Observation of mixing-inducedCPviolation in the neutralBmeson system

This article describes an observation of mixing-induced CP violation and a measurement of the CP violation parameter, sin2φ₁, with the Belle detector at the KEKB asymmetric e⁺e^- collider. Using a data sample of 29.1 fb^-1 recorded on the Υ(4S) resonance that contains 31.3 million BB̄ pairs, we reconstruct decays of neutral B mesons to the following CP eigenstates: J/ψKS⁰, ψ(2S)KS⁰, χc1KS⁰, ηcKS⁰, J/ψKL⁰ and J/ψK*0. The flavor of the accompanying B meson is identified by combining information from primary and secondary leptons, K^± mesons, Λ 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 sin2φ₁=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 sin2φ₁ is ruled out by more than six standard deviations.