Search for a heavy neutrino state in the decay π+→μ++νμ

Abstract We have searched for an admixture of a heavy neutrino state ν x in the decay π + → μ + + ν μ by analyzing the momentum spectrum of a surface muon beam in a magnetic spectrometer. No indications for such a neutrino state were found. In the mass range 0.53 MeV ≤ m ν x ≤ 1.0 MeV, not covered by previous experiments, we obtain upper limits of the branching fraction Γ ( π + → μ + ν x )/[ Γ ( π + → μ + ν 2 )+ Γ ( π + → μ + ν x )] between 22 % and 1.6 % (c.l.=0.9).