Observation of large CP violation in the neutral B meson system -: art. no. 091802

We present a measurement of the standard model CP violation parameter sin2 phi (1) based on a 29.1 fb(-1) data sample collected at the Y(4S) resonance with the Belle detector at the KEKB asymmetric-energy e(+)e(-) collider. One neutral B meson is fully reconstructed as a J/psiK(S), psi (2S)K(S), X(c1)K(S), eta (c)K(S), J/psiK(L), or J/psiK*(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 sin2 phi (1) = 0.99 +/-0.14(stat) +/-0.06(syst). We conclude that we have observed CP violation in the neutral B meson system.