A measurement of the CP violation parameter sin 2ϕ1 based on a 29.1 fb-1 data sample recorded at the ϒ(4S) resonance with the Belle detector at the KEKB asymmetric e+ e- collider is reported. One neutral B meson is fully reconstructed as a J/ψKS, ψ(2S)KS, χc1KS, ηcKS, J/ψKL or J/ψK*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 sin 2ϕ1 = 0.99±0.14(stat) ±0.06(sys). Since this value is more than 6σ from zero, we can conclude that CP symmetry is violated in the neutral B meson system.
Observation of Matter-Antimatter Asymmetry in the Neutral B Meson System