Observation ofCPViolation inB0→K+π−andB0→π+π−

We report observations of $CP$ violation in the decays $B^0\to K^{+}{\pi }^{-}$ and $B^0\to {\pi }^{+}{\pi }^{-}$ in a sample of $383\times {10}^6$ ${\rm Y}(4S)\to B\overline{B}$ events. We find $4372\pm 82$ $B^0\to K^{+}{\pi }^{-}$ decays and measure the direct $CP$-violating charge asymmetry ${\mathcal{A}}_{K\pi }=-0.107\pm 0.018(\mathrm{stat}{)}_{-0.004}^{+0.007}(\mathrm{syst})$, which excludes the $CP$-conserving hypothesis with a significance of 5.5 standard deviations. In the same sample, we find $1139\pm 49$ $B^0\to {\pi }^{+}{\pi }^{-}$ decays and measure the $CP$-violating asymmetries $S_{\pi \pi }=-0.60\pm 0.11(\mathrm{stat})\pm 0.03(\mathrm{syst})$ and $C_{\pi \pi }=-0.21\pm 0.09(\mathrm{stat})\pm 0.02(\mathrm{syst})$. $CP$ conservation in $B^0\to {\pi }^{+}{\pi }^{-}$ $(S_{\pi \pi }=C_{\pi \pi }=0)$ is excluded at a confidence level $1-\mathrm{C}.\mathrm{L}.=8\times {10}^{-8}$, corresponding to 5.4 standard deviations.