We present an improved measurement of the standard model CP violation parameter $\sin 2{\phi }_1$ (also known as $\sin 2\beta )$ based on a sample of $85\times {10}^6$ $B\overline{B}$ pairs collected at the ${\rm Y}\mathrm{}\left(4S\right)\mathrm{}$ resonance with the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider. One neutral B meson is reconstructed in a $J/\psi K_S^0,$ $\psi {\mathrm{}\left(2S\right)K}_S^0,$ ${\chi }_{c1\mathrm{}}{K}_S^0,$ ${\eta }_c{K}_S^0,$ $J/\psi K^{*0},$ or $J/\psi K_L^0$ CP-eigenstate decay channel and the flavor of the accompanying B meson is identified from its decay products. From the asymmetry in the distribution of the time interval between the two B meson decay points, we obtain $\sin 2{\phi }_1=0.719\mathrm{}\pm 0.074\left(\mathrm{stat}\right)\pm 0.035\left(\mathrm{syst}\right).\mathrm{}$

• Received 16 August 2002

DOI:https://doi.org/10.1103/PhysRevD.66.071102