muonium MuSEUM High-Field Precise spectroscopy of the muonium HFS under a high magnetic field MuSEUM Zero-Field Muonium HFS spectroscpy under a near-zero magnetic field muonic helium Muonic Helium HFS High-precision measurements of the muonic helium HFS Muonic Helium SEOP Spin exchange optical pumping for muonium helium repolarization