New muonic He atom HFS measurements at J-PARC MUSE

Muonic He ( μ He) is a hydrogen-like atom composed of a helium atom with one of its electrons replaced by a negative muon. Measurements of the ground-state hyperfine structure (HFS) interval of the μ He atom is a sensitive tool to test the three-body atomic system, bound-state quantum electrodynamics (QED) theory, and determine fundamental constants of the negative muon magnetic moment and mass. It allows a test of the CPT theorem in muons by comparing these constants with positive muons. Precise measurements of the ground-state HFS interval using a microwave magnetic resonance technique are now in progress at J-PARC by the MuSEUM collaboration. Furthermore, a new experimental approach to recover the muon polarization lost during the muon cascade process is being investigated by repolarizing μ He atoms using a spin-exchange optical pumping (SEOP) technique. The first laser repolarization experiment was recently performed. An overview of the different features of these new μ He atom HFS measurements and the latest results are presented.