Observation Of Direct Cp Violation In B Kpi Decays

The authors present an updated search for direct CP violation in the decay B° --> Kpi−. Using 227 million [Upsilon](4S) --> B{bar B} decays collected with the BaBar detector at the PEP-II asymmetric-energy Ee− collider at SLAC, the authors observe a total signal yield of n(K-pi+) + n(K+pi−) = 1606 ± 51 decays and measure the asymmetry [n(K−pi+)-n(K+pi−)]/[n(K−pi+)+n(K+pi−)] = 0.133 ± 0.030(stat) ± 0.009(syst). This measurement establishes direct CP violation in the B° meson system at the level of 4.2 standard deviations.