Search for mixing of muonium and antimuonium

A search is described for mixing of muonium ({mu}{sup +}{ital e}{sup {minus}}) and antimuonium ({mu}{sup {minus}}{ital e}{sup +}). Thermal muonium was produced by stopping muons in a SiO{sub 2} powder target. As a conversion signature, a {mu}{sup {minus}} from antimuonium would create {sup 184}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{times}10{sup {minus}6} (90% confidence). This corresponds to a limit of 0.29{ital G}{sub {ital F}} on the effective four-fermion coupling constant between muonium and antimuonium.