[en] The muon decay into a hypothetical charged boson x and a neutrino was searched for with a Si(Li) detector. In the mass region 60 MeV < msub(x) < 85 MeV and for a lifetime tausub(x) > 20 μs an upper limit on the branching ratio GAMMA(μ+ → x+ antineutrinosub(μ))|GAMMA(μ+ → all) < 2 x 10-6 is obtained with more than 90% confidence. An interpretation yields a lower limit on tausub(x)