Anomalous magnetic moment, virtual loops, and a hint of new physics
The anomalous magnetic moment of the muon, aμ = (g−2)/2, is one of the most precisely measured quantities in physics. While Dirac's equation predicts g=2 exactly, quantum field theory corrections from virtual particle loops shift this value. The Fermilab Muon g−2 experiment measures aμ = 116592061(41)×10⁻¹¹, showing a ~4.2σ tension with the Standard Model prediction — possibly a sign of undiscovered particles or forces.