Production of Muonium in Vacuum.

Experiments relating to the discovery of the muonium atom in vacuum at energies above the thermal range are described. Muonium is a purely leptonic atom of fundamental interest. Tests of quantum electrodynamics and muon-electron symmetry are facilitated by producing muonium in a vacuum. This search for muonium in vacuum is described in two parts; an initial search for thermal muonium using the Larmor precession technique and a subsequent search employing a beam-foil method sensitive to energies less than 30 KeV. The low momentum positive muon beam at the Los Alamos Meson Physics Facility was used. With the search described in part one, muons were incident on an array of thin foils. In the target region muons and any muonium precess at different rates in a transverse magnetic field. Muonium precession is evidenced by a more rapidly oscillating asymmetry in the decay positron distribution. In all cases only free muon asymmetries were observed with an experimental limit of 5% for the fraction of incident muons forming muonium. Since this spin precession method is sensitive only to polarized, thermal muonium it was necessary to perform a less restrictive search for muonium in vacuum. More energetic muonium in vacuum was discovered with the beam-foil method. Primary muons were incident on the following single foil targets: Be, C, Al, Cu, Ag, and Au. The neutral muonium component of the secondary beam emerging from the foil was isolated with a magnetic field deflecting charged components. A NaI(T1) crystal was used downstream to record the Michel energy spectrum of positrons associated with muon decay. Both He studies and a Monte Carlo simulation show that the muonium kinetic energy is approximately less than 20 KeV. The efficiency of conversion to vacuum muonium was determined to be about 0.04%. This conversion efficiency is effectively integrated over the broad secondary beam energy distribution with mean energy near 1 MeV and over atomic energies without regard to muon polarization. Near isotropic multiple scattering of muonium by the foil limited detection efficiency. Vacuum muonium formation is apparently analagous to that of hydrogen using primary protons. Conversion efficiencies to hydrogen decrease strongly with increasing proton velocity. Our muonium formation results are consistent with proton data scaled for equal velocity. Production of the negative muonium ion and development of metastable muonium beam are considered.