Search for massive neutrinos in ${\mathrm{\pi }}^{+}$→${\mathrm{\mu }}^{+}$ν decay

Evidence of massive neutrinos was sought in the ${\mathrm{\pi }}^{+}$→${\mathrm{\mu }}^{+}$x decay spectrum in the mass region ${\mathit{M}}_{\mathit{x}}$=30–33.91 MeV/${\mathit{c}}^2$. Upper limits on the branching ratio ${\mathit{R}}_{\mathrm{\mu }\mathit{x}}$≤(4–6)×${10}^{\mathrm{-}5}$ (90% C.L.) were set for this region, which correspond to the neutrino mixing parameter ‖${\mathit{U}}_{\mathrm{\mu }\mathit{x}}$${\mathrm{\Vert }}^2$<${10}^{\mathrm{-}3}$–${10}^{\mathrm{-}4}$. © 1995 The American Physical Society.