Search for muonium-to-antimuonium conversion

A new measurement is described which sets an upper limit on the coupling constant for conversion of muonium (${\mu }^{+}{e}^{-}$) to antimuonium (${\mu }^{-}{e}^{+}$) of $42G_F$, where $G_F$ is the Fermi coupling constant. Muonium atoms were formed in layers of fine silica powder separated by vacuum drift regions. The presence of ${\mu }^{-}$ was searched for using the signature of a Ca $2P-1S$ muonic x ray from calcium oxide layers adjacent to the drift regions. Results were also obtained with an Ar target.