High precision measurements of two Zeeman hyperfine transitions in the ground state of muonium in a strong magnetic field have been made at LAMPF using microwave magnetic resonance spectroscopy and a resonance line narrowing technique. These determine the most precise values of the ground state hyperfine structure interval of muonium $\Delta \nu =4\mathrm{}463302765\left(53\right)\mathrm{Hz}$ $\left(12\mathrm{ppb}\right)$, and of the ratio of magnetic moments ${\mu }_{\mu }/{\mu }_p=3.183\mathrm{}34513\left(39\right)$ $\left(120\mathrm{ppb}\right)$, representing a factor of 3 improvement. Values of the mass ratio $m_{\mu }/m_e$ and the fine structure constant $\alpha$ are derived from these results.

• Received 21 August 1998

DOI:https://doi.org/10.1103/PhysRevLett.82.711