Formation of the muonic helium atom

A full description is given of an experiment in which the muonic helium atom, ⁴Heμ^-e^-, was first observed. The method of the experiment was to study the precession of the magnetic moment of negative muons stopped in 14-atm helium with a 2% Xe admixture. A signal at the characteristic Larmor frequency of the atom was observed with an amplitude which corresponds to a muon residual polarization of 5.0±{}0.7%. In pure helium at 14 atm, precession characteristic of free muons with a polarization of 5.8±{}1.3% was found. The hyperfine structure of ⁴Heμ^-e^- and the various processes involved in the formation of this atom are also discussed.