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

Abstract

A search for massive neutrinos has been made in the decay $\ddot{I}€\to e^{+}\hat{I}½$. No evidence was found for extra peaks in the positron energy spectrum indicative of pion decays involving massive neutrinos ($\ddot{I}€\to e^{+}\hat{I}{½}_h$). Upper limits (90 \% C.L.) on the neutrino mixing matrix element $|U_{ei}{|}^2$ in the neutrino mass region 60--135 MeV/$c^2$ were set, which are %representing an order of magnitude improvement over previous results.