The LHCb experiment at the Large Hadron Collider (LHC) at CERN is designed to search for indirect evidence of physics beyond the Standard Model (SM) in CP violation and rare decays of beauty and charm hadrons. One of the key measurements is the CP violating phase Φₛ, that arises from the interference between the decay of B⁰ₛ mesons into CP eigenstates directly and via B⁰ₛ- ̅B̅⁰ₛ mixing In this thesis, the phase Φₛ is measured in the B⁰ₛ → J/ψK⁺ K⁻ decay channel using 1.9 fb−¹ of proton-proton collisions collected at a centre-of-mass energy of √s = 13 TeV. Other parameters that are measured are the average decay width of the B⁰ₛ meson with respect to that of the B⁰d meson, Γₛ − Γd, the difference in decay widths of the heavy and light mass eigenstates of the B⁰ₛ- ̅B̅⁰ₛ system, ∆Γₛ, and a further parameter that describes the CP violation in interference between mixing and decay, |λ| .The following results are obtained: Φₛ = -0:080 ± 0:041 ± 0:006 rad Γₛ − Γd = 0.0041 ± 0:0024 ± 0:0015 ps⁻¹ ∆Γₛ = 0:0764 ± 0:0077 ± 0:0026 ps⁻¹ |λ| = 1:014 ± 0:016 ± 0:006 These are 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 Φₛ yields the most precise measurements to date: Φₛ = -0.042 ± 0.025 rad Γₛ = 0.6566 ± 0.025 ps⁻¹ ∆Γₛ = 0:0811 ± 0.0048 ps⁻¹ |λ| = 0:993 ± 0:010.