Search for massive neutrinos inπ→eνdecay

We describe the details of an experiment designed to search for the admixture of massive neutrinos into electron-flavor neutrinos by observation with high resolution of the positron momentum spectrum in $\pi \to e+\nu$ decay. No indication was found for such an admixture and the corresponding mixing amplitude was constrained below the $U_{\mathrm{ei}}^2\le {10}^{-6\mathrm{}}$ region for neutrinos of ∼30 MeV/${\mathit{c}}^2$ mass.