Rabi-Oscillation Spectroscopy of the Hyperfine Structure of the Muonium Atom for Fundamental Physics

Muonium (Mu) is a hydrogenlike atom comprising a positive muon and an electron, both pointlike leptons. Spectroscopy of Mu is a promising method in the search for new physics in particle-physics research. Microwave spectroscopy of the Mu hyperfine structure (HFS) gives a test of QED and provides the most precise estimation of the magnetic moment and the mass of the muon.

MuSEUM Collaboration is studying Mu-HFS at the J-PARC muon facility in Japan, aiming at a precision one order of magnitude better than that in the last experiment performed in 1990s at Los Alamos. In addition to our technical hardware developments to reduce systematic uncertainties, we have made significant improvements in the data analysis. In contrast to standard spectroscopic analysis which determines the resonance center frequency by fitting the resonance curve, our new spectroscopic technique does not require any frequency sweep. The resonance frequency can be obtained directly in the time domain without Fourier transformation, by fitting a simulated function to the time evolution of the Rabi oscillation at a fixed microwave frequency. This technique, named Rabi-oscillation spectroscopy, can improve the precision by eliminating systematic uncertainties due to power fluctuations.

Current achievements and near-future strategies of our experiments will be presented. This new spectroscopy will be a powerful tool for the study of other exotic atoms, and also applicable to short-lived systems like radioactive nuclei.