Study of the decay K^+→π^+ν ν^^- in momentum region 140 < P_π < 199MeV/c

Experiment E949 at Brookhaven National Laboratory has observed three new events consistent with the decay K + → π + vv in the pion momentum region 140 < P π < 199 MeV/c in an exposure of 1.71 × 10 12 stopped kaons with an estimated total background of 0.93 ± 0.17(stat) +0.32 ―0.24 (syst) events. This brings the total number of observed K + → π + vv events to seven. Combining this observation with previous results, assuming the pion spectrum predicted by the standard model, results in a branching ratio of B(K + → π + vv) = (1.73 +1.15 ―1.05 ) × 10 ―10 . An interpretation of the results for alternative models of the decay K + → π + + nothing is also presented.