Search for heavy neutrinos in \(K^+\to\mu^+\nu_H\) decays

Evidence of a heavy neutrino, \(\nu_H\), in the \(K^+\to\mu^+\nu_H\) decays was sought using the E949 experimental data with an exposure of \(1.70\times 10^{12}\) stopped kaons. With the major background from the radiative \(K^+\to\mu^+\nu_\mu\gamma\) decay understood and suppressed, upper limits (90% C.L.) on the neutrino mixing matrix element between muon and heavy neutrino, \(|U_{\mu H}|^2\), were set at the level of \(10^{-7}\) to \(10^{-9}\) for the heavy neutrino mass region 175 to 300 MeV/\(c^2\).