a first study of the time-dependent CP-violating asymmetry in hadronic B0s meson decays into an φφ pair. Both φ mesons decayed in turn into a K+K- pair. The invariant mass spectrum for the four kaons is presented in the right image above showing a clear accumulation of B0s → φφ decays at the B0s mass. Using about 880 events the CP-violating phase φs was measured to be in the interval of [-2.46,-0.76] rad at 68% confidence level.
The results presented in this news represent the most precise measurements to date. They are in agreement with the Standard Model prediction. The parameter region in which the signs of new physics can still hide is significantly reduced. More details can be found in the LHCb presentations in La Thuile and in the LHCb papers here. Read also the CERN Courier article.
26 February 2013: X(3872) looks more and more exotic.
[ X(3872) JPC = 1++ ]
The LHCb Collaboration presented today at the Rencontres de Physique de la Vallée d'Aoste, La Thuile, Italy, an important result which makes the exotic nature of the X(3872) particle very probable.
In the quark model of particle physics proposed in 1964 by Murray Gell-Mann and George Zweig mesons, like the π (pion), are formed from quark and anti-quark pairs and baryons (like the proton) from three quarks. This model is very successful. Particles which cannot be described in this model, known as exotic states, have been searched for since nearly 50 years ago. Their existence has not yet been firmly established (except of a special case of pionium). One of the most famous candidates for such an exotic state is called the X(3872). It was discovered in B+ meson decay into an X(3872) and a K+ meson by the BELLE collaboration almost 10 years ago. Its existence was confirmed later by the CDF, D0 and BaBar experiments. LHCb has previously reported studies of the X(3872) in the data sample taken in 2010, see 27 October 2010 news. Particles are classified according to their quantum numbers JPC. An analysis by the CDF experiment has limited the possible values of X(3872) JPC to either 1++ or 2-+.
LHCb physicists have observed the X(3872) particle in the decay of a B+ meson into an X(3872) and a K+ meson. The X(3872) was observed in the invariant mass of a J/ψ particle and a π+π- pair, while the J/ψ was identified from its decay into μ+μ- pair. The image at the left hand side shows the difference between the invariant mass of the π+π-J/ψ combination and the J/ψ showing clearly the X(3872) and ψ(2S) enhancements over the smooth background distribution (the ψ(2S) particle decays also into a π+, π-, and J/ψ).
click the image for higher resolution
LHCb physicists made a sophisticated analysis of the whole B+ decay chain in 5 dimensions and unambiguously determined the quantum numbers of X(3872) to be 1++. The other previously allowed assignment of 2-+ was rejected with statistical significance over 8σ. About 300 signal events were selected among about 60 million million (6*1013) pp collisions seen by the LHCb detector at LHC during the 2011 data taking period. Details of analysis can be found on the LHCb staff page.
The exotic nature of the X(3872) would be unambiguously determined if its quantum numbers could not be described by the quark-antiquark combination. However, this is not the case. In fact, the mass of 3872 MeV is located in a region in which many charm quark-antiquark states are present bound by a strong force in an atom-like system called charmonium. The X(3872) has the quantum numbers of an as yet unobserved charmonium state called χc1(23P1). However, the charmonium spectrum is very well understood and the mass of the X(3872) makes this assignment very unlikely. Possible exotic explanations of the X(3872) nature include a DD* or multi quark anti-quark system such as a diquark-diantiquark tetraquark or charmonium-molecule mixture.
14 February 2013: End of 2013 data taking period.
Today at 7:25 LHCb physicists have observed the last collision at the LHC. After a very succesfull period of proton with lead ion collisions the last few days of data taking were reserved for proton-proton collisions at 2.76 TeV. The two year shut-down period, called LS1, will start two days later in order to set-up LHC for doubling the proton-proton collision energy to 13 TeV at March 2015. LHCb Collaboration congratulates and thanks LHC team for excellent performance.
20 January 2013: Start of 2013 data taking period.
Today at 15:11 LHCb physicists have observed the first 2013 collisions of protons with lead ions in the LHCb detector. The 2013 data taking period has started. The proton - lead ion collisions were alredy observed by the LHCb on 13 September 2012 during the short test run.
15 November 2012: LHCb thanks LHC.
LHC collider has delivered at 9:50 today 2 fb-1 of luminosity to LHCb this year corresponding to about 100 million million of proton-proton collisions visible at LHCb. This was possible thanks to an exceptional performance of the machine and an impressive commitment from everybody involved in the operation of LHC in this very long running year. The image shows how the LHCb Collaboration thanks the LHC operation team on the LHC status screens which are visible in the control rooms all over CERN and on the Web. The collider is currently running with very high efficiency. This can be seen in the image where very long proton-proton collisions periods are interleaved by short set-up intervals.
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12 November 2012: First evidence for the B0s →μμ decay.
[ Branching ratio B0s →μμ = (3.2+1.5-1.2)x10-9 ]
The LHCb Collaboration has presented today at the Hadron Collider Particle Symposium in Kyoto the result of the branching ratio measurement of the B0s meson decay into μ+μ- pair to be (3.2+1.5-1.2)x10-9. Both experimental and theoretical physicists were impatiently waiting for this result, an important milestone of the LHCb program. The significance of the measurement is 3.5σ and therefore is classified as the first evidence for the B0s →μμ decay. The result is in agreement with the Standard Model prediction of (3.54±0.30)x10-9 (for experts: this number takes into account a correction to the value (3.23±0.27)x10-9 due to the finite width difference of the B0s system). LHCb physicists had previously presented 15 March this year the lowest published limit of 4.5x10-9 for this decay, which allowed to squeeze strongly the parameters of supersymmetric extensions of the Standard Model (SUSY) - see 30 March 2012 news. The measurement presented today squeezes the parameter space even more.
The total 1.0 fb-1 of data taken in 2011 and 1.1 fb-1 of the data accumulated this year were used to obtain this result. A special event selection (BDT for experts) was used to classify data into bins with different ratios of B0s →μμ decays and background contributions. The μ+μ- invariant mass spectrum for the bins with the smallest background contribution is shown in the left image. The solid blue line shows that the data distribution presented as black dots is well understood and can be separated into different components presented with the help of different colour lines. The dashed red narrow distribution shows the B0s →μμ contribution around the B0s mass of 5366 MeV/c2.
The green dashed distribution shows a possible contribution from the B0 →μμ contribution around the B0 mass of 5280 MeV/c2. Within the Standard Model the branching ratio for this decay is expected to be about 30 times smaller than that for the B0s decay. A small excess of data over the background and Standard Model rate is observed, but is consistent with the Standard Model expectation. LHCb physicists have set a limit of 9.4x10-10 for this branching ratio.
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A typical B0s →μμ decay candidate event is shown above. The two muon tracks from B0s decay are seen as a pair of purple tracks traversing the whole detector in the left image above. The right image shows the zoom around the proton-proton collision point, origin of many particle tracks. The two muon purple tracks originate from the B0s decay point located 14 mm from the proton-proton collision.
The precision of these results will be improved using an additional 1 fb-1 of data or more that will be available by the end of this year thanks to the strong and continuous support from the LHC operations team for the LHCb physics program.
More details can be found in the LHCb presentation in Kyoto and in the LHCb paper here and also in the CERN seminar. Read also the CERN Bulletin article in English and French, the CERN Courier article, the CERN Quantum Diaries blog in English and French and in the Scientific American blog.
7 November 2012: Oscillating charm and beauty.
[ Δmd = 0.5156 ± 0.0051 ± 0.0033 ps-1 ]
[ x'2 = (-0.9 ± 1.3)x10-4 ; y' = (7.2 ± 2.4)x10-3 ]
A fascinating feature of quantum mechanics has been reported 15 March 2011 on this page. The strange beauty particle (matter) B0s composed of a beauty antiquark (b) bound with a strange quark s turns into its antimatter partner composed of a b quark and an s antiquark (s) with a frequency of about 3 million million times per second (3*1012). This feature is called "oscillations" or "mixing". The LHCb Collaboration has just published the first observation of similar oscillations of charm mesons D0 composed of a charm quark and an anti-up quark (D0-D0 oscillations) and the most precise measurement of a parameter defining the frequency of the oscillations of beauty mesons B0 composed of a beauty antiquark (b) bound with a d quark (B0-B0 oscillations).
The B0 decays into D+π- and J/ψK*0 were used to study B0-B0 oscillations. The images below show the asymmetry which is proportional to the difference between the number of events in which the matter (or antimatter) B0 particle decayed with the same flavour identity with which it was produced, and the number of events in which it did not, as a function of its lifetime. The B0-B0 oscillations are clearly visible. LHCb physicists have parametrized them with a value Δmd = 0.5156 ± 0.0051 ± 0.0033 ps-1 corresponding to the frequency of about 80 thousand million times per second (8*1010), about 37 times slower than B0s-B0s oscillations. The B0-B0 oscillations have been previously measured at LEP, Tevatron and B factories. The LHCb result is currently the most precise measurement of the Δmd parameter.
click the image for higher resolution
D0-D0 oscillations are very slow, over one hundred times the average lifetime of a D0 meson, so the full oscillation period cannot be observed. Instead, it is necessary to look for small changes in the flavour mixture (matter or antimatter) of the D0 mesons as a function of the time at which they decay. One of the best channels to search for this mixing is the D0 decay to the Kπ final state. The initial matter-antimatter identity can be identiﬁed by the charge of the accompanying pion in the decay D*+→D0π+ or D*-→D0π-. The mixing effect (oscillation) appears as a decay-time dependence of the ratio R between the number of reconstructed “wrong-sign” (WS; D0→K+π-) and “right-sign” (RS; D0→K-π+) processes. In the absence of mixing, R is predicted to be constant as a function of the D0 decay time t, while, in case of mixing, it is predicted to be an approximately parabolic function of t. The left image below shows the WS over RS ratio R, as a function of decay time, from a total of 36 thousand WS and 8.4 million RS decays selected from the 1.0 fb-1 of data recorded in 2011. The horizontal dashed line shows the no-mixing hypothesis, the solid line is the best ﬁt to data when mixing is allowed. The clear time dependence observed excludes the no-mixing hypothesis by 9.1 σ.
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Comparison of values parameterizing the D0-D0 oscillations, y' and x'2, obtained by different experiments, are presented in right image above. The LHCb results exclude the no-mixing hypothesis by more than 5 σ for the first time and therefore can be classified as the first observation of this effect.
Since the Standard Model predictions for the mixing parameters have large uncertainties, the next step will be to focus on cleaner observables to search for possible new physics contributions. In particular, LHCb is now well placed to investigate whether there is a CP violating contribution to the oscillations, in contrast to the Standard Model expectation. This will be achieved by studying charm mixing in this and other decay channels and exploiting the large increase in available statistics following the successful 2012 LHC run.
5 October 2012: Measurement of the γ angle.
[ γ = (71.1+16.6-15.7)° only B±→DK decays ]
[ γ = 85.1° uncertainty regions [61.8,67.8]° and [77.9,92.4]°, B±→DK and B±→Dπ decays ]
LHCb is an experiment set up to explore what happened after the Big Bang that allowed matter to survive and build the Universe we inhabit today. Therefore LHCb physicists are measuring differences between properties of matter and antimatter, called CP violation by experts. CP violation was discovered experimentally in K meson decays in 1964. M. Kobayashi and T. Maskawa proposed in 1973 a mechanism which could incorporate CP violation within the Standard Model with not less than 6 quarks; they were awarded the Nobel prize of Physics in 2008 for this idea. The size of this violation is set by the parameter η, which is shown as the y axis in the figures below. Constraints on η and the related parameter ρ (the x-axis) are measured in various ways in different experiments as shown in the compilation made by the CKMfitter group in the left image below. The constraints show that in fact the values of ρ and η within the small colored region in the center of the images are compatible with the experimental results and confirm in this way the Kobayashi and Maskawa Standard Model mechanism of CP violation. However, since this mechanism does not explain the large quantity of matter observed in the Universe physicists are searching for other sources of CP violation outside the Standard Model.
An interesting possibility is to measure precisely the angle γ of the triangle shown in the right image below in processes in which the new physics contribution is possible and in processes in which it is not. Differences between measurements in these two cases would be a sign of new physics. The measurement of the angle γ in different processes is one of most important goals of the LHCb experiment. The value of this angle is known up to now with precision of only about 10 or 12°, as seen in the right image below, using the combination of results from other experiments.
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LHCb physicists have just presented at the 7th International Workshop on the CKM Unitarity Triangle, Cincinnati, Ohio, USA, the measurement of the angle γ in processes where the contribution of new physics is not expected. These measurements will set a base for comparison with the measurement where observation of new physics effects is possible. The B±→DK and B±→Dπ decays were used with D mesons decays into KK, ππ, K0Sππ, K0SKK or Kπππ. The value of the angle γ = (71.1+16.6-15.7)° was obtained using only B±→DK decay results in the analysis. The image at the left hand side shows the confidence level of the signal as a function of angle γ, for the combination of B±→DK modes. The peak of the distribution gives the measured central value, and the width gives the error. The best value of the angle γ = 85.1° was obtained with the two corresponding uncertainty regions [61.8,67.8]° and [77.9,92.4]° (at 68% CL) when the B±→Dπ decays were included in addition.
The 2011 data sample (1.0 fb-1) was used in this analysis. The precision of the γ angle measurement is already comparable with that achieved by other experiments and the value of this angle is now known to a precision of less than 10 degrees, when this result is averaged with the latest results of other experiments. LHCb physicists are working on analyses of other decay modes that can help improve the precision. It is interesting to note that the data collected by LHCb in 2012 already exceeds the sample from 2011 and by the end of the year the total dataset should have more than tripled.
3 October 2012: Matter antimatter asymmetry in three-body charmless B decays becomes more and more interesting.
[ ACP(B±→π±π+π-) = +0.120 ± 0.020 ± 0.019 ± 0.007 ]
[ ACP(B±→π±K+K-) = -0.153 ± 0.046 ± 0.019 ± 0.007 ]
Earlier this year, LHCb physicists reported at the ICHEP2012 Conference (see 7 July 2012 (2) news) the first evidence of inclusive CP asymmetry (differences between properties of matter and antimatter) in the charmless three-body B meson decays B±→K±π+π- and B±→K±K+K- in which the b-quark decays into a u,d or s-quark instead of its dominant decay into a charm c-quark. It was interesting to note that much larger asymmetries were observed in some small special regions, like invariant mass squared of the π+π- pair in the K±π+π- decay lower than 1 GeV2, or of the K+K- pair in the K±K+K- final state between 1.2 and 2 GeV2 (7 July 2012 (2) news).
click the image for higher resolution
These measurements have now, at the 7th International Workshop on the CKM Unitarity Triangle, Cincinnati, Ohio, USA, been complemented with results from the rarer B±→π±π+π- and B±→π±K+K- decays, finding evidence of even larger CP violation. A remarkable feature of the new LHCb results is that the CP violation effects appear to arise in some special kinematical regions that are not dominated by contributions from narrow resonances. For example, in B±→π±K+K- decays a broad feature at low K+K- invariant mass that was previously observed by the BaBar collaboration [PRL 99 (2007) 221801] appears to be present only in B+ decays, as shown by the filled triangles distribution in the left figure above, and not in the B- distribution (open triangles). This points to some interesting hadronic dynamics that could generate the observed direct CP violation. LHCb is now starting detailed studies of these channels, that can also exploit the three times larger data sample available after 2012 running, to further understand these effects. The results of these analysis will also establish whether the observed CP violation is consistent with the Standard Model expectation or has a more exotic origin.
The middle and right images above show the π±K+K- invariant mass distribution for B- and B+ decays with m2K+K- < 1.5 GeV2/c4. The difference between properties of matter and antimatter, the CP violation, is clearly seen as a difference between the height of two peaks located at the B mass (at 9σ level for experts).
25 September 2012: Searching for new physics in rare kaon decays.
[ Branching ratio K0S →μμ < 9x10-9 at 90% CL ]
Having previously set the world's most restrictive limits on the dimuon decays of D0, B0 and B0s mesons, LHCb physicists have turned their attention to the search for similar decays of another member of the particle zoo, the K0S meson. Using the 2011 data sample, LHCb has set a limit on the branching ratio B(K0S→μ+μ-) to be less than 9x10-9, a factor of 30 improvement over the previous most restrictive limit measured in 1973. [For experts: the limit is at 90% confidence level.] While several techniques used were common with the search for B0s→μ+μ- (see news items of 8 April and 22 July 2011 and 5 March 2012 for more details), the challenge of this measurement lies in the specificity of kaon decays, which are very different from B decays for which LHCb detector was optimized.
The image below shows the invariant mass distribution of selected μ+μ- pairs, candidates for K0S decay. The dashed lines indicate the signal region around the K0S mass, where no significant signal is seen.
click the image for higher resolution
K meson decays into pairs of muons played a very important role in the history of particle physics. There are two types of K mesons: the short-lived, K0S ("K-short") and the long-lived, K0L ("K-long"). The results of branching ratio measurements of the K-long decay into muon pairs in the early 1970s disagreed strongly with the predictions of particle physics theory based on existence of three quarks u, d and s. The calculated branching ratios were of the order of 10-4 while the experimental limits were about 4 orders of magnitude lower. In order to solve this problem Glashow, Iliopoulos and Maiani proposed the existence of an additional quark, called a charm quark – a 1970s version of a new physics model. In the mechanism they proposed (GIM mechanism) a destructive virtual contribution of this new quark reduced very strongly the K-long decay rate into muon pairs. The discovery of the J/ψ state in November 1974 gave the first evidence for the existence of charm quark and at the same time confirmation of the GIM mechanism.
40 years later LHCb physicists are searching for K-short decays into muon pairs, again on the look-out for new physics. The branching ratio is calculated to be 5x10-12 within the framework of the Standard Model of particle physics with 6 quarks. Although the new limit 9x10-9 is still three orders of magnitude above the prediction, it starts to approach the level where new physics effects might begin to appear. Moreover, the data collected by LHCb in 2012 already exceed the sample from 2011 and by the end of the year the total dataset should have more than tripled.
13 September 2012: First proton-lead ion collisions at LHCb.
During the night, at 1:30 am, LHCb recorded the first proton-lead ion collisions at the LHC. The proton-lead physics data taking is planned to take place in January and February 2013. Today the LHC operational team made tests of collisions in order to prepare the set-up of the LHC collider and the LHC experiments for next year. Note that collisions of protons with lead ions are more difficult than proton-proton or lead-lead collisions. In fact the speed of protons and lead ions is slightly different, even though it is close to the speed of light at LHC. The LHC operators succeeded in making the proton path length inside LHC ring slightly longer than the lead-ion path length in order to compensate for this difference.
click the image for higher resolution
A typical proton-lead collision event at LHCb is shown in the left image above. Note that the lead ions arrive at the LHCb collision point from the right hand side and the protons from the left hand side. The right image shows the invariant mass spectrum of the decay products of Λ and Λ particles indicating excellent prospects for physics analysis of p-lead collision data early next year.
Read also the CERN Courier article.
21 August 2012: LHCb has doubled its recorded luminosity.
The data sample used to obtain the LHCb results presented on this page was obtained using the integrated luminosity of 1.11 fb-1 recorded by LHCb during the 2010-2011 data taking period. The same inegrated luminosity has just been recorded this year, which corresponds to doubling the total available integrated luminosity. The data sample which can be used for physics analysis has more than doubled in the same time, since it is expected that the cross-sections for bb and cc quark-antiquark pair production have increased by 15% and 12%, respectively, at the increased pp collision energy of 8 TeV this year. LHCb physicists expect to more than triple their data set this year by the end of pp collision run at December, which should allow them to obtain even more interesting physics results.
You can follow live the progress of delivered and recorded luminosity at LHCb, the number of proton-proton collisions visible at LHCb, as well as the number of bb and cc quark pairs produced within the LHCb acceptance at the image above on this page.
3 August 2012: 1 fb-1 of luminosity has been delivered to LHCb this year.
"This is great achievement considering that it comes about two months earlier than last year. Once more, the excellent performances of the machine, the skill and the commitment of the whole LHC team, made possible this result. We have also the exciting perspective of getting another 1 fb-1, or more, this year. This increased sample will allow us to push further our knowledge of Standard Model, and find finally where new physics is hiding so well." - said Pierluigi Campana, LHCb spokesperson.
12 July 2012: LHCb film shortlisted by the European Science TV and New Media Festival 2012.
"LHCb - A Beauty Experiment", a short documentary on LHCb, has been shortlisted by the European Science TV and New Media Festival 2012. The festival takes place in association with the ESOF 2012 Science Congress in Dublin at Trinity College on 13-15th July 2012. The goal of the festival is to help writers to develop TV drama that involves science and technology. It is interesting to note that the LHCb film was already shortlisted by the NHK Japan Prize Festival in October 2011. Watch the film on YouTube in different resolutions and with subtitles in 15 different languages.
Screen shots from the film show the LHCb control room on 23 November, 2009 (left) and on 30 March, 2010 (right).
7 July 2012 (1): Measurement of the flavour-specific matter antimatter asymmetry.
[ assl = (-0.24 ± 0.54 ± 0.33)% ]
Physicists from the LHCb experiment have today released results that help to shed light on one of the most significant experimental discrepancies with the Standard Model of particle physics. In 2010, and with an update in 2011, the D0 experiment, analyzing data taken at the proton-antiproton collider Tevatron at Fermilab, reported an interesting observation: that the number of events containing two positively charged muons is lower than the number of events containing two negatively charged muons, see 2010 Fermilab Press Release. The observed difference was close to 1%, measured with almost the full D0 data sample of 9 fb-1. Like-signed dimuons can be produced from the decay of particles containing the b quark, which can mix between their particle and antiparticle states. A difference between the number of positive and negative dimuons would be an indication of CP violation. The D0 result differs by 3.9σ from the tiny value predicted in the framework of the Standard Model and could indicate the presence of a new physics contribution. This difference can be expressed as an asymmetry, Absl, where the label "b" indicates decay of particles containing b-quarks and "sl" (semileptonic) indicates that the decay involves leptons, in this case muons. The decaying matter (antimatter) B particles are composed of b-antiquarks(quarks) and d- or s-quarks(antiquarks). D0 physicists could not distinguish which type of decaying particles is at the origin of the measured asymmetry, therefore they present the measured asymmetry Absl in the image below as an inclined band across the plane of individual asymmetries in the decays of Bd and Bs mesons, labelled adsl and assl, respectively. The vertical band shows the measurement of the adsl asymmetry by the BaBar and Belle collaborations working at the Υ(4S) resonance, which is in agreement with the Standard Model calculations shown as the SM point in the image.
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LHCb physicists have presented today at the ICHEP2012 Conference in Melbourne the most precise determination to date of the corresponding asymmetry for the Bs meson, assl. The LHCb result assl = (-0.24 ± 0.54 ± 0.33)% is shown as the horizontal blue band in the image above. The B0s semileptonic decays into D±sμ∓ final states were studied to obtain this result. The charmed D±s mesons were reconstructed in the φπ± mode. More details can be found in the LHCb presentation in Melbourne and in the LHCb Conference Contribution here. The LHCb result is consistent with the Standard Model prediction and does not confirm the deviation from the Standard Model reported by the D0 experiment. The D0 experiment previously published a result using Dsμ events, shown as the horizontal yellow band.
The full 2011 LHCb data sample was used to obtain this result. LHCb physicists expect to more than triple their data sample this year.
7 July 2012 (2): Evidence for matter antimatter asymmetry in three-body charmless B decays.
[ ACP(B±→K±π+π-) = +0.034 ± 0.009 ± 0.004 ± 0.007 ]
[ ACP(B±→K±K+K-) = -0.046 ± 0.009 ± 0.005 ± 0.007 ]
LHCb physicists have presented today at the ICHEP2012 Conference in Melbourne measurements of differences between properties of matter and antimatter (CP violation asymmetry) in the charmless three-body B meson decays B±→K±π+π- and B±→K±K+K-. The dominant B meson decays involve a beauty b-quark decay into a charm c-quark. In the rarer charmless decays (without charmed mesons) discussed here the b-quark decays into a u,d or s-quark. LHCb physicists have measured the charge asymmetry ACP obtained from the difference between negative and positive B event rates. The numerical values of measured asymmetries are shown above. The significance is 2.8σ for the B±→K±π+π- decay and 3.7σ for the B±→K±K+K- channel. The latter is the first evidence of inclusive CP asymmetry in charmless three-body B± decays.
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The results presented here are obtained by integrating (summing) the asymmetries over all kinematical variables of particles observed in the decays of B mesons. It is interesting to note that much larger asymmetries are observed in some small special regions, like invariant mass squared of the π+π- pair in the K±π+π- decay lower than 1 GeV2, or of the K+K- pair in the K±K+K- final state between 1.2 and 2 GeV2, as seen in the images above. Note that the value plotted in the figures is the raw asymmetry ARAWCP, and does not include all the small corrections that are included in the numerical results. LHCb physicists are planning further study of this intriguing feature.
The full 2011 data sample was used to obtain this result. LHCb physicists expect to more than triple their data sample this year.
25 May 2012: Puzzling asymmetries.
The decay of the beauty meson B into an excited K meson K* and a μ+ and μ- pair is considered as an important channel for new physics search, see 13 March 2012 and 22 July 2011 news. Different distributions and branching fractions have been studied for these B meson decays and compared with the Standard Model predictions. Recently LHCb physicists have studied in addition the differences in the results of measurements of neutral B meson decays into K*0μ+μ- and charged B+ meson decays into K*+μ+μ-. Theoretical uncertainties of the Standard Model calculations are strongly reduced in these differences. Physicists call these differences "asymmetries" and since this case involves differences between the decays of particles with different charges, it is called an "isospin asymmetry" AI. The Standard Model calculations predict this isospin asymmetry to be small, as shown with the help of the color line in the left image below, where the prediction is presented as a function of the square of the di-muon invariant mass (q2). The experimentally measured distribution, shown by points with error bars, is consistent with this prediction.
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A surprise came when the physicists made similar analysis by replacing the excited kaon K* by its ground state K. The negative asymmetry is clearly visible in the right image above (4.4σ different from zero after integration (summation) over the whole q2 region). The Standard Model calculation is not yet available for this asymmetry, but is, as in the Bs→K*μμ mode, expected to be very close to zero.
The images shown above were obtained with the full 2011 data sample. These results have been made possible by the strong and continuous support from the LHC operations team for the LHCb physics program, and the data sample is expected to more than double by the end of this year. In the meantime theorists will analyze this puzzling result in order to establish whether this effect can be accommodated in the framework of the Standard Model, or whether a new physics explanation is required.
16 May 2012: First observation of two excited states of Λb.
The quark model, independently proposed by physicists Murray Gell-Mann and George Zweig in 1964 to classify the strongly interacting particles called hadrons, is very successful. In this model baryons are composed of three quarks and mesons are composed of quark-antiquark pairs. The simplest baryon, the proton, which is the nucleus of the hydrogen atom, is composed of three light quarks uud while its neutral partner the neutron is composes of udd quarks. By replacing one of the d quarks by a heavier strange quark s we obtain a Λ particle composed of uds quarks. Furthermore by replacing in the Λ the s quark by a charm quark c or a beauty quark b we obtain a Λc or a Λb particle.
The three quarks forming the Λ, Λc and Λb are in their lowest quantum mechanical state. Like electrons in atoms quarks can form excited states with different values of angular momentum and quark spin orientation. These excited states were previously observed for the Λ and Λc particles. They were, however, never observed for the Λb particle.
The LHCb collaboration has made first observations of two excited states of Λb.
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The Λb excited states have been reconstructed in three steps. In the first step the Λc+ particles were reconstructed through their decay into a proton p, a negative K- meson and a positive π+ meson. In the second step the Λc particles were combined with negative π- mesons in order to form the Λb particles. The Λb signal is clearly seen as the enhancement in the left image above showing the Λc+π- invariant mass spectrum. Finally the Λb particles have been combined with a pair of opposite sign pions π+π-. In the right image above two enhancements are clearly seen corresponding to the two Λb excited states with masses of 5912 and 5920 MeV, about 6 times the proton mass.
LHCb physicists have observed about 16 Λb(5912)→Λbπ+π- decays (4.9σ significance) and about 50 Λb(5920)→Λbπ+π- decays (10.1σ) among about 60 million million (6*1013) pp collisions seen by the LHCb detector at LHC during the 2011 data taking period.
27 April 2012: The rarest B decay ever observed.
The LHCb collaboration has made the first observation of the decay B+ → π+μ+μ-. With a branching ratio of about 2 per 100 million decays, this is the rarest decay of a B hadron ever observed, see CERN Courier article.
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The image shows the invariant-mass distribution of selected π+μ+μ- combinations, showing the clear peak corresponding to B+ decays (green long-dash). Also shown are the components in the fit from partially reconstructed decays (red dotted) and misidentified K+μ+μ- (black dashed) and the total (blue solid line). Candidates for which the μ+μ- pair is consistent with coming from a J/ψ or ψ(2S) decay have been excluded.
6 April 2012: LHCb looks forward to electroweak physics.
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The two images above show the complementarity of measurements made by different LHC experiments. The left image shows the x-Q2 kinematic region explored by the LHCb experiment (pink), compared with ATLAS and CMS (light green), as well as previous experiments. The right image shows LHCb's measurement of W charge asymmetry, shown as a function of lepton pseudorapidity, compared with theoretical predictions, and with results from the ATLAS and CMS experiments. More details and explanations can be found in the CERN Courier article.
30 March 2012: LHCb strongly squeezes SUSY parameter space.
Results presented by the LHCb Collaboration at the Rencontres de Moriond EW and QCD conferences allowed theorists to squeeze strongly the parameters of supersymmetric extensions of the Standard Model (SUSY), the most popular new physics model. The simplest version of this model, called the Minimal Supersymmetric Standard Model (MSSM), predicted the frequencies with which Bs and Bd mesons decay into pairs of oppositely charged muons to have values significantly different from the Standard Model (SM) prediction. This is shown in the left image below, which was presented by David Straub (SNS and INFN, Pisa) at the Moriond EW conference. The predictions for both frequencies (branching ratios BR) depend on different parameters of the MSSM and cover nearly all of the left image surface. The LHCb results, see 5 March 2012 news, limit the predictions that are still allowed to a small region around the SM expected value. It is interesting to note that certain combinations of MSSM parameters allow lower BR values than those predicted by the SM. The LHCb measurements of the parameter φs, which sets the scale for the difference between properties of matter and antimatter for the strange beauty Bs mesons, see 5 March 2012 news, also strongly limits the SUSY parameter space that is still allowed, as shown by the vertical lines on the right image below.
SUSY contributions to observables that can be measured in experiments depend, in general, on more than 100 free parameters. Therefore in order to be able to analyse experimental data physicists are using a simplified model, the Constrained MSSM (CMSSM), with 5 parameters m0, m1/2, A0, tan β and μ/|μ|. Nazila Mahmoudi (Clermont-Ferrand and CERN) presented the left image below at the Moriond QCD conference. The parameter space below and to the left of the red line is excluded by the results of searches for direct production of SUSY particles at the CMS experiment, while the large yellow region shows the parameter space excluded by the analysis of Bs →μμ decays at LHCb. The image is made for a relatively high value of the parameter tan β=50. The LHCb exclusion region is smaller at lower values of tan β as can be seen in the right image below, made with tan β=35. The green regions on both images are still not excluded by LHCb's measurements.
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The images above illustrate that direct and indirect SUSY searches at LHC are complementary and show that the allowed parameter space is characterized by low tan β and high m0 and m1/2 within the CMSSM. LHCb's results have ruled out large deviations in several observables, but now LHCb physicists can start to probe the most interesting parameter region of new physics.
13 March 2012 (1): B0d →K*μμ, a first measurement of the zero-crossing point.
[ Zero crossing point B0d →K*μμ = 4.9+1.1-1.3 GeV2 ]
LHCb physicists have continued their search for physics beyond the Standard Model using the B0d decay into a K* meson (an excited kaon), and a μ+ and μ-. New physics contributions can change various distributions that describe the decay process. For example, the number of decays as a function of the square of the di-muon invariant mass (q2) and the di-muon forward-backward asymmetry (AFB) can both be affected in many new physics scenarios. The variable AFB indicates whether more or fewer muons of one sign are observed in the same direction as the K* than opposite to it. The distribution of AFB in function of q2 is shown below. The results have been announced today at the Rencontres de Moriond QCD conference.
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The point at which the AFB distribution is crossing zero is well predicted within the Standard Model, and any deviation could indicate a possible contribution from new physics. LHCb physicists have already presented at the 2011 summer conferences a first indication that the asymmetry is changing sign. Today, using three times higher statistics, they presented the first measurement of the zero-crossing point of 4.9+1.1-1.3 GeV2. This value, showed by the hatched vertical region in the Figure, is in agreement with the Standard Model prediction showed by the colored line.
The LHCb Collaboration aims to more than double its data set this year. With the new data, the zero-crossing point will be measured with higher precision, and a possible difference with the Standards Model prediction may be discovered.
Read more in the LHCb staff page.
13 March 2012 (2): A first measurement of the CP asymmetry in the decay B0s →K+K-.
In a fascinating world of quantum mechanics the strange beauty particle (matter) B0s turns into its antimatter partner about 3 million million times per second (3*1012), see 15 March 2011 news. Therefore CP violation effects, which are differences between the properties of matter and antimatter, can appear as variations with decay time. A name "time-dependent CP violation" is therefore used.
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The left figure shows the CP asymmetry in the B0d decay into π+π- pair and the right figure shows the asymmetry in the B0s decay into K+K- pair. It can be clearly seen, by comparing time scales of both figures, that the B0d mesons oscillate much slower than the B0s mesons. A new analysis from LHCb has measured the CP asymmetry separated into two components: the difference in the decay rate of B meson and anti-B meson (physicists call it "direct CP violation") and the quantum-mechanical phase difference between the B meson and anti-B meson decay ("mixing-induced"). These two components appear as a cosine-like and as a sine-like oscillation in the asymmetry plots above.
The results, announced today at the Rencontres de Moriond QCD conference, for the B0d → π+π- decay are Adirπ+π- = 0.11 ± 0.21 ± 0.03 and Amixπ+π- = -0.56 ± 0.17 ± 0.03, meaning the oscillation appears to come mainly from the sine-like component. These are the first measurements of these quantities at a hadron collider, and are consistent with previously published results from other experiments. The results for the B0s → K+K- decay - which are measured for the first time ever - are AdirK+K- = 0.02 ± 0.18 ± 0.04 and AmixK+K- = 0.17 ± 0.18 ± 0.05.
About 2/3 of data taken in 2011 were used in this analysis. The LHCb Collaboration aims to analyse three times more data by the end of this year. This will allow to see if CP violation occurs in the B0s → K+K- decay, and see if the amounts of direct and mixing-induced CP violation are as expected by the Standard Model.
Read more in the LHCb staff page.
5 March 2012 (1): Search for New Physics, an important milestone
[ Branching ratio B0s →μμ < 4.5x10-9 at 95% CL ]
The LHCb collaboration has announced today at the Rencontres de Moriond EW conference one of the most hotly anticipated results from the LHC. LHCb has shown that the frequency with which a Bs meson decays into a pair of oppositely charged muons is not larger than 4.5 times out of one billion decays. Theorists have calculated that, in the Standard Model, this decay should occur about 3 times in every billion, but that if new particles predicted by theories such as supersymmetry exist, the decay could occur much more often (see news items of 8 April and 22 July 2011 for more details). The new results represent a milestone in the search for "new physics" beyond the Standard Model.
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The left figure shows the mass calculated from the two muons for events that survive all selection requirements. The blue area shows the shape of random combinations of muons, while the red area shows the shape expected for real Bs decays (with the hatched area indicating the uncertainty on the sum of the two contributions). The data are seen to be consistent with a small excess over the background-only hypothesis. This excess is slightly less than, but consistent with, the Standard Model expectation, as shown in the right figure. The blue points show how likely the data distribution is for a given rate of Bs decay. The black dashed line shows the expected shape of the curve for the Standard Model rate, with the green band indicating the uncertainty. The horizontal lines allow to set limits with different degrees of certainty: for experts, the solid red line gives the 95% C.L.
Measuring the rate of this Bs decay has been a major goal of particle physics experiments in the past decade, with the limit on its decay rate being gradually improved by CDF, D0, LHCb and CMS experiments. The latest results by LHCb set the tightest limits to date. More data is needed to finally discover if the decay occurs at a rate above, below, or at that predicted by the Standard Model. LHCb aims to more than double the size of its data set in 2012, which could be enough to finally answer this question.
5 March 2012 (2): Heavier strange-beauty lives longer and improved φs measurements.
[ φs = -0.002 ± 0.083 ± 0.027 rad]
LHCb physicists have reported today at the Rencontres de Moriond EW conference important progress in measurement of the difference between properties of matter and antimatter for the strange beauty Bs mesons. The new results improve on those presented at the summer 2011 conferences. The size of this difference is controlled by the parameter φs, which is predicted to be small in the Standard Model. However, effects of new particles not predicted by the Standard Model can make the measured value much larger. The progress is shown in the image below: the remaining allowed region is shown in yellow and compared to the previous results from LHCb in blue, and from CDF and D0 experiments in green and red, respectively.
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In the mysterious world of quantum mechanics the strange-beauty meson Bs matter antimatter system is alternatively described as a heavy and light mass Bs meson system. The mass difference Δms determinates the matter antimatter oscillation frequency, see 15 march 2011 news. The difference of their lifetimes was measured together with the value of φs. Since it was not previously know if the heavier or lighter Bs mesons live longer, LHCb physicists have obtained two possible values for φs corresponding to two blue regions in the image above. (The axis label ΔΓs corresponds to the difference of inverse lifetimes between the heavy and light Bs meson, physicists call it width (Γ) difference.) Recently LHCb physicists have succeeded to measure that the heavier strange-beauty Bs mesons live longer and in this way eliminated one of two blue regions in the image. Sophisticated quantum mechanical interference effects were used in this measurement. Experts can read details of the analysis in the published article.
The full sample of data collected in 2011, three times larger than that used in summer 2011, was used to obtain the result φs = -0.001 ± 0.101 ± 0.027 rad using Bs decays into into a J/ψ meson and a φ meson; combining it with the measurement of the Bs decay into J/ψ and f0(980) LHCb physicists have obtained the final result φs = -0.002 ± 0.083 ± 0.027 rad.
2012 data taking period will start soon. Search for new physics in the small yellow region will continue.
5 March 2012 (3): First evidence for CP violation in the decays of Bs mesons.
Measurement of CP violation, which describes differences between the properties of matter and antimatter, is a very important goal of LHCb. CP violation is well established in the K0 and B0 meson systems. Recent results from the LHCb collaboration have also provided evidence for CP violation in the decays of D0 mesons. Consequently, there now remains only one neutral heavy meson system, the B0s, where CP violation has not been seen.
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The LHCb collaboration has recently submitted for publication the first evidence for CP violation in the decays of B0s mesons. The paper (available here for experts), submitted to Physical Review Letters, confirms the preliminary results shown at the EPS-HEP meeting in Grenoble, see 22 July 2011 news. The B meson decays into K and π mesons were studied. The decay into a positive K meson red track and a negative π green track is show in the event display above. The four plots on the right hand side show the Kπ invariant mass distribution divided into different components as shown by the legend in the top-right figure. The different charge combination of K and π indicates if the decaying B particle is a matter or an antimatter particle. The two upper plots show that the decay rates of B0d mesons are different, as known from previous experiments. The lower two plots show that the difference is also visible for the B0s mesons. The measured CP asymmetry for the B0d mesons ACP = -0.088 ± 0.011 ± 0.008 constitutes the most precise measurement available to date and the first observation (6σ) at a hadron collider. The measured CP asymmetry for the B0s mesons ACP = 0.27 ± 0.08 ± 0.02 is the first evidence (3.3σ) for CP violation in the decays of these mesons.
LHCb physicists have used 1/3 of data collected in 2011 in this analysis and expect to have ten times more data by the end of this year.
1 December 2011: LHCb looks to the future.
After a very succesful 2011 data taking period the LHCb Collaboration is preparing next year's operation. The first 2012 collisions should be observed in April. At the same time LHCb physicists are also actively working on the longer term future in which data will be taken at a much higher rate, see CERN Courier article.
14 November 2011: CP violation in charm decays.
[ ΔACP = (-0.82 ± 0.21 ± 0.11)% ]
The LHCb Collaboration has presented today at the Hadron Collider Particle Symposium in Paris possible first evidence for CP violation, the difference between behaviour of matter (particles) and antimatter (antiparticles), in charm decays. The study of CP violation in both charm and beauty particle decays is central to the LHCb physics programme. In the Standard Model CP violation is expected to be very small in the charm sector, whereas new physics effects could generate enhancements.
In this new analysis the LHCb physicists have used data collected in the first half of the 2011 run to study the differences in decay rates of neutral D meson particles composed of a charm quark c bound with an up antiquark (u) and D meson antiparticles (D) composed of a charm antiquark (c) bound with an up quark (u). The decays of D*+ mesons into D mesons and π+, and D*- mesons into D mesons and π- were used to select the D and D mesons. In the next step of the analysis the difference (asymmetry ACP) between the decay rates of D and D mesons into K+K– pairs as well as into π+π- pairs was measured. By determining the difference, ΔACP, in CP asymmetries for the K+K- and π+π- decays, the analysis strongly suppresses possible measurement biases which could arise through effects related to particle production, selection etc. The following preliminary result is obtained:
ΔACP = (-0.82 ± 0.21 (stat.) ± 0.11 (sys.) )% [ 3.5 sigma significance for experts ]
A very interesting period now begins. LHCb physicists are analysing the remainder of the data collected in 2011. If the result is confirmed theoretical work will be required to establish whether this effect can be accommodated in the framework of the Standard Model, or whether a new physics explanation is required.
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The figures show the invariant mass distribution of the K-K+ (around 1.4 million) and π-π+ (around 0.4 million) pairs. The distributions are centered at the D meson mass of 1865 MeV.
30 October 2011: End of 2011 proton-proton collision data taking period.
The 2011 proton-proton collision data taking period has ended today. LHC collider and LHCb experiment have been working extremaly well. LHCb has recorded all in all an impressive 1.1 fb-1 out of a 1.22 fb-1 delivered at 3.5 TeV.
“We’ve got from the LHC the amount of data we dreamt of at the beginning of the year and our results are putting the Standard Model of particle physics through a very tough test ” said LHCb Spokesperson Pierluigi Campana. “So far, it has come through with flying colours, but thanks to the great performance of the LHC, we are reaching levels of sensitivity where we can see beyond the Standard Model. The researchers, especially the young ones, are experiencing great excitement, looking forward to new physics.”
3 October 2011: 1 fb-1 of luminosity has been delivered to LHCb.
This is a very important milestone for LHCb which will allow LHCb physicists to reach an unprecedented accuracy in most of the core physics processes that are under study.
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The left hand image shows Mike Lamont (Operations Group Leader), Pierluigi Campana (LHCb Spokesperson), Steve Myers (Director for Accelerators and Technology), and Paul Collier (Head of the Beams Department) celebrate the LHCb milestone. The right hand image shows the zoom on the computer screen located above them and showing the LHC screen congratulating LHCb for its new record.
3 October 2011: LHCb film shortlisted by Japan Prize Festival
"LHCb - A Beauty Experiment", a short documentary on LHCb, has been shortlisted by the NHK Japan Prize Festival. Every year, the festival awards the very best in global educational media. Watch the film at YouTube in different resolutions and with subtitles.
27 August 2011: φs: different properties of matter and antimatter for Bs mesons
[ φs = 0.03 ± 0.16 ± 0.07 ]
LHCb physicists have presented today the most precise measurement of φs (the Bs mixing phase, for experts) at the Lepton Photon conference in Mumbai (India). The value of φs is precisely predicted in the Standard Model and sets the scale for the difference between properties of matter and antimatter for Bs mesons, known to physicists as CP violation. The predicted value is small and therefore the effects of new physics could change its value significantly - see the analogy in the 8 April news.
The decay of the strange-beauty particle B0s, composed of a beauty antiquark (b) bound with a strange quark (s), into a J/ψ meson and a φ meson was used for this measurement. The J/ψ meson decays in turn into a μ+μ- pair, and the φ decays to K+K– pair. In order to make this difficult measurement LHCb physicists had to analyse the Bs decay particles in 3 dimensions as well as to measure precisely the fast oscillations of strange beauty - see 15 March news.
The "artist's view" below shows the result of the φs measurement in a plane together with the correlated measurement of another value, ΔΓs. The results of the measurement favour two regions, one of which is located around φs = -0.036 ± 0.002 rad, the Standard Model prediction. The LHCb measurement is in agreement with the Standard Model prediction but the shaded region representing the LHCb result indicates that there is still room for a new physics contribution. The hints for a larger contribution of new physics suggested by the CDF and D0 experiments at Fermilab, also shown in the figure, are not confirmed.
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In February the LHCb Collaboration made first observation of the Bs decay into J/ψ f0(980) - see 27 February news. This decay contributed to the φs measurement.
Using both Bs decays LHCb physicists have obtained the value φs = 0.03 ± 0.16 ± 0.07.
Read also CERN Courier article.
22 July 2011 (1): Hunting for New Physics continues
[ Branching ratio B0s →μμ < 1.2x10-8 at 90% CL and 1.5x10-8 at 95% CL ]
LHCb physicists continue their search for new physics, see 8 April 2011 news for introduction. They have presented updated results during the International Europhysics Conference on High Energy Physics (EPS-HEP) at Grenoble this week. The LHCb physicists have succeeded in setting the limit for an enhanced decay rate of the strange beauty particle B0s, composed of a beauty antiquark (b) bound with a strange quark s, into a μ+ and μ- pair, as low as about 4 times the rate calculated within the Standard Model (limits 1.2x10-8 at 90% CL and 1.5x10-8 at 95% CL for experts). This result was obtained from the analysis of about 8 times more data than in the previous analysis.
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The computer reconstructed images above show the most significant event compatible with the strange beauty B0s decay into muon pair seen as a pair of purple tracks traversing the whole detector. The right hand image shows a close-up around the proton-proton interaction point from which many tracks originate. The B0s decays about 1 cm from the proton-proton collision point into two muons (purple tracks). The invariant mass of muon pair corresponds to the B0s mass. The number of observed B0s candidates is slightly above the background predictions and compatible with the expected signal predicted by the Standard Model (SM) theory. This analysis is much more sensitive than previous experiments, and although large deviations (by more than a factor 4) from the SM are excluded, there is still plenty of room for new physics contributions. LHCb physicists are expecting to be able to analyse about three times as many events by the end of 2011, and about ten times more by the end of 2012, to give a final answer for the possibility of a new physics contribution to this interesting rare decay.
22 July 2011 (2): Hunting more for New Physics
LHCb Physicists have also presented results of their search for new physics using the B0d (composed of a beauty antiquark (b) bound with a down quark d) decay into an excited K meson, K*, and a μ+ and μ-. The partial rate as a function of the square of the di-muon invariant mass (q2) and the di-muon forward-backward asymmetry (AFB) can both be affected in many new physics scenarios. The variable AFB indicates whether more or fewer muons of one sign are observed in the forward direction than in the backward direction. The distribution of AFB in function of q2 is shown below.
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The LHCb precision measurements shown in black are in agreement with the Standard Model (SM) theory and indicate for the first time that the asymmetry is changing sign as predicted by the SM. The results of measurements by other experiments are presented in the figure on the right hand side.
22 July 2011 (3): Different properties of matter and antimatter
An important part of the LHCb physics programme is reserved for studying differences between the properties of matter and antimatter (CP violation for experts). At the EPS-HEP meeting in Grenoble this week the LHCb physicists have presented distributions in which these differences can be clearly seen.
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The blue line follows the measured points. After background subtraction (slightly sloping black line from left to right) the Kπ invariant mass distribution is divided into different components shown in different colours. The different charge combination of K and π indicates if the decaying B particle is a matter or an antimatter particle. The red curve in the upper plots shows the decay rate of B0d (matter, left plot) and anti-B0d (antimatter, right plot). The horizontal red dotted line helps to show that the decay rates of matter and antimatter B particles to Kπ mesons are different. The green distributions in lower plots show that the difference between matter and antimatter decay rate is also observed in B0s decays to Kπ.
1 June 2011: Pierluigi Campana - new spokesperson for the LHCb collaboration
Pierluigi Campana from the Istituto Nazionale di Fisica Nucleare in Frascati, begins his 3-year tenure as LHCb spokesperson this June. He replaces Andrei Golutvin, from Imperial College London and Russia’s Institute for Theoretical and Experimental Physics. As the new voice for the collaboration, Campana will lead the experiment through what should prove to be a very exciting phase.
8 April 2011: Hunting for New Physics
Quantum mechanics allows energy non-conservation during a very short time, typical in particle collisions. This opens up the possibility to study signal for the existence of particles for which there is insufficient energy to produce them directly. This feature is used by LHCb physicists to search for heavy particles expected in new physics models - models that describe physics outside the Standard Model (SM) of particle physics. Since the effects of new physics are expected to be very small, LHCb physicists are looking for modifications of the properties of very rare SM processes which can be calculated with high precision. The rare decay of the strange beauty particle B0s, composed of a beauty antiquark (b) bound with a strange quark s, into a μ+ and μ- pair is an excellent candidate. Only one out of 3*109 B0s mesons should decay into a μ+μ- pair according to precise SM calcutions. This rate could be higher if new physics particles, such as those in models with an extended Higgs sector, for example, were to influence this decay.
LHCb physicists have not seen these decays in the data taken during the 2010 run and were able to set a limit which is about 19 times higher than the SM prediction, that is, the limit close to the one set by the Fermilab experiments CDF and D0 after many years of data taking. The figure below was used to set this limit at different levels of statistical probability.
click in the image to get it in higher resolution
LHCb physicists expect to observe the B0s mesons decay into μ+μ- pairs with the rate calculated within the SM using data taken this year and in 2012. An earlier observation of a higher rate will give interestng evidence for new physics.
Read more details in the CERN Courier article.
25 March 2011: LHCb movie
LHCb live and LHCb physics are presented in a short movie (about 15 min) "LHCb - the beauty experiment", available at YouTube and CDS. The YouTube version allows you to choose subtitle language (cc just below the movie window), as well as the image resolution including High Definition version 1080p.
15 March 2011: Oscillating Strange Beauty
or how matter turns into antimatter and back
Using data collected in proton–proton collisions at the LHC at a centre-of-mass energy of 7 TeV LHCb has observed a fascinating feature of quantum mechanics. The strange beauty particle (matter) B0s composed of a beauty antiquark (b) bound with a strange quark s turns into its antimatter partner composed of a b quark and an s antiquark (s) about 3 million million times per second (3*1012). The B0s particles have been identified through their decay into strange charm Ds particles (composed of a charm quark c bound with a strange s antiquark) and one or three πs. Of course, LHCb observes B0s particles and antiparticles only during their short lifetime in which they travel about 1 cm in the LHCb detector.
The plot shows the observation of these oscillations when all data have been folded into one oscillation period. The variable Amix is proportional to the difference between the number of events in which the produced matter(antimatter) B0s particle had the same identity during its decay, and the number of events in which it had not, as a function of its lifetime.
click in the image to get it in higher resolution
The B0s oscillations were observed for the first time by the Fermilab experiments CDF and D0 in 2006, see Press Release article. The oscillation parameters measured by the LHCb Collaboration show agreement with those measured at Fermilab.
27 February 2011: LHCb makes first observations of interesting B0s decays
Using data collected in proton–proton collisions at the LHC at a centre-of-mass energy of 7 TeV, the LHCb experiment has observed two new rare decay modes of B0s mesons for the first time. The decay B0s → J/ψ f0(980) will be important for studying differences between properties of matter and anti-matter (CP violation for experts) in the B0s system, while the decay B0s → D*–s2Xμ+ν will be valuable for testing predictions of strong interaction (QCD) theory.
The first new decay mode observed is of the decay B0s → J/ψ f0(980). The B0s consists of a b antiquark (b) bound with an s quark, and can decay to a J/ψ (cc) together with an ss state, which can be a φ or, more rarely, an f0. While the φ decays to K+K–, the f0 decays to π+π–. The figure shows the enhancement in the π+π– invariant mass distribution in the region of 980 MeV indicating an observation of f0(980).
click in the image to get it in higher resolution
The observation of these two new decay modes demonstrates that the LHCb experiment is already competitive in the field of heavy flavour physics. Great progress is expected with the larger data sample due from the coming run, with the potential to constrain, or even observe, new physics.
Read more details in the CERN Courier article, which also includes description of the second decay.
Read more details in the National Science Foundation article.
27 October 2010: Exotic mesons
In 1964 Murray Gell-Mann and George Zweig proposed the quark model (QM) in which mesons, like π mesons, are formed from quark and anti-quark pairs and baryons (like protons) from three quarks. The model is very succesful, but recently particles which could not be classified in this model have been discovered. LHCb has observed one of these exotic state candidates called X(3872) using its decay into a J/ψ meson (see 6 September news) and a π+ π- pair. The J/ψ decays in turn into a μ+ and μ- pair. The invariant mass of J/ψ π+ π- is shown in the figure below. The left enhancement at the mass of 3686 GeV is consistent with the QM bound state ψ' of charm and anti-charm quarks, the right one at the mass of 3872 GeV has properties that are very difficult to reconcile with the Gell-Mann Zweig QM. Possible explanations include a meson-meson molecule (DD* for experts) or multi quark anti-quark system (diquark-diantiquark tetraquark meson for experts).
this plot was made using all data taken in 2010, click in the image to get it in higher resolution, click here to see original plot.
The particle with the mass of 3872 Gev was first observed by the BELLE collaboration in 2003 and is called X(3872). The observation of this particle at this early stage of data taking by LHCb confirms the excellent performance of the LHCb detector and data analysis.
13 October 2010: LHCb control room in action
LHCb physicists discussing data taking.
click in the image to see other photos.
24 September 2010: Young scientists at LHCb
The young scientists took part in the LHCb shifts during the European researchers’ night on Friday 24 September.
click in the image to see other photos.
6 September 2010: Beautiful atoms
The LHCb has observed beautiful atoms. The atoms are bound states of the beauty quark and anti-beauty quark. The atoms are bound by the strong force, the force which also binds quarks inside proton. The beautiful atom is 10 times heavier than the proton (yes, we can create mass from energy using famous Einstein formulae E=mc2), has a size sligtly smaller than the size of the proton but about 100 000 times smaller than the size of the hydrogen atom which is composed of a proton and an electron and is bound by the electromagnetic force. Just like ordinary atoms beauty and anti-beauty quarks form different quantum states with different angular momenta and different spin orientations (see figure below right). Only the states marked 1S, 2S and 3S are observed at LHCb by detecting their decay into a μ+ and μ- pair (left).
The figures above show the invariant mass of μ+ and μ- particles (left) and the schematic view of the beautifull atom quantum states (right), click in images to get them in higher resolution. The invariant mass plot was made using all data taken in 2010, click here to see original plot.
The states 1S, 2S and 3S do not decay into Beauty Particles since their mass is lower than the sum of masses of Beauty and anti-Beauty particles (BB threshold in the figure). On the other hand the state 4S does decay. This feature is used by the experiments BABAR and BELLE producing the 4S state at e+e- colliders as a source of Beauty and anti-Beauty particles.
click in images to get them in higher resolution.
22 July 2010: From a B to Z, LHCb explores the particle alphabet
LHCb has unveiled pictures of a Z boson inside the experiment. This boson is one of the best understood of all particle species. It shows us how the forces of electricity, magnetism and radiation are connected inside the Standard Model, our theory of particle physics. Measurements of how often we see Z bosons inside LHCb will provide a sensitive test of how well our theory describes this particle at the record breaking energies of the LHC.
click in images to get them in higher resolution
In this picture the Z boson has decayed immediately to two muons μ, shown by the thick white lines which point to the green muon chamber hits in the outer circle of the Eolas display (described in 10 June 2010 News). Not much else happens inside LHCb when a Z is at work – only a few other particles are visible – and this makes it an easy particle to find. We’re looking forward to collecting more of them now, and really testing how well the Standard Model performs for us.
12 July 2010: LHCb is younger!
The average age of the LHCb Collaboration members has strongly decreased after arrival of the LHCb summer students seen below in front of the Globe of Science and Innovation. The Summer Students follow the lectures given at the first floor of the Globe in the morning. During the breaks they can visit the new CERN exhibition "Universe of Particles" located on the ground floor and the rest of the time they make an important contribution to the LHCb data taking and data analysis.
click in image to get it in higher resolution
10 June 2010: The W boson in action
LHCb has taken its first snapshots of the W boson in action. This particle conveys the weak force, which makes certain forms of radioactivity possible. It is shown here having decayed to a muon μ (shown as a straight white line, pointing to the filled green muon detector hit circles in the 2D picture, and as a red line pointing to blue muon hits in the 3D picture), which we see, and a neutrino ν, which we don't, with very little else around it.
click in images to get them in higher resolution
Eolas (gaelic for 'knowledge') is a 2D view of a collision inside LHCb. It is a transmogrified view, chosen to illustrate where particles deposit energy as they fly outwards from the collision point. The radius represents flight through the detector along the beam direction - through the tracking detectors, then the first muon chamber, then the electromagnetic and hadronic calorimeters, and finally the last four muon chambers. The φ angle represents the angle in the x,y direction perpendicular to the beam. Information is colour coded. Particle tracks are shown by the dashed lines. The transverse momentum of the particle is shown by the solid white long along this path - the higher this is, the longer the solid white bar is. Yellow bars show energy deposited in the electromagnetic calorimeter, cyan energy deposited in the hadronic calorimeter. Deposits in muon chambers are illustrated by green circles. If these are filled, they are associated with a particle track passing through them.
We will use samples of W bosons to test our theory of particle physics, the Standard Model, to high precision. This is exciting because we don't know yet if our theory holds at LHC energies - if it doesn't, if there are new particles to find in nature, we'll see W bosons behaving in a way we don't expect. With these first snapshots taken, we're on our way to finding out.
7 May 2010: Strange Beauty and Charm
LHCb has reconstructed an event having all characteristics of a Strange Beauty Particle decay! A computer view of this event is shown below. The Strange Beauty Particle (called Bs) is composed of an anti-quark b (b is for beauty) and a quark s (s is for strange). It is produced by the collision of two 3.5 TeV protons from the LHC at a location marked as "PV" (Primary Vertex), together with many other particles (not shown). The Bs decays after travelling about 1.5 mm into three particles called μ-, Ds+ and neutrino ν at a place marked "SV" (Secondary Vertex). The ν is not detected since it can even traverse the whole Earth without any interaction. The Charm Particle Ds+ is composed of a c quark (c is for charm) and anti-quark s. The Ds+ particle decays in turn after travelling 6.5 mm into three long lived particles K+, K- and π+ in a place called "TV" (Tertiary Vertex). The K+, K- and π+ are traversing the LHCb detector where the tracking system is used to reconstruct their trajectories with such a very high precision that it is clear that the particles come from three different places called vertices.
click in image to get it in higher resolution
21 April 2010: First reconstructed Beauty Particle
LHCb has reconstructed its first Beauty Particle! You can see below a computer view of this event in two projections (images on the left hand side). The Beauty Particle (called B+) is composed of an anti-quark b (that has a very short lifetime of 1.5 thousandth of a nanosecond!) and a quark u. It is produced by the collision of two very high energy protons from the LHC at a location marked as "Primary vertex", together with many other particles (shown in black). The B+ decays after travelling about 2mm into two particles (called J/ψ and K+) at a place marked "B decay vertex". The J/ψ particle decays in turn immediately into two long lived particles called μ+ and μ-. The μ+ , μ- and K+ are traversing the LHCb detector where the tracking system is used to reconstruct their trajectories with such a very high precision, that it is clear they do not come from the primary vertex. The fact that the reconstructed tracks do not cross exactly in two points reflects experimental precision of computer reconstruction. The real particle tracks originate at the two vertices. The images on the right hand side show the same event when the tracks from the "Primary vertex" are forced to come from the "Primary vertex".
click in images to get them in higher resolution
The LHCb physicists have collected about 10 million proton-proton collisions in order to find this first Beauty Particle. The reconstruction of each event is not easy, there are about 100 particle tracks reconstructed in this event, see full event display below.
More details: LHCb physicists have calculated invariant mass of μ+ and μ- particles from the "B decay vertex" and found that it correspond to the J/ψ mass, see below invariant mass distribution of all μ+ and μ- pairs with the peak corresponding to the J/ψ decays. The reconstructed invariant mass of J/ψ and K+ is 5.32 GeV, in agreement with to the known B+ mass, 5.5 times higher than the colliding proton mass but 650 times smaller than the colliding proton energy (yes, we can create mass from energy using famous Einstein formulae E=mc2).
Both proton beams made a full turn of LHC on Feb. 28th. A new period of great measurements with LHCb has started again and will continue for 18-24 months. On March 18th both beams have been accelerated to 3.5 TeV,
see TV footage with preparation for collisions and photo above taken just after the first collisions at 12:59 on March 30 (click on picture to get it in higher resolution).
On March 8th, during during International Women's Day, many many women have been present in the LHCb control roon, ...
Below you may find interesting computer reconstructed events observed with the LHCb detector during the LHC restart period in November and December 2009. Click on picture to see it in higher resolution version.
Protons have ended to circulate at LHC on December 16th.
1.2 TeV collisions at LHCb
On December 14th 1.2 TeV proton beams have collided at LHCb during LHC machine studies. Many LHCb subdetectors, except for sensitive silicon detectors, recorded the world's highest energy pp collisions. Other events can be found here.
proton interactions with gas
Position of proton interactions inside the vertex detector with residual gas are shown in blue or red, proton-proton interactions in green (more details).
So called "strange particles" are produced in the proton-proton collisions and decay inside LHCb detector into two other particles reconstructed as red tracks (more details).
High multiplicity events
A high multiplicity event with three muon tracks (green) recorded on December 12th. Other events can be found here.
More proton-proton collisions
On December 8th many long tracks were reconstructed using the detectors along the whole length of the LHCb. Collision vertex is clearly observed (bottom left). The tracks are curved in the magnetic field allowing measurement of the track momentum (top left). Other events can be found here.
LHCb RICH detectors are used to identify particles. The circles show possible position of measured points for different kinds of particles traversing the detector. The measured points clearly choose one possibility for every circle and in this way allow to identify particles.
First proton-proton collisions
A proton-proton collision candidate event. On November 23 protons from two beams circulated at LHC and have collided at LHCb.
LHC news video youTube
First proton-proton collisions
Tracks originate from the expected region inside LHCb Vertex Locator detector VELO.
Animation (click on picture): pp collisions and proton beam gas collisions recorded on Nov. 23, 2009. Other reconstructed events can be found here.
First reconstructed pi0's
pi0 is the short lived particle decaying into two photons which were measured in the electromagnetic calorimeter ECAL. The plot above shows the nearly perfectly reconstructed pi0 mass.
First proton interactions
... reconstructed Vertex Locator VELO track. Other reconstructed events can be found here.
First proton interactions
... not yet proton-proton interaction but interactions of the protons with residual gas inside LHC ring. Tracks reconstructed during data taking.
A splash from the LHC beam
LHC has restarted on November 21st. Two LHC beams have made a full turn of the LHC. Afterwards, after synchronization with the LHC accelerating system (RF capture in technical language), the beams made few hundred turns. During LHC operation LHCb has recoded splash events. The movie (click on picture) shows what LHCb detector has recorded every 25ns (1/(40 000 000) s) for a particular splash event; see individual events here.
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