Measurement of CP-violation in decays of strange beauty mesons

The Standard Model (SM) describes fundamental particles and interactions between them, except gravity. However, it lacks explanations for several phenomena, including the origin of particle masses, dark matter, and the predominance of matter over antimatter in the Universe. Sakharov's conditions, which summarize the prerequisites for generating a matter-dominated Universe, require the presence of a violation of CP-symmetry. In principle, CP-violation is present in the SM within the weak interaction. However, the observed matter-antimatter asymmetry significantly exceeds the asymmetry generated within the SM. Understanding the dominance of matter over antimatter in the Universe requires further searches for extra contributions to the CP-violating phenomena. The Bs → J/ψφ is a crucial decay mode for such investigations, strongly sensitive to new CP-violating processes via the CP-violating phase present in the decay. The discrepancy between the measured and predicted CP-violating phase in this decay would indicate the existence of new CP-violating processes in the Universe. This thesis presents the world's most precise measurement of the amount of CP-violation in Bs → J/ψφ decays up to date. The measurement relies on the simultaneous observation of the decay time and the angles of decay products. Therefore, an accurate description of the decay time and angular-dependent detector inefficiencies and biases is necessary. The precision was achieved by collecting a high-purity dataset of 5.7 inverse fb in the LHCb experiment and improving the measurement methodology. The CP-violating phase is measured to be -0.038 +/- 0.021 (stat.) +/- 0.006 (syst.) mrad. The uncertainties are large, and the value is compatible with the SM prediction within these uncertainties. The main limitation of the precision is the size of the collected dataset. Therefore, more data is necessary to probe the extra CP-violating resources within Bs → J/ψφ decays.