Observation ofB±→pp¯K±

We report the observation of the decay mode $B^{\pm }\to p\overline{p}{K}^{\pm }$ based on an analysis of $29.4{\mathrm{fb}}^{-1\mathrm{}}$ of data collected by the Belle detector at KEKB. This is the first example of a $b\to s$ transition with baryons in the final state. The $p\overline{p}$ mass spectrum in this decay is inconsistent with phase space and is peaked at low mass. The branching fraction for this decay is measured to be $B(B^{\pm }\to p\overline{p}{K}^{\pm })=\left[{4.3}_{-0.9\mathrm{}}^{+1.1\mathrm{}}\right(\mathrm{stat})\pm 0.5(\mathrm{syst}\left)\right]\times {10}^{-6\mathrm{}}$. We also report upper limits for the decays $B^0\to p\overline{p}{K}_S$ and $B^{\pm }\to p\overline{p}{\pi }^{\pm }$.