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selected physics results: φ_s, B⁰_s →μμ, B⁰_d →K^*μμ, ΔA_CP

16 May 2012: First observation of two excited states of Λ_b.

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 Λ_c or a Λ_b particle.

The three quarks forming the Λ, Λ_c and Λ_b 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 Λ_c particles. They were, however, never observed for the Λ_b particle.

The LHCb collaboration has made first observations of two excited states of Λ_b.

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The Λ_b excited states have been reconstructed in three steps. In the first step the Λ_c⁺ particles were reconstructed through their decay into a proton p, a negative K^- meson and a positive π⁺ meson. In the second step the Λ_c particles were combined with negative π^- mesons in order to form the Λ_b particles. The Λ_b signal is clearly seen as the enhancement in the left image above showing the Λ_c⁺π^- invariant mass spectrum. Finally the Λ_b 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 Λ_b excited states with masses of 5912 and 5920 MeV, about 6 times the proton mass.

LHCb physicists have observed about 16 Λ_b(5912)→Λ_bπ⁺π^- decays (4.9σ significance) and about 50 Λ_b(5920)→Λ_bπ⁺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.

Read more in CERN Bulletin article and LHCb publication.

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.

Measurements of the production of W and Z bosons, see 10 June and 22 July 2010 news, allowed LHCb physicists to explore electroweak physics, see CERN Courier article.

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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 B_s and B_d 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 φ_s, which sets the scale for the difference between properties of matter and antimatter for the strange beauty B_s 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, m_1/2, 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 B_s →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 m_1/2 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): B⁰_d →K^*μμ, a first measurement of the zero-crossing point.

[ Zero crossing point B⁰_d →K^*μμ = 4.9^+1.1_-1.3 GeV² ]

LHCb physicists have continued their search for physics beyond the Standard Model using the B⁰_d 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 (A_FB) can both be affected in many new physics scenarios. The variable A_FB 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 A_FB 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 A_FB 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 B⁰_s →K⁺K^-.

In a fascinating world of quantum mechanics the strange beauty particle (matter) B⁰_s 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 B⁰_d decay into π⁺π^- pair and the right figure shows the asymmetry in the B⁰_s decay into K⁺K^- pair. It can be clearly seen, by comparing time scales of both figures, that the B⁰_d mesons oscillate much slower than the B⁰_s 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 B⁰_d → π⁺π^- decay are A^dir_π⁺π^- = 0.11 ± 0.21 ± 0.03 and A^mix_π⁺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 B⁰_s → K⁺K^- decay - which are measured for the first time ever - are A^dir_K⁺K^- = 0.02 ± 0.18 ± 0.04 and A^mix_K⁺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 B⁰_s → 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 B⁰_s →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 B_s 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 B_s 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 B_s 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 B_s 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.

Read more in CERN Press Release CERN Bulletin article, in the CERN Quantum Diaries blog and also in the LHCb staff page.

5 March 2012 (2): Heavier strange-beauty lives longer and improved φ_s measurements.

[ φ_s = -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 B_s mesons. The new results improve on those presented at the summer 2011 conferences. The size of this difference is controlled by the parameter φ_s, 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 B_s matter antimatter system is alternatively described as a heavy and light mass B_s meson system. The mass difference Δm_s determinates the matter antimatter oscillation frequency, see 15 march 2011 news. The difference of their lifetimes was measured together with the value of φ_s. Since it was not previously know if the heavier or lighter B_s mesons live longer, LHCb physicists have obtained two possible values for φ_s corresponding to two blue regions in the image above. (The axis label ΔΓ_s corresponds to the difference of inverse lifetimes between the heavy and light B_s meson, physicists call it width (Γ) difference.) Recently LHCb physicists have succeeded to measure that the heavier strange-beauty B_s 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
φ_s = -0.001 ± 0.101 ± 0.027 rad using B_s decays into into a J/ψ meson and a φ meson; combining it with the measurement of the B_s decay into J/ψ and f₀(980) LHCb physicists have obtained the final result φ_s = -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.

Read more in CERN Press Release CERN Bulletin article, in the CERN Courier article, in the CERN Quantum Diaries blog and also in the LHCb staff page.

5 March 2012 (3): First evidence for CP violation in the decays of B_s 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 B⁰_s, 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 B⁰_s 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 B⁰_d mesons are different, as known from previous experiments. The lower two plots show that the difference is also visible for the B⁰_s mesons. The measured CP asymmetry for the B⁰_d mesons A_CP = -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 B⁰_s mesons A_CP = 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.

Read more in CERN Press Release CERN Bulletin article, in the CERN Quantum Diaries blog and also in the LHCb staff page.

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.

[ ΔA_CP = (-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 A_CP) 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, ΔA_CP, 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:

ΔA_CP = (-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.

Read more in CERN Bulletin article in English and French, in the CERN Courier, in the CERN Quantum Diaries blog in English and French and also in the LHCb staff page.

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.”

Read more in CERN Press release in English and French.

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.

Read more in CERN Bulletin article in English and French.

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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: φ_s: different properties of matter and antimatter for B_s mesons

[ φ_s = 0.03 ± 0.16 ± 0.07 ]

LHCb physicists have presented today the most precise measurement of φ_s (the B_s mixing phase, for experts) at the Lepton Photon conference in Mumbai (India). The value of φ_s is precisely predicted in the Standard Model and sets the scale for the difference between properties of matter and antimatter for B_s 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 B⁰_s, 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 B_s 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 φ_s measurement in a plane together with the correlated measurement of another value, ΔΓ_s. The results of the measurement favour two regions, one of which is located around φ_s = -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 B_s decay into J/ψ f₀(980) - see 27 February news. This decay contributed to the φ_s measurement.

Using both B_s decays LHCb physicists have obtained the value φ_s = 0.03 ± 0.16 ± 0.07.

Read also CERN Press Release in English and French.

Read also CERN Bulletin article in English and French.

Read also CERN Courier article.

22 July 2011 (1): Hunting for New Physics continues

[ Branching ratio B⁰_s →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 B⁰_s, 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 B⁰_s 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 B⁰_s decays about 1 cm from the proton-proton collision point into two muons (purple tracks). The invariant mass of muon pair corresponds to the B⁰_s mass. The number of observed B⁰_s 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 B⁰_d (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 (A_FB) can both be affected in many new physics scenarios. The variable A_FB indicates whether more or fewer muons of one sign are observed in the forward direction than in the backward direction. The distribution of A_FB 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 B⁰_d (matter, left plot) and anti-B⁰_d (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 B⁰_s 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.

Read more details at the LHCb Collaboration Web page and in the CERN Bulletin article in English and in French.

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 B⁰_s, 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 B⁰_s 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.

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LHCb physicists expect to observe the B⁰_s 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) B⁰_s 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 B⁰_s particles have been identified through their decay into strange charm D_s particles (composed of a charm quark c bound with a strange s antiquark) and one or three πs. Of course, LHCb observes B⁰_s 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 A_mix is proportional to the difference between the number of events in which the produced matter(antimatter) B⁰_s particle had the same identity during its decay, and the number of events in which it had not, as a function of its lifetime.

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The B⁰_s 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 B⁰_s 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 B⁰_s mesons for the first time. The decay B⁰_s → J/ψ f₀(980) will be important for studying differences between properties of matter and anti-matter (CP violation for experts) in the B⁰_s system, while the decay B⁰_s → D^*–_s2Xμ⁺ν will be valuable for testing predictions of strong interaction (QCD) theory.

The first new decay mode observed is of the decay B⁰_s → J/ψ f₀(980). The B⁰_s 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).

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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.

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13 October 2010: LHCb control room in action

LHCb physicists discussing data taking.

click in the image to see other photos.

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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.

Read CERN Bulletin article in English and French, as well as the one in the Symmetry Breaking.

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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 beauty-anti-beauty atom, called "Upsilon" was discovered in 1977 at the proton-antiproton collider at Fermilab near Chicago.

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.

The charm and anti_charm quarks form two bound atom states 1S and 2S called J/ψ and ψ' observed at LHCb through their decay into a μ⁺ and μ^- pair (left) and a e⁺ and e^- pair (right).

click in images to get them in higher resolution.

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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.

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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

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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.

The W boson was discovered in 1983 at CERN by the UA1 and UA2 experiments giving the Nobel Prize to Carlo Rubbia and Simon van der Meer.

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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 B_s) 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 B_s decays after travelling about 1.5 mm into three particles called μ^-, D_s⁺ 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 D_s⁺ is composed of a c quark (c is for charm) and anti-quark s. The D_s⁺ 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

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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).

see comments in articles: NewScientist internet, magazine and ZDNet.