Search for heavy neutrinos inK+→μ+νHdecays

Evidence of a heavy neutrino, ν H , in the K + → μ + ν H decays was sought using the E949 experimental data with an exposure of 1.70 × 1 0 12 stopped kaons. With the major background from the radiative K + → μ + ν μ γ decay understood and suppressed, upper limits (90% C.L.) on the neutrino mixing matrix element between the muon and heavy neutrinos, | U μ H | 2 , were set at the level of 1 0 - 7 to 1 0 - 9 for the heavy neutrino mass region 175 to 300 MeV / c 2 .