The Standard Model provides a very precise prediction of the muon’s magnetic anomaly aµ= (gµ - 2)/2, the deviation from 2 of the gyromagnetic ratio gµ. In his seminal 1926 paper, P.A.M. Dirac predicted that for electrons ge= 2, but experiments then revealed that gewas slightly larger than 2. The reason was to be found in Quantum Mechanics, and the first radiative correction to ge , calculated by Julian Schwinger, explained a deviation of order 0.1 %. Today, the Standard Model predicts the value of aµto a precision of ± 0.3 parts per million (ppm). Dedicated experiments have measured aμ to ± 0.54 ppm precision. Therefore, precision measurements of the anomaly provide a stringent test of the Standard Model’s completeness, since Nature knows about all forces that could contribute to the muon’s magnetism, including those from New Physics that has not yet been discovered.
I will review the intellectual history that began with the discovery of spin and the g-factor of the electron and its role on the development of Modern Physics. Then I will focus on the measurements of the muon magnetic anomaly, which include particle counting techniques, and nuclear magnetic resonance (NMR) around the 45 m circumference of the ring to monitor and measure the precision magnetic field. A new experiment with a precision goal of ± 0.14 ppm is now in progress now at Fermilab, which should clarify whether or not differences that have surfaced between theory and experiment are statistically significant at the five standard deviation level.