Search for exotic muon decays

Recently, it has been proposed that the observed anomaly in the time distri bution of neutrino induced reactions, reported by the KARMEN Collaboration, can be interpreted as a signal from an exotic muon decay branch mu(+)-->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 th e kinematical limit. We have performed a search for the X particle by study ing for the first time the very low energy part of the Michel spectrum in m u(+) decays. Using a HPGe detector setup at the mu E4 beamline at PSI we fi nd branching ratios 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). (C) 1999 Elsevier Science B .V. All rights reserved.