LHCb results on rare leptonic decays of B-mesons

Processes where B meson decays into several leptons in the final state are very powerful sources of new knowledge due to it sensitivity to potential contributions from new mediators and other new physics beyond the Standartd Model (SM). The key features of the decays are: • Their time-integrated branching fractions are predicted in the SM with small uncertainty due to absence of hadrons in final state • The decays are sensitive probes for physics beyond the SM • Within the Standard Model (SM) of particle physics, fully leptonic decays of B mesons are very rare


Introduction
Processes where B meson decays into several leptons in the final state are very powerful sources of new knowledge due to it sensitivity to potential contributions from new mediators and other new physics beyond the Standartd Model (SM). The key features of the decays are: • Their time-integrated branching fractions are predicted in the SM with small uncertainty due to absence of hadrons in final state • The decays are sensitive probes for physics beyond the SM • Within the Standard Model (SM) of particle physics, fully leptonic decays of B mesons are very rare The search for the rare leptonic decays of B-mesons was performed on the data obtained on LHCb detector.The LHCb detector is a single-arm forward spectrometer covering the pseudorapidity range 2 < η < 5, designed for the study of particles containing b or c quarks. The tracking system provides a measurement of the momentum of charged particles with a relative uncertainty that varies from 0.5% at low momentum to 1.0% at 200GeV/c. Different types of charged hadrons are distinguished using information from two ring-imaging Cherenkov detectors. Photons, electrons and hadrons are identified by a calorimeter system consisting of scintillating-pad and preshower detectors, an electromagnetic and a hadronic calorimeter. Muons are identified by a system composed of alternating layers of iron and multiwire proportional chambers [6] [7].
LHCb data can be divided into 2 different parts, corresponding to the LHC Run1 and Run2, see figure 1. Those parts have a different kinematics and usually can not be processed in exactly the same way. So it's a common situation, when only part of the data is processed in the analysis.

Search for the decays B
The analysis is performed with proton-proton collision data corresponding to integrated luminosities of 1.0 f b −1 and 2.0 f b −1 recorded with the LHCb detector at centre-of-mass energies of 7 and 8 TeV, respectively. The τ leptons are reconstructed through the decay τ → π − π + π − ν τ [2]. B 0 s and B 0 mesons can not be discerned due to the ν in the final state, which can not be detected by the LHCb detector. Analysis strategy is built on segregating events into 3 different groups according to the pions position on the figure 4.
Assuming no contribution from . Two-dimensional distribution of the invariant masses m π + π − of the two oppositely charged two-pion combinations for simulated B 0 s → τ + τ − candidates. The distribution is symmetric by construction. The vertical and horizontal lines illustrate the sector boundaries [2]. CL [10]. These results correspond to the first direct limit on B(B 0 s → τ + τ − ) and the world's best limit on B(B 0 → τ + τ − ).

Search for the lepton-flavour violating decays
A search for the lepton-flavour violating (LFV) decays B 0 (s) → e ± µ ∓ was performed using pp collision data collected at centre-of-mass energies of 7 and 8TeV, corresponding to a total integrated luminosity of 3fb −1 [3]. Two normalisation channels were used: the B 0 → K + π − decay which has a similar topology to that of the signal, and the B + → J/Ψ K + decay, with J/Ψ → µ + µ − ,which has an abundant yield and a similar purity and trigger selection [3].
The result fit was performed independently for the different BDT bins. Also 2 different approaches were taken into account: the direct one and the one where the momentum of the final state particles was corrected using the information about bremsstrahlung photons.

Search for the lepton-flavour-violating decays
A search for B 0 s → τ ± µ ∓ and B 0 → τ ± µ ∓ decays is performed using data corresponding to an integrated luminosity of 3 fb −1 of proton-proton collisions, recorded with the LHCb detector in 2011 and 2012 [4]. The τ lepton is reconstructed in the τ → π − π + π − ν τ channel. Again, B 0 s and B 0 mesons can not be discerned due to the ν in the final state, which can not be detected by the LHCb detector.  Figure 5. Distributions of the invariant mass of the B 0 (s) → e ± µ ∓ candidates, m e ± µ ∓ , divided into bins of BDT response and two bremsstrahlung categories (left) without and (right) with bremsstrahlung photons recovered. The result of the fit is overlaid and the different components are detailed. The edges of the range that was examined only after finalising the selection and fit procedure are delimited by gray dashed vertical lines. This region includes 90% of the potential signal candidates. Given the result obtained from the fit, the B 0 → e ± µ ∓ component is not visible in the plots [3]. [4]. These are the first limit on B(B 0 s → τ ± µ ± ) and the world's best limit on B(B 0 → τ ± µ ± ).

Search for the rare decay
A search for the rare leptonic decay B + → µ + µ − µ + ν µ is performed using proton-proton collision data corresponding to an integrated luminosity of 4.7 fb −1 collected by the LHCb experiment. The search is carried out in the region where the lowest of the two µ + µ − mass combinations is below 980 MeV/c 2 . The branching fraction of a B + → µ + µ − µ + ν µ signal is obtained by normalising to the B + → J/Ψ(→ µ + µ − )K + decays [5].
No signal is observed for the B + → µ + µ − µ + ν µ decay, see figure µ + µ − mass combinations is below 980 MeV/c 2 . The limit for the full kinematic region stays the same under the assumption that the decay is dominated by intermediate vector mesons [5].

Conclusions
Lot of rare B mesons decays were studied by the LHCb team. All results are consistent with the Standard Model. Nearly all results presented are either unique or the most accurate for the time. The data points are shown as black points with the total fit overlaid as a red solid line [5].