Accurate Determination of theμ+Magnetic Moment

Abstract

Using a precession technique, the magnetic moment of the positive mu meson is determined to an accuracy of 0.007%. Muons are brought to rest in a bromoform target situated in a homogeneous magnetic field, oriented at right angles to the initial muon spin direction. The precession of the spin about the field direction, together with the asymmetric decay of the muon, produces a periodic time variation in the probability distribution of electrons emitted in a fixed laboratory direction. The period of this variation is compared with that of a reference oscillator by means of phase measurements of the "beat note" between the two. The magnetic field at which the precession and reference frequencies coincide is measured with reference to a proton nuclear magnetic resonance magnetometer. The ratio of the muon precession frequency to that of the proton in the same magnetic field is thus determined to be 3.1834\ifmmode\pm\else\textpm\fi{}0.0002. Using a re-evaluated lower limit to the muon mass, this is shown to yield a lower limit on the muon $g$ factor of 2(1.00122\ifmmode\pm\else\textpm\fi{}0.00008), in agreement with the predictions of quantum electrodynamics.