Direct CP Violating Asymmetry in B0→K+π- Decays

We present a measurement of the direct CP violating asymmetry in the decay B⁰→K⁺π^- using a data sample of 227×10⁶ Υ(4S)→BB¯ decays collected with the BABAR detector at the PEP-II asymmetric-energy e⁺e^- collider at SLAC. We observe a total signal yield of nK^-_π⁺+nK⁺_π^-=1606±51 decays and measure the asymmetry (nK^-_π⁺-nK⁺_π^-)/(nK^-_π⁺+nK⁺_π^-)=-0.133±0.030(stat)±0.009(syst). The probability of observing such an asymmetry in the absence of direct CP violation is 1.3×10^-5, corresponding to 4.2 standard deviations.