Search for spontaneous conversion of muonium to antimuonium

An experimental search for the spontaneous conversion of muonium to antimuonium is described. A 10 MeV/c ${\mu }^{+}$ beam of average intensity 3×${10}^5$ ${\mathrm{s}}^{-1\mathrm{}}$ was used to produce muonium atoms in vacuum by charge capture in a thin foil. The novel method applied here for detection of an antimuonium atom was a triple coincidence of two muonic x-ray photons from the atomic capture of its ${\mu }^{-}$ by a high-$Z$ target and a secondary electron ejected from the target upon the impact of the antimuonium atom. No antimuonium events were observed and the resulting 90% confidence level upper limit on the coupling constant leading to the conversion, $G_{M\overline{M}}$, is $6.9G_F$, where $G_F$ is the Fermi coupling constant.