Search for Admixtures of Massive Neutrinos in the Decayπ+→μ++ν¯

Positive pions were stopped in a Ge detector and the energy of their decay muons was measured with a resolution of 6.7 keV. The energy spectrum was analyzed to obtain an upper limit for the branching ratio for the decay into a muon and a heavy neutrino. For neutrino masses between 2.5 and 10 MeV/${\mathit{c}}^2$ this upper limit is a few parts in ${10}^4$.