The Muonium Atom as a Probe of Physics beyond the Standard Model
  The observed interactions between particles are not fully explained in the successful theoretical description of the standard model to date. Due to the close confinement of the bound state muonium ($M = \mu^+ e^-$) can be used as an ideal probe of quantum electrodynamics and weak interaction and also for a search for additional interactions between leptons. Of special interest is the lepton number violating process of sponteanous conversion of muonium to antimuonium. 
L. Willmann and K. Jungmann (Physikalisches Institut, Universitaet   Heidelberg, Heidelberg, Germany)