Evidence of the K+→μ+ννν decay was searched for using E949 (Brookhaven National Laboratory, USA) experimental data with an exposure of 1.70×1012 stopped kaons. The data sample is dominated by the background process K+→μ+νμγ. An upper limit on the decay rate Γ(K+→μ+ννν)<2.4×10-6Γ(K+→all) at 90% confidence level was set assuming the standard model muon spectrum. The data are presented in such a way as to allow calculation of rates for any assumed μ+ spectrum.
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