Improved search for massive neutrinos in ${\pi }^{+}\to e^{+}\nu$ decay

Evidence of massive neutrinos was sought in the ${\pi }^{+}\to e^{+}\nu$ decay spectrum with the background from the $\pi \to \mu \to e$ decay chain highly suppressed. Upper limits (90% C.L.) were set on the neutrino mixing parameter ${\left|U_{\mathrm{ei}}\right|}^2<{10}^{-7\mathrm{}}$ in the mass region $50<\mathrm{}{m}_{\nu }<130\mathrm{}$ MeV/${\mathit{c}}^2$.