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 ${\mathit{e}}^{\mathrm{-}}$ and the atomic ${\mathit{e}}^{+}$. No conversion events were seen, which sets an improved upper limit of 6.5×${10}^{\mathrm{-}7}$ (90% C.L.) on the conversion probability. The corresponding limit on the coupling constant, using a V-A form for the conversion Hamiltonian, is ${\mathit{G}}_{\mathit{M}\mathit{M}\mathrm{¯}}$<0.16${\mathit{G}}_{\mathit{F}}$ (90% C.L.), where ${\mathit{G}}_{\mathit{F}}$ is the Fermi coupling constant.