Improved search for massive neutrinos in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow><mml:mi>π</mml:mi></mml:mrow><mml:mrow><mml:mo>+</mml:mo></mml:mrow></mml:msup></mml:mrow><mml:mo>→</mml:mo><mml:mrow><mml:msup><mml:mrow><mml:mi>e</mml:mi></mml:mrow><mml:mrow><mml:mo>+</mml:mo></mml:mrow></mml:msup></mml:mrow><mml:mi>ν</mml:mi></mml:math>decay

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