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/$c^2$. 
A.V. Artamonov, B. Bassalleck, B. Bhuyan, E.W. Blackmore, D.A. Bryman,   S. Chen, I-H. Chiang, I.-A. Christidi, P.S. Cooper,...
hep-ex