Direct measurement of muonium hyperfine splitting at J-PARC

Muonium is a bound state of a positive muon and an electron. It is an ideal two-body system for the precision test of bound-state theory and fundamental symmetries. The ground state hyperfine splitting (HFS) of muonium provides the most rigorous test of bound-state QED theory and the most precise method to determine the muon-to-electron mass ratio. The measurement precision of the muonium HFS was mostly statistically limited because the continuous muon beam was utilized. At J-PARC, a new precision spectroscopy of the muonium HFS was performed with the high-intensity pulsed muon beam and the high-rate capable positron detector. As a result of the experiment, the muonium HFS resonance was observed and the muonium hyperfine structure interval of ∆ν = 4.463292(22) GHz was obtained with the relative precision of 4.9 ppm. The result was consistent with the previous ones obtained at Los Alamos National Laboratory with the continuous muon beam. This article reports on the new measurement at J-PARC and its first result.