Search for exotic muon decays

The observed anomaly in the time distribution of neutrino induced reactions , reported by the KARMEN collaboration, has recently been interpreted as a signal from an exotic muon decay branch mu(+) --> e(+)X. This hypothesis gi ves an acceptable fit to the KARMEN data if the weakly interacting massive boson X has a mass of mx = 103.9 MeV/c(2), close to the kinematical limit o f the proposed decay. We have performed a search for the X particle by stud ying the very low energy part of the Michel spectrum in mu(+) decays. Using a HPGe detector setup at the mu E4 beamline at PSI we find branching ratio s BR(mu(+) --> e(+)X) < 5.7 . 10(-4) (90% C.L.) for most of the region 103 MeV/c(2) < m(x) < 105 MeV/c(2) [1].