Improved Search for Heavy Neutrinos in the Decay $\pi\rightarrow e\nu$

A search for massive neutrinos has been made in the decay $pirightarrow enu$. No evidence was found for extra peaks in the positron energy spectrum indicative of pion decays involving massive neutrinos ($pirightarrow e^+ nu_h$). 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 at the $10^{-8}$ level.