Search for Heavy Neutrinos in $\ddot{I}€\to \hat{I}¼\hat{I}½$ Decay

Abstract

Heavy neutrinos were sought in pion decays $\ddot{I}{€}^{+}\to \hat{I}{¼}^{+}\hat{I}½$ by examining the observed muon energy spectrum for extra peaks in addition to the expected peak for a massless neutrino. No evidence for heavy neutrinos was observed. Upper limits were set on the neutrino mixing matrix $|U_{\hat{I}¼i}{|}^2$ in the neutrino mass region of 15.7--33.8 MeV/c${}^2$, improving on previous results by an order of magnitude.