DirectCPViolating Asymmetry inB0→K+π−Decays

We present a measurement of the direct $CP$ violating asymmetry in the decay $B^0\to K^{+}{\pi }^{-}$ using a data sample of $227\times {10}^6$ ${\rm Y}(4S)\to B\overline{B}$ decays collected with the BABAR detector at the PEP-II asymmetric-energy $e^{+}{e}^{-}$ collider at SLAC. We observe a total signal yield of $n_{K^{-}{\pi }^{+}}+n_{K^{+}{\pi }^{-}}=1606\pm 51$ decays and measure the asymmetry $(n_{K^{-}{\pi }^{+}}-n_{K^{+}{\pi }^{-}})/(n_{K^{-}{\pi }^{+}}+n_{K^{+}{\pi }^{-}})=-0.133\pm 0.030(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t})\pm 0.009(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t})$. The probability of observing such an asymmetry in the absence of direct $CP$ violation is $1.3\times {10}^{-5}$, corresponding to $4.2$ standard deviations.