In-medium Electromagnetic Form Factors and Spin Polarizations

TL;DR

This paper develops a framework linking fermion spin to electromagnetic fields via form factors, showing how medium effects lift degeneracies in spin couplings, with calculations of in-medium QCD corrections at one-loop order.

Contribution

It introduces a formulation of spin-electromagnetic coupling using form factors and demonstrates medium-induced degeneracy lifting through one-loop QCD corrections.

Findings

Vacuum form factors reproduce spin polarization effects.

Degeneracy in spin couplings is lifted in medium.

In-medium corrections differentiate magnetic field couplings.

Abstract

We formulate the coupling between fermion spin and background electromagnetic fields using form factors. We show that the vacuum form factors at tree level reproduce the spin polarization effects found in chiral kinetic theory. The vacuum form factors corresponding to spin couplings to perpendicular electric field, parallel and perpendicular magnetic field are degenerate. The degeneracy is expected to be lifted in medium. As an example, we calculate the in-medium QCD radiative correction to the form factors at one-loop order, where we find partial lift of the degeneracy: the spin couplings to parallel and perpendicular magnetic field are different, but the spin couplings to perpendicular electric and parallel magnetic field remain the same.