Observation of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow><mml:mi mathvariant="italic">B</mml:mi></mml:mrow><mml:mrow><mml:mn>0</mml:mn></mml:mrow></mml:msup></mml:mrow></mml:math>decay to two charmless mesons

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