Search for mixing of muonium and antimuonium

A search is described for mixing of muonium (${\mu }^{+}{e}^{-}$) and antimuonium (${\mu }^{-}{e}^{+}$). Thermal muonium was produced by stopping muons in a Si${\mathrm{O}}_2$ powder target. As a conversion signature, a ${\mu }^{-}$ from antimuonium would create ${}_{}{}^{184}{}_{}{}^{}\mathrm{Ta}$ in an adjacent tungsten foil. The surface layer of the sample was chemically extracted and counted in a low-background germanium spectrometer; no conversion events were observed. The resulting upper limit on the probability that a muonium atom spontaneously converts to antimuonium is 2.1×${10}^{-6\mathrm{}}$ (90% confidence). This corresponds to a limit of $0.29G_F$ on the effective four-fermion coupling constant between muonium and antimuonium.