First observation of the negative muonium ion produced by electron capture in a beam-foil experiment

The negative muonium ion Mu^-{≡}μ⁺{e}^{{{-}}}e^- has been produced by double electron pickup as a beam of positive muons passes through a foil. A clean signal was detected by electrostatically accelerating and analyzing the ions in a magnetic time-of-flight spectrometer. The Mu^- beam at the focal plane of our spectrometer had an intensity 1.4×{}10^-5 times that of the incident μ⁺ beam or 2 s^-1, which corresponds to a Mu^-/μ⁺ conversion probability of 0.5×{}10^-4 at the foil in agreement with the calculated value. The kinetic energies of Mu^- at the foil are less than several keV as expected.