Observation of resonance line narrowing for old muonium

Resonance line narrowing up to 1/2 of the natural linewidth has been observed for microwave magnetic-resonance transitions between Zeeman levels of ground-state muonium at a strong magnetic field of 1.7 T. The observed lines are in good agreement with predicted line shapes and are useful for a precision determination of Δν and ${\mathrm{\mu }}_{\mathrm{\mu }}$/${\mathrm{\mu }}_{\mathit{p}}$.