New Precise Value for the Muon Magnetic Moment and Sensitive Test of the Theory of the hfs Interval in Muonium

Measurements of Zeeman transitions in the ground state of muonium at strong magnetic field have yielded values for the hfs interval, $\Delta \nu =4463\mathrm{}302.35\left(52\right)\mathrm{}$ kHz (0.12 ppm) and for the muon magnetic moment, $\frac{{\mu }_{\mu }}{{\mu }_p}=3.183\mathrm{}3403\left(44\right)\mathrm{}$ (1.4 ppm), of considerably higher precision than previous results. The theoretical expression for $\Delta \nu$, including our measured value of $\frac{{\mu }_{\mu }}{{\mu }_p}$, disagrees with the experimental value by 2.5 standard deviations. The electronic $g_J$ density shift for muonium in Kr has been measured.