Observation of Muonium in Vacuum

The first observation of muonium (μ⁺e^-) in vacuum, produced when a low-energy positive muon beam impinges on a thin foil target, is reported. It is found that μ⁺e^- emerges from the foil surface predominantly with energies of less than 20 keV. The results show that the Clinton P. Anderson Meson Physics Facility stopped-muon channel operating at $E=4.0$ MeV with an average flux of 3×{}10⁶{{{μ}}}⁺$/s can produce roughly ${10}³μ⁺{e}^{{-}}$/s in vacuum from thin targets of Be, C, Al, Cu, Ag, and Au.