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Abstract

We study the (ambi-)twistor model for spinning particles interacting via
electromagnetic field, as a toy model for studying classical dynamics of
gravitating bodies including effects of both spins to all orders. We compute
the momentum kick and spin kick up to one-loop order and show precisely how
they are encoded in the classical eikonal. The all-orders-in-spin effects are
encoded as a dynamical implementation of the Newman-Janis shift, and we find
that the expansion in both spins can be resummed to simple expressions in
special kinematic configurations, at least up to one-loop order. We confirm
that the classical eikonal can be understood as the generator of canonical
transformations that map the in-states of a scattering process to the
out-states. We also show that cut contributions for converting worldline
propagators from time-symmetric to retarded amount to the iterated action of
the leading eikonal at one-loop order.