Observation ofB0decay to two charmless mesons

We report results from a search for the decays ${\mathit{B}}^0$→${\mathrm{\pi }}^{+}$ ${\mathrm{\pi }}^{\mathrm{-}}$, ${\mathit{B}}^0$→${\mathit{K}}^{+}$ ${\mathrm{\pi }}^{\mathrm{-}}$, and ${\mathit{B}}^0$→${\mathit{K}}^{+}$ ${\mathit{K}}^{\mathrm{-}}$. We find 90% confidence level upper limits on the branching fractions, ${\mathit{B}}_{\mathrm{\pi }\mathrm{\pi }}$<2.9×${10}^{\mathrm{-}5}$, ${\mathit{B}}_{\mathit{K}\mathrm{\pi }}$<2.6×${10}^{\mathrm{-}5}$, and ${\mathit{B}}_{\mathit{K}\mathit{K}}$<0.7×${10}^{\mathrm{-}5}$. While there is no statistically significant signal in the individual modes, the sum of ${\mathit{B}}_{\mathrm{\pi }\mathrm{\pi }}$ and ${\mathit{B}}_{\mathit{K}\mathrm{\pi }}$ exceeds zero with a significance of more than 4 standard deviations, indicating that we have observed charmless hadronic B decays.