First measurement of $C\!P$ violation in $B^0_s\!\rightarrow J\!/\psi K^0_{\text{S}}$ decays with the LHCb experiment

This thesis presents the first measurement of $C\!P$ violation in decays of $B^0_s$ and $\overline{B^0_s}$ mesons into the final state $J\!/\psi K^0_{\text{S}}$, where the $C\!P$ observables allow to constrain the contribution of penguin topologies in $B^0\!\rightarrow J\!/\psi K^0_{\text{S}}$ decays. The measurement is performed on a dataset corresponding to an integrated luminosity of 3 $\text{fb}^{-1}$ of proton-proton collisions, which were recorded by the LHCb experiment at centre-of-mass energies of 7 and 8 TeV. Using an unbinned maximum-likelihood fit, the $C\!P$ observables are measured as \begin{equation*} \begin{alignedat}{2} A_{B^0_s\!\rightarrow J\!/\psi K^0_{\text{S}}}^{\Delta\mathit{\Gamma}} &= \phantom{-}0.49 \pm \,{}^{0.77}_{0.65} &&\text{\,(stat)} \pm 0.06 \text{\,(syst)} \,\text{,}\\ C_{B^0_s\!\rightarrow J\!/\psi K^0_{\text{S}}} &= -0.28 \pm 0.41 &&\text{\,(stat)} \pm 0.08 \text{\,(syst)} \,\text{,}\\ S_{B^0_s\!\rightarrow J\!/\psi K^0_{\text{S}}} &= -0.08 \pm 0.40 &&\text{\,(stat)} \pm 0.08 \text{\,(syst)} \,\text{.} \end{alignedat} \end{equation*} Furthermore, the ratio of branching fractions $\mathcal{B}\left(B^0_s\!\rightarrow J\!/\psi K^0_{\text{S}}\right)/\mathcal{B}\left(B^0\!\rightarrow J\!/\psi K^0_{\text{S}}\right)$ is determined as \begin{equation*} 0.0431 \pm 0.0017 \text{\,(stat)} \pm 0.0012 \text{\,(syst)} \pm 0.0025 \,(f_{s}/f_{d}) \,\text{,} \end{equation*} where the last uncertainty is due to the limited knowledge of the ratio $f_{s}/f_{d}$ of $B^0_s$ to $B^0$ meson hadronisation fractions.