Measurement of Time-Dependent $CP$-Violating Parameters in $B^0\to K_S^0{K}_S^0$ Decays

We report a measurement of the $CP$-violating parameters in $B^0\to K_S^0{K}_S^0$ decays based on a data sample of $657\times {10}^6$ $B\overline{B}$ pairs collected at the ${\rm Y}(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider. In this Letter, one neutral $B$ meson is fully reconstructed in the $B^0\to K_S^0{K}_S^0$ decay mode, and the flavor of the accompanying $B$ meson is identified by its decay products. The $CP$-violating parameters are measured from the asymmetry in the distributions of the proper-time interval between the two $B$ decays: ${\mathcal{S}}_{K_S^0{K}_S^0}=-{0.38}_{-0.77}^{+0.69}(\mathrm{stat})\pm 0.09(\mathrm{syst})$ and ${\mathcal{A}}_{K_S^0{K}_S^0}=-0.38\pm 0.38(\mathrm{stat})\pm 0.05(\mathrm{syst})$.

Figure 1

(a) $\Delta E$, (b) $M_{\mathrm{bc}}$, and (c) $\mathcal{R}$ projections for the $B^0\to K_S^0{K}_S^0$ candidate events (a) with $\mathcal{R}>0.6$ and $5.27\mathrm{GeV}/c^2<M_{\mathrm{bc}}$, (b) with $\mathcal{R}>0.6$ and $|\Delta E|<0.1\mathrm{GeV}$, and (c) in the signal region. The solid histogram and curves show the fit projections and the hatched areas show the background component. The points with error bars are the data.

Figure 2

(a) $\Delta t$ distribution and (b) raw asymmetry ${\mathcal{A}}_{CP}$ for the $B^0\to K_S^0{K}_S^0$ candidate events in the signal region with good tags ($r>0.5$) and $\mathcal{R}>0.6$. In (a), the dashed and solid curves show the fit results with $q=\pm 1$, respectively. The dotted curve shows the background component. In (b), the solid curve shows the fit projection.