Observation of mixing-induced CP violation in the neutral B meson system

This article describes an observation of mixing-induced CP violation and a measurement of the CP violation parameter, sin 2phi(1), with the Belle detector at the KEKB asymmetric e(+)e(-) collider. Using a data sample of 29.1 fb(-1) recorded on the Y(4S) resonance that contains 31.3 million B (B) over bar pairs, we reconstruct decays of neutral B mesons to the following CP eigenstates: J/psiK(S)(0), psi(2S)K(S)(0), chi(c1)K(S)(0), eta(c)K(S)(0), J/psiK(L)(0) and J/psiK(*0). The flavor of the accompanying B meson is identified by combining information from primary and secondary leptons, K(+/-) 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 2phi(1)=0.99+/-0.14(stat)+/-0.06(syst) is obtained by applying a maximum likelihood fit to the 1137 candidate events. We conclude that there is large CP violation in the neutral B meson system. A zero value for sin 2phi(1) is ruled out by more than six standard deviations.