Magnetic Moment of the Free Muon

The magnetic moment of the positive $\mu$ meson has been measured in several target materials by a magnetic resonance technique. Muons were brought to rest with their spins parallel to a magnetic field. A radio-frequency pulse was applied to effect a spin reorientation which was detected by counting the decay electrons emerging after the pulse in a fixed direction. Results are expressed in terms of a $g$ factor which for a spin 1/2 particle is the ratio of the actual moment to $\frac{e\hslash }{2m_{\mu }c}$. The most accurate result obtained in a CH${\mathrm{Br}}_3$ target, is that $g=2(1.0026\mathrm{}\pm 0.0009)$ compared to the theoretical prediction of $g=2\mathrm{}\left(1.0012\right)\mathrm{}$. Less accurate measurements yielded $g=2.005\mathrm{}\pm 0.005$ in a copper target and $g=2.00\mathrm{}\pm 0.01$ in a lead target.