Search for the rare decay $K^+tomu^+ ubar u u$

Evidence of the $K^+tomu^+ ubar u u$ decay was searched for using E949 experimental data with an exposure of $1.70times 10^{12}$ stopped kaons. The data sample is dominated by the backgrond process $K^+tomu^+ u_mugamma$. An upper limit on the decay rate $Gamma(K^+tomu^+ ubar u u)< 2.4times 10^{-6}Gamma(K^+to all)$ 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 $mu^+$ spectrum.