First Observation of the Ground-State Hyperfine-Structure Resonance of the Muonic Helium Atom

The first measurement of the hfs interval ..delta nu.. for the muonic helium atom (/sup 4/He/sup + +/..mu../sup -/e/sup -/)/sup 0/ is reported. In terms of its electronic structure, it is a heavy isotope of hydrogen. Polarized atoms are formed by stopping polarized negative muons in a helium-gas target at 19.4 atm with a 1.5% admixture of Xe. The ground-state hfs splitting ..delta nu.. was measured through observation of a microwave magnetic-resonance transition at zero magnetic field. After ccorrection for the hfs pressure shift, we determine ..delta nu..=4464.95(6) MHz (13 ppm).