Measurement of the CP-Violating Phase phi(s) in the Decay B-s(0) -> J/psi phi

We present a measurement of the time-dependent CP-violating asymmetry in B-s(0) -> J/psi phi decays, using data collected with the LHCb detector at the LHC. The decay time distribution of B-s(0) -> J/psi phi is characterized by the decay widths Gamma(H) and Gamma(L) of the heavy and light mass eigenstates, respectively, of the B-s(0) - (B) over bar (0)(s) system and by a CP-violating phase phi(s). In a sample of about 8500 B-s(0) -> J/psi phi events isolated from 0.37 fb(-1) of pp collisions at root s = 7 TeV, we measure phi(s) = 0.15 +/- 0.18(stat) +/- 0.06(syst) rad. We also find an average B-s(0) decay width Gamma(s) (Gamma(L) + Gamma(H))/2 = 0.657 +/- 0.009(stat) +/- 0.008(syst) ps(-1) and a decay width difference Delta Gamma(s) Gamma(L) - Gamma(H) = 0.123 +/- 0.029(stat) +/- 0.011(syst) ps(-1). Our measurement is insensitive to the transformation (phi(s), Delta Gamma(s)) bar right arrow (pi - phi(s), - Delta Gamma(s)).