The Eikonal Phase and Spinning Observables

We set up the Kosower, Maybee, O'Connell formalism for generic spin following a previous field theory construction and use it to extract a next-to-leading order eikonal formula for the linear-in-spin momentum impulse, $\Delta p^{\mu}$, and spin kick, $\Delta S^{\mu \nu}$. We explicitly test the formalism for the case of electrodynamics and find agreement with previous QFT and worldline results. This includes the case where multiple spins propagate with allowed spin transitions, or equivalently where no spin supplementary condition is imposed on the worldline equations of motion.