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

Abstract

The time-dependent $CP$ asymmetry in $B_s^0 \to J/ψK^+K^-$ decays is measured using $pp$ collision data, corresponding to an integrated luminosity of $3.0$fb$^{-1}$, collected with the LHCb detector at centre-of-mass energies of $7$ and $8$TeV. In a sample of 96 000 $B_s^0 \to J/ψK^+K^-$ decays, the $CP$-violating phase $ϕ_s$ is measured, as well as the decay widths $Γ_{L}$ and $Γ_{H}$ of the light and heavy mass eigenstates of the $B_s^0-\bar{B}_s^0$ system. The values obtained are $ϕ_s = -0.058 \pm 0.049 \pm 0.006$ rad, $Γ_s \equiv (Γ_{L}+Γ_{H})/2 = 0.6603 \pm 0.0027 \pm 0.0015$ps$^{-1}$, and$ΔΓ_s \equiv Γ_{L} - Γ_{H} = 0.0805 \pm 0.0091 \pm 0.0032$ps$^{-1}$, 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 $B_s^{0} \to J/ψπ^+π^-$ decays gives $ϕ_s = -0.010 \pm 0.039 $rad. All measurements are in agreement with the Standard Model predictions. For the first time the phase $ϕ_s$ is measured independently for each polarisation state of the $K^+K^- $ system and shows no evidence for polarisation dependence.

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