Measurement of the CP-violating phase <inf>s</inf> in the decay Bs0→J/ψ

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).