Search for a light dark photon in muonium decay

About 0.85 × 105 events involving stopped μ + and muonium decay were observed in a nuclear photoemulsion with the aim of searches for a light dark photon (DPh) in the decay process DPh → e + e −. With a probability of about 10−5, no event of the decay μ+ → е + ν μ ν е accompanied by an electron–positron pair was observed. In the interval of 1.1 MeV ≤ m DPh ≤ 60 MeV, the mixing parameter e 2 was estimated at about 10−5 to 10−4.