New Bounds from Searching for Muonium to Antimuonium Conversion
  A new upper limit for the probability of spontaneous muonium to antimuonium conversion was established at ${\rm P_{M\bar{M}}} \leq 8.2 \cdot 10^{-11}$ (90%C.L.) in 0.1~T magnetic field, which implies consequences for speculative extensions to the standard model. Coupling parameters in R-parity violating supersymmetry and the mass of a flavour diagonal bileptonic gauge boson can be significantly restricted. A Z$_8$ model with radiative mass generation through heavy lepton seed and the minimal version of 331-GUT models are ruled out. 
L.Willmann (1), P.V.Schmidt (1), H.P.Wirtz (2), R.Abela (3), V.Baranov   (4), J.Bagaturia (5), W.Bertl (3), R.Engfer (2), A....