Electron Spin

Does the vacuum affect the spin of a particle such as an electron?

Answer

Yes. Although the vacuum influence on the electron spin is extremely small, the same effect of the vacuum on the muon’s spin has been measured at Brookhaven National Laboratory. The interaction magnitude is predicted by the Standard Model (SM) of Leptons and Quarks and their interactions. All fundamental particle-antiparticle pairs momentarily appear in the vacuum and disappear sporadically, so the electron (and muon) see them all, if only for a fleeting moment. This vacuum “soup” is slightly magnetic, so it increases the magnetic moment of the electron or muon to g = 2 (1 + a). The small correction of about 0.12 percent is called the anomalous moment but is often referred to as “g-2.” Its measurement with gradually increasing accuracy presents spectacular agreement with calculation to better than 24 parts per billion.

The muon is 206 times heavier than the electron, so the muon’s magnetic moment is 206 times smaller, but the virtual particles in the quantum soup can be more massive. As a result, the anomalous moment is 40,000 times more sensitive to undiscovered particles and new physics at short distances. There is agreement to 4 parts per million that must be regarded as the best test of the theory, but there is also a small discrepancy that needs to be explained, a difference in mean values of the experiments and the theory by 2.6 standard deviations.

The muon g-2 result cannot at present be explained by the established SM. Recalculations of the predicted theoretical value continue, and corrections have been done. Moreover, the g-2 calculation involves three of the four fundamental interactions weak, electromagnetic, and color so there are many Feynman diagrams that contribute.

Perhaps this unresolved g-2 difference is the harbinger of new physics beyond the SM, such as new quarks, or supersymmetric particles, or a surprise in the vacuum.