  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.
