Measurement of the Time-DependentCPAsymmetry in theB0→ϕK0Decay

We present a measurement of the time-dependent $CP$ asymmetry for the neutral $B$-meson decay $B^0\to \varphi K^0$. We use a sample of approximately $114\times {10}^6$ $B$-meson pairs taken at the ${\rm Y}(4S)$ resonance with the BABAR detector at the PEP-II $B$-meson factory at SLAC. We reconstruct the $CP$ eigenstates $\varphi K_S^0$ and $\varphi K_L^0$, where $\varphi \to K^{+}{K}^{-}$, $K_S^0\to {\pi }^{+}{\pi }^{-}$, and $K_L^0$ is observed via its hadronic interactions. The other $B$ meson in the event is tagged as either a $B^0$ or ${\overline{B}}^0$ from its decay products. The values of the $CP$-violation parameters are $S_{\varphi K}=0.47\pm 0.34{\mathrm{(}\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{)}}_{-0.06}^{+0.08}\mathrm{(}\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{)}$ and $C_{\varphi K}=0.01\pm 0.33\mathrm{(}\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{)}\pm 0.10\mathrm{(}\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{)}$.