Observation ofB±→pp¯K±

We report the observation of the decay mode ${B}^{\\ifmmode\\pm\\else\\textpm\\fi{}}\\ensuremath{\\rightarrow}p\\overline{p}{K}^{\\ifmmode\\pm\\else\\textpm\\fi{}}$ based on an analysis of $29.4{\\mathrm{fb}}^{\\ensuremath{-}1}$ of data collected by the Belle detector at KEKB. This is the first example of a $b\\ensuremath{\\rightarrow}s$ 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 $B({B}^{\\ifmmode\\pm\\else\\textpm\\fi{}}\\ensuremath{\\rightarrow}p\\overline{p}{K}^{\\ifmmode\\pm\\else\\textpm\\fi{}})\\phantom{\\rule{0ex}{0ex}}=\\phantom{\\rule{0ex}{0ex}}[{4.3}_{\\ensuremath{-}0.9}^{+1.1}(\\mathrm{stat})\\ifmmode\\pm\\else\\textpm\\fi{}0.5(\\mathrm{syst})]\\ifmmode\\times\\else\\texttimes\\fi{}{10}^{\\ensuremath{-}6}$. We also report upper limits for the decays ${B}^{0}\\ensuremath{\\rightarrow}p\\overline{p}{K}_{S}$ and ${B}^{\\ifmmode\\pm\\else\\textpm\\fi{}}\\ensuremath{\\rightarrow}p\\overline{p}{\\ensuremath{\\pi}}^{\\ifmmode\\pm\\else\\textpm\\fi{}}$.