Microwave Spectroscopy of the Muonium 2S1/2-2P3/2 Fine Structure Interval

We report a microwave spectroscopy measurement of the muonium 2S1/2-2P3/2 fine structure transition, yielding a transition frequency of 9871.0 ± 7.0~MHz, in agreement with state-of-the-art QED predictions within one standard deviation. In combination with the recent Lamb shift result, the 2P1/2-2P3/2 splitting is determined, improving the spectroscopic characterization of the $n=2$ manifold in muonium. These results provide a stringent test of bound-state QED in a purely leptonic system and establish a path toward future searches for Lorentz violation and muon-specific new physics.