  We show that the on-shell effective theory (OSEFT) is the quantum field
theory counterpart of a Foldy-Wouthuysen diagonalization of relativistic
quantum mechanics for massless fermions. Thus, it is free of the Zitterbewegung
oscillations that would yield an ill-defined meaning to the semiclassical
transport approach at short distances if derived from the pure Dirac picture.
We present a detailed derivation of the collision terms in the chiral kinetic
theory using the OSEFT. Collision integrals are derived up to order 1/E, where
E is the energy of an on-shell fermion. At this order, the collision terms
depends on the spin tensor of the fermion, and in the presence of chiral
imbalance, it describes how a massless fermion of a given helicity interacts
differently with the transverse photons of different circular polarization. In
order to back up our results, we check that they allow us to reproduce the
fermion decay rate in an ultradegenerate plasma with a chiral imbalance
computed directly from QED.
