Observation Of Cp Violation In The B Meson System

We present an updated measurement of time-dependent CP-violating asymmetries in neutral B decays with the BABAR detector at the PEP-II asymmetric B Factory at SLAC. This result uses an additional sample of[Upsilon](4S) decays collected in 2001, bringing the data available to 32 million B[bar B] pairs. We select events in which one neutral B meson is fully reconstructed in final state containing charmonium and the flavor of the other neutral B meson is determined from its decay products. The amplitude of the CP-violating asymmetry, which in the Standard Model is proportional to sin2[beta], is derived from the decay time distributions in such events. The result sin2[beta]= 0.59[+-] 0.14 (stat)[+-] 0.05 (syst) establishes CP violation in the B[sup 0] meson system. We also determine[lambda]= 0.93[+-] 0.09 (stat)[+-] 0.03 (syst), consistent with no direct CP violation.