Search for admixtures of massive neutrinos in the decayπ+→μ++ν

We have performed a search for admixtures of massive neutrinos in the pion decay ${\pi }^{+}$→${\mu }^{+}$+ν at the Swiss Institute for Nuclear Research (SIN). Positive pions were stopped in a high-purity germanium detector and the energy of their decay muons was measured with a resolution of 10 keV (full width at half maximum). The energy spectrum of the muons was searched for additional peaks, the existence of which would have indicated lepton mixing; no such peaks were found. The upper limit for the branching ratio for the decay of a pion into a muon and a heavy neutrino has been significantly improved for neutrino masses ranging from 1 to 16 MeV/$c^2$.