Measurement of the CP-violating phase $\phi$$_{s}$ in muon-tagged B$_{s}^{0}$ $\rightarrow$ J$/psi$ $\phi$ decays with the CMS experiment

This dissertation presents a study of the B$_{s}^{0}$ → J/ψφ decay performed with the CMS experiment and aimed at measuring the CP-violating phase φs. This weak phase parametrizes the CP violation in the interference between direct B$_{s}^{0}$ meson decays to a CP eigenstate via b → c$\bar{c}$s transitions and decays through mixing to the same final state. The Standard Model of particle physics relates φ$_{s}$ to the elements of the Cabibbo–Kobayashi–Maskawa matrix, which are precisely constrained by global fits to experimental data. Due to its clean theoretical prediction, the CP-violating phase φs is extremely sensitive to new-physics effects that can modify its value via the contribution of non-SM particles to the B$_{s}^{0}$–$\bar{B}$$_{s}^{0}$ mixing. Thus, the precise measurement of this phase is a powerful test to either establish physics phenomena beyond the Standard model or put stringent constraints on them. Moreover, by analyzing the B$_{s}^{0}$ →J/ψφ decay rate, several other interesting physics parameters can be measured, such as the decay width difference ∆Γ$_{s}$ between the light and heavy B$_{s}^{0}$ mass eigenstates, the average decay width Γ$_{s}$, the mass splitting ∆m$_{s}$, and the CP observable |λ| that describes the amount of direct CP violation in the process. Therefore, this measurement consists also of a comprehensive test of the phenomenology of the B$_{s}^{0}$ system. To measure the physics parameters, a time-dependent and flavor-tagged angular analysis of the B$_{s}^{0}$ → J/ψφ decay rate is performed. The measurement is based on a sample of 48500 reconstructed B$_{s}^{0}$ → J/ψφ → μ+μ− K+K− events collected in proton-proton collisions at $\sqrt{s}$ = 13 TeV, corresponding to an integrated luminosity of $\imath$$_{int}$ = 96.4fb$^{−1}$. Compared to the previous CMS measurement at $\sqrt{s}$ = 8 TeV, this analysis benefits from the increase in the center-of-mass energy from 8 to 13 TeV that nearly doubles the B$_{s}^{0}$ production cross section. Events are collected with a trigger that requires a J/ψ → μ+μ− candidate plus an additional third muon, which is exploited to infer the flavor of the B$_{s}^{0}$ meson at production time. A novel opposite-side muon tagger based on calibrated deep neural networks has been developed to maximize the analysis sensitivity. A high tagging power of ≈10%, corresponding to a tagging efficiency of ≈50%, is achieved, aided by the requirement of an additional muon imposed at the trigger level. The CP-violating phase φs is measured to be: φs = −11±50(stat) ±10(syst) mrad, in agreement both with the Standard Model predictions and the absence of CP violation in the decay-mixing interference. These results are significantly more precise than those from the previous CMS measurement at 8 TeV. This analysis also represents the first CMS measurement of the mass splitting ∆m$_{s}$ and the direct CP observable |λ|. The results are further combined with those obtained by CMS at $\sqrt{s}$ = 8 TeV, yielding φs = −21±44(stat) ±10(syst) mrad.