New Bounds from a Search for Muonium to Antimuonium Conversion

A new upper limit for the probability of spontaneous muonium to antimuonium conversion was established at ${P}_{\mathrm{M}\overline{\mathrm{M}}}\ensuremath{\le}8.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}$ ( $90%$ 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 flavor 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 models are disfavored.