Measurement of CP violation and the B_s^0 meson decay width difference with B_s^0\to J/ψK^+ K^- and B_s^0\to J/ψπ^+π^- decays
The time-dependent CP asymmetry in B_s^0\to J/ψK^+K^- decays is measured using $pp$ collision data at \sqrt{s}=7TeV, corresponding to an integrated luminosity of 1.0fb^-1, collected with the LHCb detector. The decay time distribution is characterised by 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 and by a CP-violating phase ϕ_s. In a sample of 27\,617 B_s^0\to J/ψK^+K^- decays, where the dominant contribution comes from B_s^0\to J/ψϕ decays, these parameters are measured to be ϕ_s = 0.07 \pm 0.09 (stat) \pm 0.01 (syst) rad, Γ_s \equiv (Γ_L+Γ_H)/2 = 0.663 \pm 0.005 (stat) \pm 0.006 (syst) ps^-1, ΔΓ_s \equiv Γ_L -Γ_H = 0.100 \pm 0.016 (stat) \pm 0.003 (syst) & ps^-1, corresponding to the single most precise determination of ϕ_s, ΔΓ_s and Γ_s. The result of performing a combined analysis with B_s^{0} \to J/ψπ^+π^- decays gives ϕ_s = 0.01 \pm 0.07 (stat) \pm 0.01 (syst) rad, Γ_s = 0.661 \pm 0.004 (stat) \pm 0.006 (syst) ps^-1, ΔΓ_s = 0.106 \pm 0.011 (stat) \pm 0.007 (syst) & ps^-1. All measurements are in agreement with the Standard Model predictions.