Precise measurements of CP-violating and mixing parameters of B0s and B0d systems with the LHCb detector.

In this dissertation, the analysis strategies and the preliminary results on the ongoing studies of CP-violating and mixing parameters of the B0s and B0d systems are presented. The analysed dataset corresponds to the proton-proton interactions collected by the LHCb experiment from 2015 to 2018 at a centre-of-mass energy of √s = 13 TeV/c^2, during the so-called Run 2, corresponding to an integrated luminosity of 5.9 fb−1. In particular, the determination of the CP-violating phase φs, of the decay-width difference ∆Γs and of the decay widths Γs and ΓH in the decay channels B0s → J/ψφ and B0s → J/ψπ+π− are discussed, together with the measurement of the mixing parameter ∆Γd, exploiting the decay channels B0d → J/ψK∗(892)0 and B0d → J/ψK0s. These measurements represent an important test of the Standard Model (SM), the current most complete theoretical description of elementary particles and their interactions, and a sensitive probe of physics phenomena beyond the SM. The analysis presented in this thesis, using the B0s → J/ψφ decay, allows to obtain a statistical precision on the φs measurement of 0.022 rad and thus it will be the world’s most precise single measurement of this parameter, with the possibility to observe CP violation in the interference between a decay with and without mixing. The preliminary measurement of ∆Γd using data collected in 2016 shows a statistical precision of 0.008 ps−1 and an achievable precision, with the full Run 2 dataset, of 0.004 ps−1. This final measurement will supersede the precision reached in the previous LHCb analysis and of the current world average value, ∆Γd = 0.0007 ± 0.0066 ps−1, representing the major contribution to the future world average combination.