New Representations of the Perturbative S-Matrix

TL;DR

This paper introduces a novel, all-encompassing framework for representing the perturbative S-matrix in quantum field theories with massless particles, utilizing tree-level amplitudes and integrable terms, extending to all orders.

Contribution

It develops a new S-matrix representation derived from Feynman expansions, involving Q-cuts and a contour prescription, applicable to all orders and non-planar theories.

Findings

Framework reproduces recent scattering equation results at one-loop.

Applicable to non-planar theories without forward limits.

Expresses loop integrands in terms of off-shell and on-shell momenta.

Abstract

We propose a new framework to represent the perturbative S-matrix which is well-defined for all quantum field theories of massless particles, constructed from tree-level amplitudes and integrable term-by-term. This representation is derived from the Feynman expansion through a series of partial fraction identities, discarding terms that vanish upon integration. Loop integrands are expressed in terms of "Q-cuts" that involve both off-shell and on-shell loop-momenta, defined with a precise contour prescription that can be evaluated by ordinary methods. This framework implies recent results found in the scattering equation formalism at one-loop, and it has a natural extension to all orders---even non-planar theories without well-defined forward limits or good ultraviolet behavior.