Factorization constraints for soft anomalous dimensions in QCD scattering amplitudes

TL;DR

This paper investigates the structure of soft and collinear singularities in massless gauge theory scattering amplitudes, revealing symmetry constraints and proposing a minimal dipole-based form for the soft anomalous dimension.

Contribution

It introduces a symmetry-based framework for factorizing singularities and constrains the soft anomalous dimension's form, especially for multi-leg amplitudes, including a minimal dipole solution.

Findings

Soft and collinear singularities can be factorized using Wilson lines.

Rescaling symmetry constrains the soft anomalous dimension.

For four or more legs, the soft anomalous dimension is a sum over dipoles, with possible higher-order corrections constrained by conformal cross ratios.

Abstract

We study the factorization of soft and collinear singularities in dimensionally-regularized fixed-angle scattering amplitudes in massless gauge theories. Our factorization is based on replacing the hard massless partons by light-like Wilson lines, and defining gauge-invariant jet and soft functions in dimensional regularization. In this scheme the factorized amplitude admits a powerful symmetry: it is invariant under rescaling of individual Wilson-line velocities. This symmetry is broken by cusp singularities in both the soft and the eikonal jet functions. We show that the cancellation of these cusp anomalies in any multi-leg amplitude imposes all-order constraints on the kinematic dependence of the corresponding soft anomalous dimension, relating it to the cusp anomalous dimension. For amplitudes with two or three hard partons the solution is unique: the constraints fully determine the kinematic dependence of the soft function. For amplitudes with four or more hard partons we present a minimal solution where the soft anomalous dimension is a sum over colour dipoles, multiplied by the cusp anomalous dimension. In this case additional contributions to the soft anomalous dimension at three loops or beyond are not excluded, but they are constrained to be functions of conformal cross ratios of kinematic variables.