Precision measurement of $CP$ violation in $B_s^0 \to J/ψ K^+K^-$ decays

The time-dependent CP asymmetry in Bs0→J/ψK+K- decays is measured using pp collision data, corresponding to an integrated luminosity of 3.0fb-1, collected with the LHCb detector at center-of-mass energies of 7 and 8 TeV. In a sample of 96000 Bs0→J/ψK+K- decays, the CP-violating phase φs is measured, as well as the decay widths ΓL and ΓH of the light and heavy mass eigenstates of the Bs0-B¯s0 system. The values obtained are φs=-0.058±0.049±0.006rad, Γs≡(ΓL+ΓH)/2=0.6603±0.0027±0.0015ps-1, and ΔΓs≡ΓL-ΓH=0.0805±0.0091±0.0032ps-1, where the first uncertainty is statistical and the second, systematic. These are the most precise single measurements of those quantities to date. A combined analysis with Bs0→J/ψπ+π- decays gives φs=-0.010±0.039rad. All measurements are in agreement with the standard model predictions. For the first time, the phase φs is measured independently for each polarization state of the K+K- system and shows no evidence for polarization dependence.