Search for mixing of muonium and antimuonium

A search is described for mixing of muonium (μ⁺e^-) and antimuonium (μ^-e⁺). Thermal muonium was produced by stopping muons in a SiO₂ powder target. As a conversion signature, a μ^- from antimuonium would create ¹⁸⁴Ta 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 (90% confidence). This corresponds to a limit of 0.29GF on the effective four-fermion coupling constant between muonium and antimuonium.