Magnetic Moment of the Free Muon

Abstract

The magnetic moment of the positive $\ensuremath{\mu}$ 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 $\frac{e\ensuremath{\hbar}}{2{m}_{\ensuremath{\mu}}c}$. The most accurate result obtained in a CH${\mathrm{Br}}_{3}$ target, is that $g=2(1.0026\ifmmode\pm\else\textpm\fi{}0.0009)$ compared to the theoretical prediction of $g=2(1.0012)$. Less accurate measurements yielded $g=2.005\ifmmode\pm\else\textpm\fi{}0.005$ in a copper target and $g=2.00\ifmmode\pm\else\textpm\fi{}0.01$ in a lead target.