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 {ital e}{sup {minus}} and the atomic {ital e}{sup +}. No conversion events were seen, which sets an improved upper limit of 6.5{times}10{sup {minus}7} (90% C.L.) on the conversion probability. The corresponding limit on the coupling constant, using a {ital V}{minus}{ital A} form for the conversion Hamiltonian, is {ital G}{sub {ital M}{ital {bar M}}}{lt}0.16{ital G}{sub {ital F}} (90% C.L.), where {ital G}{sub {ital F}} is the Fermi coupling constant.