g-factor

The g-factor (or gyromagnetic ratio) is a dimensionless quantity characterizing the magnetic moment of a particle relative to its angular momentum. For the electron, the Dirac equation predicts $g=2$, but quantum electrodynamics (QED) corrections yield $g\approx 2.002319$. The deviation $a=(g-2)/2$ is the anomalous magnetic dipole moment.

The muon g-factor is exceptionally sensitive to contributions from virtual particles not included in the Standard Model, making it a critical probe for new physics.

Fermilab Muon g-2 Experiment

• Measures the muon’s anomalous magnetic moment via spin precession in a 14-meter diameter magnetic storage ring at Fermilab.
• Combined analysis of Brookhaven (2001–2005) and Fermilab (2017–2023) data shows a 4.2σ discrepancy with the Standard Model prediction.
• The 2025 results (final dataset) confirmed the anomaly, with $a_{\mu }=116592061(41)\times 10^{-11}$ versus Standard Model $a_{\mu }=116591810(43)\times 10^{-11}$.
• This tension suggests potential new physics, such as undiscovered particles or forces beyond the Standard Model.

2026 04 14 Fermilabs Muon g 2 measurement