Large deflection scattering, soft radiation and KMOC formalism

Abstract

KMOC (Kosower, Maybee, and O'Connell) formalism is an approach to analyze classical scattering in gauge theories and gravity using a class of ``inclusive'' observables which can be computed solely from on-shell amplitudes \cite{Kosower:2018adc}. This formalism has led to striking developments in the context of perturbative scattering, which corresponds to large impact parameter scattering. As a result, in its current form, the KMOC formulae cannot be directly applied to processes for generic values of the impact parameter. However, there is a domain where the relationship between classical radiation and on-shell amplitudes can be stretched beyond large impact parameter scattering. This regime is defined by the soft expansion of outgoing radiation. It is thus natural to ask whether such soft radiative fields can be computed using the basic paradigm set by the KMOC formalism. In this short note, we show that this is indeed the case for electromagnetic memory. In particular, we compute an inclusive observable associated with soft flux at ${\cal I}^{+}$ and show that, irrespective of the details of the hard scattering, this observable defines a non-perturbative formula for the electromagnetic memory in the classical limit. We argue that the result obtained for electromagnetic memory using the KMOC paradigm is consistent with that of \cite{Laddha:2018rle}, where the classical limit of the quantum soft theorem was derived using saddle-point analysis. The gravitational case, however, is qualitatively different due to the presence of the nonlinear memory effect, which requires knowledge of the hard amplitude. Consequently, unlike the electromagnetic memory, we have not been able to show consistency of the leading soft graviton theorem and the soft inclusive gravitational flux obtained using the KMOC formalism.