Emission of heavy neutrinos in muon capture

The possible use of various muon-capture observables is assessed in the search for heavy neutrinos mixed in the dominant variety. Existing data on the triton recoil spectrum measured in the reaction ${\mu }^{-}+{}_{}{}^3{}_{}{}^{}\mathrm{He}\to {\nu }_{\mu }+{}_{}{}^3{}_{}{}^{}\mathrm{H}$ readily allow the exclusion of the emission of heavy neutrinos of mass $m_{\nu j}$ between 60 and 72 MeV/${\mathit{c}}^2$ with probability higher than ${\left|U_{\mu j}\right|}^2\cong 0.01$. This complements limits on ${\left|U_{\mu j}\right|}^2$ obtained from the reactions $\pi \to \mu {\nu }_{\mu }$ and $K\to \mu {\nu }_{\mu }$.