Search for spontaneous conversion of muonium to antimuonium

We have searched for spontaneous conversion of muonium (M) to antimuonium (anti 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 event 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 8x106 triggers indicates upper limits on the effective M → anti M four-fermion coupling constant of Gsub(Manti M) < or approx. 30 GF (90% C.L.) and Gsub(Manti M) ≤ 8 GF (90% C.L.), respectively, from the two signatures. This begins to probe predictions of the left-right symmetric theory with a doubly-charged Higgs triplet. (orig.)