We present a measurement of the time-dependent $CP$-violating asymmetry in $B_s^0\to J/\psi \varphi$ decays, using data collected with the LHCb detector at the LHC. The decay time distribution of $B_s^0\to J/\psi \varphi$ is characterized by the decay widths ${\Gamma }_H$ and ${\Gamma }_L$ of the heavy and light mass eigenstates, respectively, of the $B_s^0-{\overline{B}}_s^0$ system and by a $CP$-violating phase ${\varphi }_s$. In a sample of about 8500 $B_s^0\to J/\psi \varphi$ events isolated from $0.37{\mathrm{fb}}^{-1}$ of $pp$ collisions at $\sqrt{s}=7\mathrm{TeV}$, we measure ${\varphi }_s=0.15\pm 0.18(\mathrm{stat})\pm 0.06(\mathrm{syst})\mathrm{rad}$. We also find an average $B_s^0$ decay width ${\Gamma }_s\equiv ({\Gamma }_L+{\Gamma }_H)/2=0.657\pm 0.009(\mathrm{stat})\pm 0.008(\mathrm{syst}){\mathrm{ps}}^{-1}$ and a decay width difference $\Delta {\Gamma }_s\equiv {\Gamma }_L-{\Gamma }_H=0.123\pm 0.029(\mathrm{stat})\pm 0.011(\mathrm{syst}){\mathrm{ps}}^{-1}$. Our measurement is insensitive to the transformation $({\varphi }_s,\Delta {\Gamma }_s)\mapsto (\pi -{\varphi }_s,-\Delta {\Gamma }_s)$.

• Received 14 December 2011

DOI:https://doi.org/10.1103/PhysRevLett.108.101803

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© 2012 CERN, for the LHCb Collaboration