Measurement of the CP-violating phase ϕs in the decay Bs0->J/ψϕ

We present a measurement of the time-dependent CP-violating asymmetry in Bs0→J/ψ decays, using data collected with the LHCb detector at the LHC. The decay time distribution of Bs0→J/ψ is characterized by the decay widths Γ H and Γ L of the heavy and light mass eigenstates, respectively, of the Bs0-B̄s0 system and by a CP-violating phase s. In a sample of about 8500 Bs0→J/ψevents isolated from 0.37fb -1 of pp collisions at √s=7TeV, we measure s=0.15±0.18(stat)±0.06(syst)rad. We also find an average Bs0 decay width Γ s (Γ L+Γ H)/2=0.657±0.009(stat)±0.008(syst)ps -1 and a decay width difference ΔΓ s Γ L- Γ H=0.123±0.029(stat)±0.011(syst)ps -1. Our measurement is insensitive to the transformation ( s, ΔΓ s) (π- s,-ΔΓ s)