Exotic muon decays and the KARMEN anomaly
  An anomaly in time distribution of neutrinos from the ISIS pulsed beam stop source observed by the KARMEN collaboration is discussed. We show that the anomaly can be interpreted as a superposition of two exponentials, both having time constants consistent with the $\mu^{+}$ lifetime of 2.2 $\mu s$. % and, therefore, It is assumed that they both originate from muon decays at rest.   One of them describes the time distribution of the prompt neutrino events, while the other describes the time distribution of events from delayed decays of slowly moving ($\beta \simeq 0.02$) particles in the KARMEN calorimeter.   We propose here that these particles are produced in exotic decays of positive muons $\mu^{+}\to e^{+} + X$, resulting in the second exponential time distribution shifted by the time of flight with respect to the time distribution of neutrino events. This model gives an aceptable fit to the KARMEN data if $X$ has a mass of 103.9 MeV.   The possible decay modes of this new massive neutral particle are discussed. This hypothesis can be experimentally tested in the near future by studying the low energy part of the $e^{+}$ spectrum in the $\mu^{+}$ decays. 
S.N. Gninenko and N.V. Krasnikov