First measurement of time-dependent CP violation with B0s → π+π- decays at LHCb

A measurement of the time-dependent CP-violation parameters of the rare B0s → π+π−decay is reported in this thesis. The analysis is conducted using the sample of protonproton collisions collected with the LHCb experiment during the Run 2 of the LHC at a centre-of-mass-energy √s = 13 TeV and corresponding to an integrated luminosity of 5.7 fb−1. The CP-violation parameters are measured to be Cππ = − 0.03 ± 0.26, Sππ = − 0.20 ± 0.26, A∆Γππ = − 1.34 ± 0.34, where the uncertainty is statistical and consistent with estimations from analyses of similar channels, considering the small branching fraction of the B0s → π+π− decay. The determination of the systematic uncertainty is being finalised at the moment of discussing this thesis, but the measurement, according to previous experience with similar decays, is expected to be limited by the statistics. The results are compatible with the hypothesis of no CP violation and with the Standard Model predictions. This is the very first measurement of these quantities to date.