Study of the decay K+→π+νν¯ in the momentum region 140199  MeV/c

Experiment E949 at Brookhaven National Laboratory has observed three new events consistent with the decay K+ -> pi(+) nu(nu) over bar in the pion momentum region 140 < P-pi < 199 MeV/c in an exposure of 1.71 x 10(12) stopped kaons with an estimated total background of 0.93 +/- 0.17(stat)(-0.24)(+0.32) (syst) events. This brings the total number of observed K+ -> pi(+)nu(nu) over bar 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+ -> pi(+)nu(nu) over bar) = 1.73(+1.15)(-1.05) x 10(-10). An interpretation of the results for alternative models of the decay K+ -> pi(+) + nothing is also presented.