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Abstract

This thesis presents a study of the decay $B^0_s\rightarrow J/\psi\phi$ with the LHCb experiment. Due to the mixing between $B^0_s$ and $\bar{B}^0_s$ mesons and a final state that is accessible for both species, this decay is sensitive to CP violation originating from the interference between the mixing and the decay process. The CP violating is parametrized by the weak phase difference $\phi_s$, which can be precisely constrained within the Standard Model of particle physics based on other measurements. Thus, the measurement of this phase difference constitutes an interesting test for possible contributions of physics phenomena from beyond the Standard Model. A flavour-tagged analysis of the time-dependent decay rates of $B^0_s$ and $\bar{B}^0_s$ mesons is performed. In addition, an angular analysis is needed to disentangle the different CP components of the final state. The analysis is based on a proton-proton collision data set collected in 2015 and 2016 by the LHCb experiment, corresponding to an integrated luminosity of $1.9\,$fb$^{-1}$. The obtained value for the CP violating phase difference is $\phi_s=(−0.083 \pm 0.041_{\rm stat} \pm 0.006_{\rm syst})\,$rad, which, combined with a previous analysis of this channel by the LHCb experiment, constitutes the most precise single channel and single experiment measurement of this quantity. No significant deviation from the Standard Model expectation is observed.

Besides the measurement of $\phi_s$, the main other determined parameters are the decay width and decay-width splitting of the $B^0_s$ meson system. In contrast to previous analyses of this channel, the decay width is directly measured with respect to decay width of the $B^0$ meson.