Observation of Direct CP Violation in B0 --> K+pi- Decays

The authors present an updated search for direct CP violation in the decay B0 --> K+pi-. Using 227 million Υ(4S) --> B(bar B) decays collected with the BaBar detector at the PEP-II asymmetric-energy E+e- 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 B0 meson system at the level of 4.2 standard deviations