B flavour tagging with leptons and measurement of the CP violation phase phi_s in the B_s^0 -> J/psi phi decay at the CMS experiment

This thesis presents the development and optimization of an algorithm used to determine the flavour at production time of neutral B^0 and B_s^0 mesons. The flavour tagging algorithm developed in this thesis exploits muons and electrons produced in the semileptonic decay of the additional b-hadron produced in pp -> bbX collisions at the LHC. The charge of the lepton is used to infer the flavour of the neutral b-meson. Three simulated samples of B_s^0 -> J/psi phi, B^+ -> J/psi K^+ and B^0 -> J/psi K* decays are exploited to develop and test the algorithm. Two independent neural networks are defined for muons and electrons, trained on 24 000 and 20 400 simulated B_s^0 -> J/psi phi events respectively, to parametrize the probability of wrong flavour tag omega of the algorithm. The tagging performances are further measured and calibrated on a sample of self-tagging B^+ -> J/psi K^+ decays collected by the CMS experiment during 2012, corresponding to 20 fb^-1 . The charge of the kaon univocally determines the flavour of the B^+ at production time and allows the direct measure of the mis-identification probability. A tagging power P_tag = epsilon_tag (1 - 2 omega )^2 of 0.833 +/- 0.024 (stat.) +/- 0.012 (syst.) % is measured using muons and 0.483 +/- 0.020 (stat.) +/- 0.003 (syst.) % using electrons. Combining the two algorithms results in the overall tagging power of 1.307 ± 0.031 (stat.) ± 0.007 (syst.) %. The combined lepton flavour tagging algorithm is used in the measurement of the charge-parity (CP) violation parameters phi_s and DeltaGamma_s , sensitive to potential new physics processes not included in the standard model description. A time-dependent and flavour-tagged full angular analysis of the mu^+ mu^- K^+ K^- final state of the B_s^0 -> J/psi phi decay is performed based on the 2012 CMS dataset. A total of 49 000 reconstructed B_s^0 decays are used to extract the weak phase phi_s and decay with difference DeltaGamma_s values: phi_s = -0.075 +/- 0.097 (stat.) +/- 0.031 (syst.) rad DeltaGamma_s = 0.095 +/- 0.013 (stat.) +/- 0.007 (syst.) ps^-