Search for exotic muon decays

Abstract Recently, it has been proposed that the observed anomaly in the time distribution of neutrino induced reactions, reported by the KARMEN Collaboration, can be interpreted as a signal from an exotic muon decay branch μ+→e+X. It has been shown that this hypothesis gives an acceptable fit to the KARMEN data if the boson X has a mass of m X =103.9 MeV/c 2 , close to the kinematical limit. We have performed a search for the X particle by studying for the first time the very low energy part of the Michel spectrum in μ+ decays. Using a HPGe detector setup at the μE4 beamline at PSI we find branching ratios BR(μ+→e+X) 103 MeV/c 2 X MeV/c 2 .