Search for spontaneous conversion of muonium to antimuonium

Abstract

We have searched for spontaneous conversion of muonium (M) to antimuonium ( M ) by a method involving detection of high- Z muonic X-rays. A beam of M atoms with keV energies, produced by electron pickup by μ + from a foil, travels in vacuum and in a magnetic field-free environment to a high- Z target. The events signatures used were a double coincidence of two muonic X-rays of the target material and a triple coincidence that also required detection of secondary electrons ejected when M strikes the target. Partial analysis of our 8 × 10 6 triggers indicates upper limits on the effective M → M four-fermion coupling constant of G M M ⩽ 30G F (90% C.L.) and G M M ⩽ 8G F (90% C.L.), respectively, from the two signatures. This begins to probe predictions of the left-rightsymmetric theory with a doubly-charged Higgs triplet.