Search for Heavy Neutrinos in $K^{+}\to {\mu }^{+}{\nu }_{\mu }$ 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 $\mathrm{\mid }{U}_{\mu 4}{\mathrm{\mid }}^2$ 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.