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

We have searched for an admixture of a heavy neutrino state νx in the decay π+→μ++νμ 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 ≤mνx≤ 1.0 MeV, not covered by previous experiments, we obtain upper limits of the branching fraction Γ(π+→μ+νx)/[Γ(π+→μ+ν2)+Γ(π+→μ+νx)] between 22 % and 1.6 % (c.l.=0.9).