Search for Mixing of (${\mu }^{+}{e}^{-}$) and (${\mu }^{-}{e}^{+}$) with Fermi Coupling Strength

A search is described for muonium (${\mu }^{+}{e}^{-}$) conversion to antimuonium (${\mu }^{-}{e}^{+}$) with a coupling strength approximately equal to that of the weak interaction. A silica-powder target produced thermal muonium in a vacuum. A unique sequence of radioactivity following the capture of a ${\mu }^{-}$ in tungsten was the signal for conversion. No events were seen, implying an upper limit on the effective four-fermion weak coupling constant for conversion of $0.88G_{\mathrm{F}}$ (90% confidence level), corresponding to a branching ratio of less than 2×${10}^{-5\mathrm{}}$ with respect to normal muon decay.