Measurement of the $CP$ violating phase $\phi_{s}$ using $B^{0}_{s} \rightarrow J/\psi K^{+}K^{-}$ decays at the LHCb Experiment

The LHCb experiment at the Large Hadron Collider (LHC) at CERN is designed to search for indirect evidence of physics beyond the Standard Model in $CP$ violation and rare decays of beauty and charm hadrons. One of the key measurements is the $CP$ violating phase $\phi_s$, that arises from the interference between the decay of $B^0_s$ mesons into $CP$ eigenstates directly and via $B^0_s - \bar{B^0_s}$ mixing. In this thesis, the phase $\phi_s$ is measured in the $B^{0}_{s} \rightarrow J/ \psi K^{+}K^{-}$ decay channel using 1.9 fb$^{-1}$ of proton-proton collisions collected by the LHCb experiment in the years 2015 and 2016 at a centre-of-mass energy of $\sqrt{s} = 13$ TeV. The following results is obtained: $\phi_s = -0.080 \pm 0.041 \pm 0.006$ rad. This is the most precise measurements using a single decay channel. Combining this analysis with other decay channels measured at the LHCb experiment that are sensitive to $\phi_s$ yields the most precise measurements to date: $\phi_s = -0.042 \pm 0.025$ rad.