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A muonium particle is an exotic atom made up of a positive muon and an electron, and is given the symbol Mu or μ+e. During the muon's 2 microsecond lifetime, muonium can enter into compounds such as muonium chloride (MuCl) or sodium muonide (NaMu) [1].

Due to the mass difference between the muon and the electron, muonium is more similar to atomic hydrogen than positronium. Muonium has been used to produce muon-catalyzed fusion in which muons shield the positive charge of the nuclei so that the nuclei can fuse.