Search for Heavy Neutrinos in \(\pi ^{ + } \to \mu ^{ + }\nu \) Decay and Status of Lepton Universality Test in the PIENU Experiment

Abstract

In the present work of the PIENU experiment, heavy neutrinos were sought in pion decays \(\pi ^{ + } \to \mu ^{ + }\nu \). No evidence for extra peak was found in the muon kinetic energy spectrum and 90% confidence level upper limits were set on the neutrino mixing matrix \(|U_{\mu i}|^{2}\) in the mass range of 15.7 to 33.8 MeV/c^2, improving an order of magnitude over previous experiments. Current status of lepton universality test is also reported.