Evidence for the Decay K + → π + ν ν ¯

An event consistent with the signature expected for the rare kaon decay ${K}^{+}\\ensuremath{\\rightarrow}{\\ensuremath{\\pi}}^{+}\\ensuremath{\\nu}\\overline{\\ensuremath{\\nu}}$ has been observed. In the pion momentum region examined, $211lPl230\\mathrm{MeV}/c$, the backgrounds are estimated to contribute $0.08\\ifmmode\\pm\\else\\textpm\\fi{}0.03$ events. If the event is due to ${K}^{+}\\ensuremath{\\rightarrow}{\\ensuremath{\\pi}}^{+}\\ensuremath{\\nu}\\overline{\\ensuremath{\\nu}}$, the branching ratio is ${4.2}_{\\ensuremath{-}3.5}^{+9.7}\\ifmmode\\times\\else\\texttimes\\fi{}{10}^{\\ensuremath{-}10}$.