Marking the semi-anniversary of the Fermilab Muon g−2 experiment (E989) Run 1 result, this colloquium will review the current status of the muon magnetic anomaly, the inferred evidence of possible particles outside the Standard Model (SM), and future prospects in this active research field.
The intrinsic magnetic field of a simple object, such as a compass needle, is expressed in terms of its magnetic moment. The magnetic moment of a point particle, such as the electron, is predicted by relativistic quantum mechanics to be g = 2, in convenient dimensionless units. For the electron, this prediction fails at the part-per-thousand level; the resulting magnetic anomaly, ae = (g − 2)/2, is due to the electron’s couplings to virtual particles excited in the vacuum.
Muon, the electron’s 200 times heavier cousin, experiences far stronger couplings to massive virtual particles, including possible non-SM exotics. The SM provides a prediction for the muon magnetic anomaly aµ with sub-ppm precision. Hence, a comparably precise measurement of aµ offers a uniquely sensitive test for the presence of non-SM particles in nature. For almost 20 years a tantalizing discrepancy of ∼ 3 – 4σ has persisted between the measurements of aµ, and the SM calculations. The Fermilab Muon g−2 Run 1 result brings a much awaited update to this test, with much more to come.