THE BROOKHAVEN MUON ANOMALOUS MAGNETIC MOMENT EXPERIMENT

The E821 Experiment at the Brookhaven Alternating Gradient Synchrotron has measured the muon anomalous magnetic moment aμ to a relative precision of 0.5 parts per million. This effort required a new beamline, a super-ferric muon storage ring with a highly uniform magnetic field, a precision magnetic field measurement system, and electromagnetic calorimeters to record the electrons from muon decay, which carry the essential spin precession frequency information. Data obtained over five years resulted in more than nine billion analyzed events, in nearly equal samples of both muon charges. The experimental results aμ⁺ = 11 659 203(8) × 10⁻¹⁰ and aμ⁻ = 11 659 214(9) × 10⁻¹⁰ are consistent with each other, as predicted by the CPT invariance theorem. The combined result aμ ± = 11 659 208(6) × 10⁻¹⁰ is 0.9–2.4 standard deviations higher than predicted by theory; the range depends on the method employed to obtain the hadronic vacuum polarization term in the standard-model calculation. We review the experimental design, physical realization, and analysis procedures and compare the results to the theoretical prediction.