Rabi-oscillation spectroscopy of the hyperfine structure of the muonium atom for high-precision determination of the muon mass

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, superior to that of hydrogen for which the uncertainty in the proton radius and its internal structure gives limitations. Microwave spectroscopy of the Mu hyperfine structure (HFS) provides the most precise estimation of the magnetic moment and the mass of the muon, opening a way to search for a physics beyond the Standard Model. MuSEUM Collaboration is studying Mu-HFS at the J-PARC muon facility in Japan, aiming at a precision one order of magnitude better than before. With our new spectroscopic technique which does not require any frequency sweep or Fourier transformation, the resonance frequency can be obtained directly by fitting a simulated function to the time evolution of the Rabi oscillation at a fixed microwave frequency. This method, named Rabi-oscillation spectroscopy, can improve the precision by eliminating systematic uncertainties due to power fluctuations. After a decade of our study under zero magnetic field with fruitful results, we are about to start our experiment under a high magnetic field.