A new search for conversion of muonium to antimuonium

To search for conversion of muonium (M ≡ μ+e-) to antimuonium (anti M ≡ μ-e+) with very low background, a new signature was implemented that required the time-coincident detection of the decay e-(≤ 53 MeV) with the atomic e+(∼ 13 eV) from decay of an anti M atom. A 20 MeV/c μ+ beam was stopped in a 9 mg/cm2 SiO2 powder target. Muonium, formed in the powder, diffused into a vacuum region at thermal velocities and was observed for a coincidence of anti M decay products. Any decay e- was charge and momentum analyzed in a dipole magnet and tracked by an array of MWPCs; any atomic e+ was electrostatically collected, accelerated to 5.7 keV, and magnetically transported to a microchannel plate detector. To calibrate the signature, M was observed for the first time by coincidence of its decay e+ and its atomic e-. A maximum likelihood analysis of the position distribution of decay origins finds no anti M events and less than 2 at 90% confidence. This places an upper limit on the conversion probability per atom of S anti M < 6.5 x 10-7 (90% C.L.), which corresponds to an upper limit of GM anti M < 0.16 GF (90% C.L.) on the effective coupling constant for a (V - A) conversion coupling. In a class of left-right symmetric models, the value of GM anti M may be in this range. 116 refs., 45 figs., 10 tabs