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

A search for massive neutrinos has been made in the decay $\pi\rightarrow e^+ \nu$. No evidence was found for extra peaks in the positron energy spectrum indicative of pion decays involving massive neutrinos ($\pi\rightarrow e^+ \nu_h$)\mathrm{.}Upperlimits(90\mathrm{\%}C\mathrm{.}L\mathrm{.})ontheneutrinomixingmatrixelement$|U_{ei}|^2$intheneutrinomassregion60--135MeV\mathrm{/}$c^2$ were set, which are %representing an order of magnitude improvement over previous results.