Study of the decay K+→π+νν̄ in the momentum region 140<Pπ<199MeV/c

Experiment E949 at Brookhaven National Laboratory has observed three new events consistent with the decay ${K}^{+}\\ensuremath{\\rightarrow}{\\ensuremath{\\pi}}^{+}\\ensuremath{\\nu}\\overline{\\ensuremath{\\nu}}$ in the pion momentum region $140l{P}_{\\ensuremath{\\pi}}l199\\text{ }\\text{ }\\mathrm{MeV}/c$ in an exposure of $1.71\\ifmmode\\times\\else\\texttimes\\fi{}{10}^{12}$ stopped kaons with an estimated total background of $0.93\\ifmmode\\pm\\else\\textpm\\fi{}0.17(\\mathrm{stat}{)}_{\\ensuremath{-}0.24}^{+0.32}(\\mathrm{syst})$ events. This brings the total number of observed ${K}^{+}\\ensuremath{\\rightarrow}{\\ensuremath{\\pi}}^{+}\\ensuremath{\\nu}\\overline{\\ensuremath{\\nu}}$ events to seven. Combining this observation with previous results, assuming the pion spectrum predicted by the standard model, results in a branching ratio of $\\mathcal{B}({K}^{+}\\ensuremath{\\rightarrow}{\\ensuremath{\\pi}}^{+}\\ensuremath{\\nu}\\overline{\\ensuremath{\\nu}})=({1.73}_{\\ensuremath{-}1.05}^{+1.15})\\ifmmode\\times\\else\\texttimes\\fi{}{10}^{\\ensuremath{-}10}$. An interpretation of the results for alternative models of the decay ${K}^{+}\\ensuremath{\\rightarrow}{\\ensuremath{\\pi}}^{+}+nothing$ is also presented.