New Muonic Helium Atom HFS Measurements at J-PARC MUSE

Muonic helium (μHe) is a hydrogen-like atom composed of a helium atom with one of its

electrons replaced by a negative muon. Its ground-state hyperfine structure (HFS), from the

interaction of the remaining electron and the negative muon magnetic moment, is very similar

to muonium (bound state of a positive muon and an electron). Measurements of the μHe-

HFS interval are 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. We can verify the CPT symmetry in muons by comparing these

constants with positive muons.

Precise measurements of the muonium ground-state HFS interval using a microwave

magnetic resonance technique are now in progress at J-PARC by the MuSEUM collaboration.

The same method can be used to precisely determine the μHe-HFS and the negative muon

magnetic moment and mass.

We have successfully measured μHe-HFS at zero field using CH4 as an electron donor at J-

PARC MUSE D-line with better accuracy than previous studies. We are now planning high-

field measurements using the world's most intense pulsed negative muon beam at MUSE

and muon repolarization by SEOP technique. The latest results and future plans will be

outlined.