New search for the spontaneous conversion of muonium to antimuonium

To search for spontaneous conversion of muonium to antimuonium with very low background, a new signature was implemented that required the coincident detection of the decay products of the antimuonium atom, the energetic e^- and the atomic e⁺. No conversion events were seen, which sets an improved upper limit of 6.5×{}10^-7 (90% C.L.) on the conversion probability. The corresponding limit on the coupling constant, using a V-A form for the conversion Hamiltonian, is G_MM0.16GF (90% C.L.), where GF is the Fermi coupling constant.