Search for Spontaneous Conversion of Muonium to Antimuonium

The results of an experimental search with a new method for the spontaneous conversion of muonium to antimuonium are reported. The upper limit for $G_{\mathrm{M}\overline{\mathrm{M}}}$, the coupling constant characterizing the strength of the interaction leading to the conversion, is measured to be $G_{\mathrm{M}\overline{\mathrm{M}}}<7.5\mathrm{}{G}_{\mathrm{F}}$ (90% confidence level), where $G_{\mathrm{F}}$ is the Fermi coupling constant. This result is about a factor of 3 lower than the previous limit and begins to probe predictions of the left-right-symmetric theory with a doubly charged Higgs triplet.