Observation of Large $\mathit{CP}$ Violation in the Neutral $\mathit{B}$ Meson System

We present a measurement of the standard model $\mathrm{CP}$ violation parameter $\sin 2{\phi }_1$ based on a $29.1{\mathrm{fb}}^{-1\mathrm{}}$ data sample collected at the $\Upsilon \left(4S\right)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider. One neutral $B$ meson is fully reconstructed as a $J/\psi K_S$, $\psi \left(2S\right)K_S$, ${\chi }_{c1\mathrm{}}{K}_S$, ${\eta }_c{K}_S$, $J/\psi K_L$, or $J/\psi K^{*0\mathrm{}}$ decay and the flavor of the accompanying $B$ meson is identified from its decay products. From the asymmetry in the distribution of the time intervals between the two $B$ meson decay points, we determine $\sin 2{\phi }_1=0.99\mathrm{}\pm 0.14\left(\mathrm{stat}\right)\pm 0.06\left(\mathrm{syst}\right)$. We conclude that we have observed $\mathrm{CP}$ violation in the neutral $B$ meson system.