Improved Measurement of $CP$ Asymmetries in $B^0\to (c\overline{c})K^{0(*)}$ Decays

We present results on time-dependent $CP$ asymmetries in neutral $B$ decays to several $CP$ eigenstates. The measurements use a data sample of about $227\times {10}^6$ ${\rm Y}(4S)\to B\overline{B}$ decays collected by the BABAR detector at the PEP-II asymmetric-energy $B$ Factory at SLAC. The amplitude of the $CP$asymmetry, $\sin 2\beta$ in the standard model, is derived from decay-time distributions from events in which one neutral $B$ meson is fully reconstructed in a final state containing a charmonium meson and the other $B$ meson is determined to be either a $B^0$ or ${\overline{B}}^0$ from its decay products. We measure $\sin 2\beta =0.722\pm 0.040(\mathrm{stat})\pm 0.023(\mathrm{syst})$ in agreement with the standard model expectation.

Figure 1

Distributions for $B_{CP}$ and $B_{\mathrm{flav}}$ candidates satisfying the tagging and vertexing requirements: (a) $m_{\mathrm{ES}}$ for the final states $J/\psi K_S^0$, $\psi (2S)K_S^0$, ${\chi }_{c1}{K}_S^0$, and ${\eta }_c{K}_S^0$; (b) $\Delta E$ for the final state $J/\psi K_L^0$; (c) $m_{\mathrm{ES}}$ for $J/\psi K^{*0}$ ($K^{*0}\to K_S^0{\pi }^0$); and (d) $m_{\mathrm{ES}}$ for the $B_{\mathrm{flav}}$ sample. In each plot, the shaded region is the estimated background contribution.

Figure 2

(a) Number of ${\eta }_f=-1$ candidates ($J/\psi K_S^0$, $\psi (2S)K_S^0$, ${\chi }_{c1}{K}_S^0$, and ${\eta }_c{K}_S^0$) in the signal region with a $B^0$ tag $N_{B^0}$ and with a ${\overline{B}}^0$ tag $N_{{\overline{B}}^0}$, and (b) the raw asymmetry $(N_{B^0}-N_{{\overline{B}}^0})/(N_{B^0}+N_{{\overline{B}}^0})$, as functions of $\Delta t$. (c) and (d) are the corresponding plots for the ${\eta }_f=+1$ mode $J/\psi K_L^0$. All plots exclude Other tagged events. The solid (dashed) curves represent the fit projections in $\Delta t$ for $B^0$ (${\overline{B}}^0$) tags. The shaded regions represent the estimated background contributions.