Observation of LargeCPViolation in the NeutralBMeson System

We present a measurement of the standard model $\mathrm{CP}$ violation parameter $\mathrm{sin}2{\ensuremath{\varphi}}_{1}$ based on a $29.1{\mathrm{fb}}^{\ensuremath{-}1}$ data sample collected at the $\ensuremath{\Upsilon}(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy ${e}^{+}{e}^{\ensuremath{-}}$ collider. One neutral $B$ meson is fully reconstructed as a $J/\ensuremath{\psi}{K}_{S}$, $\ensuremath{\psi}(2S){K}_{S}$, ${\ensuremath{\chi}}_{c1}{K}_{S}$, ${\ensuremath{\eta}}_{c}{K}_{S}$, $J/\ensuremath{\psi}{K}_{L}$, or $J/\ensuremath{\psi}{K}^{*0}$ 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 $\mathrm{sin}2{\ensuremath{\varphi}}_{1}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.99\ifmmode\pm\else\textpm\fi{}0.14(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.06(\mathrm{syst})$. We conclude that we have observed $\mathrm{CP}$ violation in the neutral $B$ meson system.