Soft Constraints on KMOC Formalism

TL;DR

This paper explores how classical soft theorems impose constraints on the KMOC formalism for deriving classical gauge and gravity observables from scattering amplitudes, confirming consistency with soft photon theorems up to NLO.

Contribution

It demonstrates that classical soft theorems lead to an infinite hierarchy of constraints within the KMOC formalism, extending understanding of soft radiation effects in gauge theory and gravity.

Findings

Soft electromagnetic fields impose infinite constraints on observables.

Quantum amplitude soft radiation matches classical soft photon theorem up to NLO.

In 4D, log soft theorems generate constraints involving angular momentum and momenta.

Abstract

In this note, we investigate the implications of classical soft theorems for the formalism developed by Kosower, Maybee and O'Connell (KMOC) to derive classical observables in gauge theory and gravity from scattering amplitudes. In particular, we show that the radiative electro-magnetic field at leading order in the soft expansion imposes an infinite hierarchy of constraints on the expectation value of the family of observables generated by \textit{monomials} of linear impulse. We perform an explicit check on these constraints at next to leading order (NLO) in the coupling and as a corollary show how up to NLO, soft radiation obtained from quantum amplitudes is consistent with the (leading) classical soft photon theorem. We also argue that in 4 dimensions the classical log soft theorem derived by Saha, Sahoo and Sen generates an infinite hierarchy of constraints on the expectation value of operators which are products of one angular momentum and an arbitrary number of linear momenta.