Search for heavy neutrinos in $K^+ \rightarrow \mu^+ \nu_{\mu}$ decays

The NA62 experiment recorded a large sample of $K^+ \rightarrow \mu^+ \nu_{\mu}$ decays in 2007. A peak search has been performed in the reconstructed missing mass spectrum. In the absence of a signal, limits in the range $2 \times 10^{-6}$ to $10^{-5}$ have been set on the squared mixing matrix element $|U_{\mu4} |^{2}$ between muon and heavy neutrino states, for heavy neutrino masses in the range $300-375$ MeV/$c^{2}$. The result extends the range of masses for which upper limits have been set on the value of $|U_{\mu4} |^{2}$ in previous production search experiments.