Search for a light dark photon in muonium decay

Abstract

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_μν_е accompanied by an electron–positron pair was observed. In the interval of 1.1 MeV ≤ m_DPh ≤ 60 MeV, the mixing parameter ε² was estimated at about 10⁻⁵ to 10⁻⁴.