Study of the decays ... and first observation of the ... decay at the LHCb experiment

Resumo

The Standard Model of Particle Physics (SM) is the theory which better explains the behaviour of the elementary particles and their interactions. Among the phenomena that the SM cannot explain, one of the most interesting is the matter-antimatter asymmetry observed in the universe. In order to produce matter and antimatter at different rates, CP-symmetry violation (violation of the charge and parity symmetries) is a necessary condition. The amount of CP-violation predicted by the SM is not enough to explain the large matter-antimater imbalance. Loop-mediated penguin decays of B-mesons are among the most interesting processes to look for new physics not included in the SM. Flavour-changing neutral currents (FCNC) are very sensitive to deviations from the SM induced by NP particles circulating in the loop. Three FCNC decays are studied in this thesis: B0 -> Kst Kstb, B0s -> Kst Kstb and B0s → Phi Kstb. These decays are of the type B0(s) -> V V', where V and V' are light vector mesons, and an amplitude analysis is therefore needed to obtain the different polarisation fractions. The LHCb experiment, one of the four detectors at the LHC collider, was designed to study rare decays and CP-violation in b-hadron decays, with the hope of revealing physics beyond the SM. In this thesis, an study of the B0 -> Kst Kstb and B0s -> Kst Kstb decays is presented with 3 fb^{-1} of data taken by the LHCb experiment, 1 fb^{-1} in 2011 at sqrt(s) = 7 TeV and 2 fb^{-1} in 2012 at sqrt(s) = 8 TeV. The measurement of the polarisation fractions of these decays is performed with flavour-untagged and time-integrated amplitude analyses in a mass window of ±150 MeV/c^2 around the Kst mass. In addition to the three helicity angles, the two K± π∓ invariant masses are also included. Only the P- and S-wave contributions are considered. For the former decay, B0 -> Kst Kstb , it is the first study at LHCb. The large longitudinal polarisation obtained, fL = 0.707 ± 0.053, confirms the previous measurement from the BaBar experiment. The S-wave contribution is measured for the first time, (0.425 ± 0.051). For the latter decay, B0s -> Kst Kstb , an update of an earlier LHCb analysis is presented, corroborating the low longitudinal polarisation, fL = 0.233 ± 0.030, and the large contribution of the S-wave. Moreover, the ratio of branching ratios between the two modes, BR(B0 -> Kst Kstb)/BR(B0s -> Kst Kstb), is measured. The BR of the B0 -> Kst Kstb can be obtained, BR(B0 -> Kst Kstb) = (8.6 ± 1.9 (stat)) × 10^{−7} , that is in agreement with the previous measurement by BaBar and with the theoretical predictions. In addition, a search for the B0s -> Phi Kstb decay with the data taken in 2011 is performed. This mode is the only b -> d penguin decay into vector mesons that had not previously been observed. As a result of the work presented here, the B0s -> Phi Kstb decay is observed for the first time, with a significance of 6.5σ. The branching ratio of the decay is measured, BR(B0s -> Phi Kstb) = (1.09 ± 0.24 (stat) ± 0.11 (syst) ± 0.08 (fd/fs)) × 10^{−6}, which is found to be compatible within 1σ with theoretical predictions. An angular analysis of the B0s -> Phi Kstb decay is included. The small value obtained for the longitudinal polarisation fraction, fL = 0.46 ± 0.11 (stat) ± 0.08 (syst), follows the trend of the b → s penguin decays, and the result is smaller than the predictions.