Study of the penguin-dominated decay B0 s - K 0K 0 at LHCb

B0 s ! K 0K 0, using the first data acquired by LHCb during 2010 and 2011 from the LHC proton-proton collisions at a centre-of-mass energy p s = 7 TeV, corresponding to an integrated luminosity of 37 pb1 and 1 fb1 respectively. B0 s ! K 0K 0 is a Flavour Changing Neutral Current b ! sdd¯ process. Within the Standard Model (SM), it proceeds through a penguin transition dominated by a virtual intermediate top quark coupling to a W boson. Extensions of the SM predict additional one-loop contributions that could introduce sizeable effects on the dynamics of this transition, and produce measurable deviations from the SM predictions of its observables. The most relevant SM predictions concerning this mode have been summarised. Furthermore, the LHCb experiment has been described. LHCb is one of the experiments operating at the Large Hadron Collider (LHC) at CERN. It is dedicated to the study of CP violation and rare decays in hadrons containing b quarks. The experimental conditions during the relevant data taking period have also been explained. As a result of the work presented here, the first observation of the B0 s ! K 0K 0 decay was performed. With the data collected during 2010 a clear B0 s ! (K+p)(Kp+) signal was found with an statistical significance of more than 10 s. Normalising to the B0 d ! J/yK 0 decay channel, the branching ratio of B0 s ! K 0K 0 was measured, taking as input the polarisation fractions measured through a simplified angular analysis of the K 0 decay products. With the extended data sample collected in 2011, a more precise time integrated and untagged study was carried out of the decay B0 s ! (K+p)(Kp+). A combined angular and mass analysis was performed to asses the fraction of the different amplitudes contributing to this final state. The polarisation fractions of the B0 s ! K 0K 0 were determined, obtaining for the longitudinal component fL = 0.201 0.057(stat.) 0.040(syst.), and the S–wave contribution was measured to be large, 0.665 0.067(stat.) 0.030(syst.). The branching fraction for the vector-vector mode was also measured to be B(B0 s ! K 0K 0) = (10.6 1.8(stat.) 1.0(syst.) 0.6 ( fd/ fs)) 106, in agreement with the Standard Model prediction. Finally, a determination was performed, in a model independent way, of the 8 angular asymmetries sensitive to CP violation accessible to the untagged sample. They are four triple product asymmetries and four direct CP asymmetries that arise in the interference of the CP-even and CP-odd amplitudes contributing to the B0 s ! (K+p)(Kp+) decay. No signs of CP-violation were found, in agreement with the Standard Model expectation

Study of the kinematic dependences of Λ 0b production in pp collisions and a measurement of the Λ 0b → Λ +c π − branching fraction

The kinematic dependences of the relative production rates, fΛ0b/fd, of Λ 0b baryons and B 0 mesons are measured using Λ 0b → Λ +c π − and B¯0 →D+π− decays. The measurements use proton-proton collision data, corresponding to an integrated luminosity of 1 fb−1 at a centre-of-mass energy of 7 TeV, recorded in the forward region with the LHCb experiment. The relative production rates are observed to depend on the transverse momentum, p T, and pseudorapidity, η, of the beauty hadron, in the studied kinematic region 1.5 < p T < 40 GeV/c and 2 < η < 5. Using a previous LHCb measurement of fΛ0b/fd in semileptonic decays, the branching fraction ℬ(Λ 0b → Λ +c π −) = (4.30 ± 0.03 + 0.12− 0.11 ± 0.26 ± 0.21) × 10− 3 is obtained, where the first uncertainty is statistical, the second is systematic, the third is from the previous LHCb measurement of fΛ0b/fd and the fourth is due to the b B¯0 →D+π− branching fraction. This is the most precise measurement of a Λ 0b branching fraction to date.S

Evidence for the decay B+c→J/ψ3π+2π−

Evidence is presented for the decay B + c → J/ψ3π +2π − using proton-proton collision data, corresponding to an integrated luminosity of 3 fb−1 , collected with the LHCb detector. A signal yield of 32 ± 8 decays is found with a significance of 4.5 standard deviations. The ratio of the branching fraction of the B + c → J/ψ3π +2π − decay to that of the B+ c → J/ψπ+ decay is measured to be B (B+ c → J/ψ3π +2π −) B B + c → J/ψπ+ = 1.74 ± 0.44 ± 0.24, where the first uncertainty is statistical and the second is systematicS

Search for the decay D0→π+π−μ+μ−

A search for the D0→π+π−μ+μ− decay, where the muon pair does not originate from a resonance, is performed using proton–proton collision data corresponding to an integrated luminosity of 1.0 fb−1 recorded by the LHCb experiment at a centre-of-mass energy of 7 TeV. No signal is observed and an upper limit on the relative branching fraction with respect to the resonant decay mode D0→π+π− φ(→μ+μ− ), under the assumption of a phase-space model, is found to be B D0→π + π − μ + μ − /B D0→π + π − φ →μ + μ − < 0.96 at 90% confidence level. The upper limit on the absolute branching fraction is evaluated to be B(D0→π+π−μ+μ− ) < 5.5×10−7 at 90% confidence level. This is the most stringent to date.S

Effective lifetime measurements in the B0s→K+K−, B0→K+π−and B0s→π+K−decays

Measurements of the effective lifetimes in the B0s→K+K−, B0→K+π−and B0s→π+K−decays are presented using 1.0fb−1of ppcollision data collected at a centre-of-mass energy of 7TeV by the LHCb experiment. The analysis uses a data-driven approach to correct for the decay time acceptance. The measured effective lifetimes are τB0s→K+K−=1.407±0.016(stat)±0.007(syst)ps, τB0→K+π−=1.524±0.011(stat)±0.004(syst)ps, τB0s→π+K−=1.60±0.06(stat)±0.01(syst)ps. This is the most precise determination to date of the effective lifetime in the B0s→K+K−decay and provides constraints on contributions from physics beyond the Standard Model to the B0smixing phase and the width difference Γs.S

Measurement of the Ξ−b and Ω−b baryon lifetimes

Using a data sample of ppcollisions corresponding to an integrated luminosity of 3fb−1, the Ξ−band Ω−bbaryons are reconstructed in the Ξ−b→J/ψΞ−and Ω−b→J/ψΩ−decay modes and their lifetimes measured to be τ Ξ − b = 1.55+0.10 −0.09 (stat)±0.03 (syst) ps, τ Ω − b = 1.54+0.26 −0.21 (stat)± 0.05 (syst) ps. These are the most precise determinations to date. Both measurements are in good agreement with previous experimental results and with theoretical predictionsS

Evidence for the decay X(3872) →ψ(2S)γ

Evidence for the decay mode X(3872) →ψ(2S)γin B+→X(3872)K+decays is found with a sig-nificance of 4.4 standard deviations. The analysis is based on adata sample of proton–proton collisions, corresponding to an integrated luminosity of3fb−1, collected with the LHCb detector, at centre-of-mass energies of 7and8TeV. The ratio of the branching fraction of the X(3872) →ψ(2S)γdecay to that of the X(3872) →J/ψγdecay is measured to be B(X(3872)→ψ(2S)γ) B(X(3872)→J/ψγ) = 2.46± 0.64±0.29, where the first uncertainty is statistical and the second is systematic. The measured value does not support apureD¯ D∗molecular interpretation of the X(3872)state.S

Study of Υ production and cold nuclear matter effects in pPb collisions at √sNN = 5 TeV

Production of Υ mesons in proton-lead collisions at a nucleon-nucleon centre-of-mass energy √sNN = 5 TeV is studied with the LHCb detector. The analysis is based on a data sample corresponding to an integrated luminosity of 1.6 nb−1. The Υ mesons of transverse momenta up to 15 GeV/c are reconstructed in the dimuon decay mode. The rapidity coverage in the centre-of-mass system is 1.5 < y < 4.0 (forward region) and −5.0 < y < −2.5 (backward region). The forward-backward production ratio and the nuclear modification factor for Υ(1S) mesons are determined. The data are compatible with the predictions for a suppression of Υ(1S) production with respect to proton-proton collisions in the forward region, and an enhancement in the backward region. The suppression is found to be smaller than in the case of prompt J/ψ mesons.S

A study of CP violation in B± →DK± and B±→Dπ± decays with D→K0SK±π∓ final states

A first study of CP violation in the decay modes B±→[KS 0 K±π∓] D h±and B±→[KS 0 K∓ π±] D h±, where h labels a K or π meson and D labels a D 0 or D‾ 0 meson, is performed. The analysis uses the LHCb data set collected in pp collisions, corresponding to an integrated luminosity of 3 fb− 1. The analysis is sensitive to the CP-violating CKM phase γ through seven observables: one charge asymmetry in each of the four modes and three ratios of the charge-integrated yields. The results are consistent with measurements of γ using other decay modesS

Angular analysis of charged and neutral B → Kμ + μ − decays

The angular distributions of the rare decays B + → K + μ + μ − and B0→K0Sμ+μ− are studied with data corresponding to 3fb−1 of integrated luminosity, collected in proton-proton collisions at 7 and 8 TeV centre-of-mass energies with the LHCb detector. The angular distribution is described by two parameters, F H and the forward-backward asymmetry of the dimuon system A FB, which are determined in bins of the dimuon mass squared. The parameter F H is a measure of the contribution from (pseudo)scalar and tensor amplitudes to the decay width. The measurements of A FB and F H reported here are the most precise to date and are compatible with predictions from the Standard Model.S