We present a measurement of the standard model $\mathrm{CP}$ violation parameter $\sin 2{\phi }_1$ (also known as $\sin 2\beta$) based on a $10.5{\mathrm{fb}}^{-1\mathrm{}}$ data sample collected at the $\Upsilon \left(4S\right)$ resonance with the Belle detector at the KEKB asymmetric $e^{+}{e}^{-}$ collider. One neutral $B$ meson is reconstructed in the $J/\psi K_S$, $\psi \left(2S\right)K_S$, ${\chi }_{c1\mathrm{}}{K}_S$, ${\eta }_c{K}_S$, $J/\psi K_L$, or $J/\psi {\pi }^0$ $\mathrm{CP}$-eigenstate decay channel and the flavor of the accompanying $B$ meson is identified from its charged particle decay products. From the asymmetry in the distribution of the time interval between the two $B$-meson decay points, we determine $\sin 2{\phi }_1{=0.58\mathrm{}}_{-0.34\mathrm{}}^{+0.32\mathrm{}}\left(\mathrm{stat}{)}_{-0.10\mathrm{}}^{+0.09\mathrm{}}\right(\mathrm{syst})$.

• Received 9 February 2001

DOI:https://doi.org/10.1103/PhysRevLett.86.2509