This article describes an observation of mixing-induced $\mathrm{CP}$ violation and a measurement of the $\mathrm{CP}$ violation parameter, $\sin 2{\phi }_1,$ with the Belle detector at the KEKB asymmetric $e^{+}{e}^{-}$ collider. Using a data sample of $29.1\hspace{0.5em}{\mathrm{fb}}^{-1}$ recorded on the ${\rm Y}\mathrm{}\left(4S\right)\mathrm{}$ resonance that contains 31.3 million $B\overline{B}$ pairs, we reconstruct decays of neutral B mesons to the following $\mathrm{CP}$ eigenstates: $J/\psi K_S^0,$ $\psi {\mathrm{}\left(2S\right)K}_S^0,$ ${\chi }_{c1\mathrm{}}{K}_S^0,$ ${\eta }_c{K}_S^0,$ $J/\psi K_L^0$ and $J/\psi K^{*0}.$ The flavor of the accompanying B meson is identified by combining information from primary and secondary leptons, $K^{\pm }$ mesons, $\Lambda$ baryons, slow and fast pions. The proper-time interval between the two B meson decays is determined from the distance between the two decay vertices measured with a silicon vertex detector. The result $\sin 2{\phi }_1=0.99\mathrm{}\pm 0.14\left(\mathrm{stat}\right)\pm 0.06\left(\mathrm{syst}\right)$ is obtained by applying a maximum likelihood fit to the 1137 candidate events. We conclude that there is large $\mathrm{CP}$ violation in the neutral B meson system. A zero value for $\sin 2{\phi }_1$ is ruled out by more than six standard deviations.

• Received 13 February 2002

DOI:https://doi.org/10.1103/PhysRevD.66.032007