Muonium (Mu) is a hydrogenlike exotic atom composed of a positive muon and an electron ( \( \mu ^+\) e \( ^-\) ), and is a suitable probe with its hyperfine structure (HFS) for a precise determination of the magnetic moment and thus the mass of the muon, a lepton particle, which leads to a rigorous test of bound-state quantum electrodynamics (QED) and weak interactions as well as a search for any possible new physics beyond the Standard Model of particle physics. MuSEUM (Muonium Spectroscopy Experiment Using Microwave) Collaboration has so far succeeded in measuring the ground-state HFS of the Mu atom under the zero magnetic field, and is now aiming at a precision of 2 ppb ( \(2 \times 10^{-9}\) ) under strong magnetic fields to surpass the last world record, using a newly operational high-intensity muon beam line at J-PARC in Japan. In contrast to conventional spectroscopy which determines the resonance center frequency by fitting the resonance curve, our new technique named Rabi-oscillation spectroscopy does not require any frequency scanning, eliminating systematic uncertainties due to possible power fluctuations. The resonance frequency can be obtained directly from the time evolution of the Rabi oscillation at a fixed frequency of the applied electromagnetic wave. We are also studying the muonic helium atom ( \( \mu ^- \alpha \) e \( ^-\) ), which is also hydrogenlike with a pseudo-proton composed of a negative muon and a helium nucleus ( \( \mu ^- \alpha \) ), using the same method of hyperfine spectroscopy. We have very recently started our experiments for Mu under a strong and highly uniform magnetic field of 1.7 T produced by a superconducting solenoid, with satisfying spectroscopic results. Current achievements and near-future strategies of our experiments are presented.
