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 $1.70\times 10^{12}$stoppedkaons\mathrm{.}Withthemajorbackgroundfromtheradiative$K^+\to\mu^+\nu_\mu\gamma$ decay understood and suppressed, upper limits (90% C.L.) on the neutrino mixing matrix element between muon and heavy neutrino, $\mathrm{\mid }{U}_{\mu H}{\mathrm{\mid }}^2$, were set at the level of ${10}^{-7}$ to ${10}^{-9}$ for the heavy neutrino mass region 175 to 300 MeV/$c^2$.