Search for heavy neutrinos in K + → μ + ν H decays

Here, evidence of a heavy neutrino, νH, in the K+→μ+νH decays was sought using the E949 experimental data with an exposure of 1.70 × 1012 stopped kaons. With the major background from the radiative K+→μ+νμγ decay understood and suppressed, upper limits (90% C.L.) on the neutrino mixing matrix element between the muon and heavy neutrinos, |UμH|2, were set at the level of 10–7 to 10–9 for the heavy neutrino mass region 175 to 300 MeV/c2.