Direct Measurement of Muonium 1S-2S Frequency: Vacuum vs 1 atm – Precision Gas-Effect Test

Proposal for a direct laser spectroscopy measurement of the muonium 1S-2S transition frequency (1058.9 nm, 282 THz) in ultra-high vacuum (≤10⁻¹² torr) versus 1 atm air. Targets ~10⁻¹⁰ Hz shift using split-beam interferometry and lock-in amplification for phase-lag detection. Feasible with PSI Mu-MASS or J-PARC facilities, frequency combs, and controlled gas injection. Minimizes collisions, thermal drift, and residuals via quadratic extrapolation and UHV isolation. Raw empirical test—no theoretical interpretation.