Infrared singularities and the collinear limits of multi-leg scattering amplitudes

TL;DR

This paper investigates the structure of multi-particle collinear limits in massless scattering amplitudes at high perturbative orders, demonstrating conditions for factorisation and deriving new constraints on the soft anomalous dimension.

Contribution

It provides a detailed analysis of collinear factorisation at three- and four-loop orders, establishing conditions that ensure factorisation in multi-collinear limits and extending these results to massive particles.

Findings

Strict collinear factorisation is realized at four loops for multiple particles.

Conditions on the soft anomalous dimension ensure factorisation in all two-particle collinear limits.

New constraints on the soft anomalous dimension are derived assuming factorisation extends to massive particles.

Abstract

Scattering amplitudes are expected to admit a factorised structure in special kinematic limits, such as the Regge, soft and collinear limits. However, less is known about the precise mechanisms through which factorisation of $n$-particle scattering amplitudes is realised at high perturbative orders, where more complex structures arise. Starting with the soft anomalous dimension, in this work we investigate the multi-particle collinear limits of massless amplitudes at three- and four-loop orders. Using colour conservation and rescaling symmetry, we show how strict collinear factorisation of multiple massless final-state coloured particles is realised, and provide results for the corresponding splitting amplitude soft anomalous dimensions. In particular, we demonstrate through four loops that the conditions on the structure of the soft anomalous dimension that are required by strict collinear factorisation in all two-particle collinear limits, are sufficient to guarantee such factorisation also in any multiple collinear limit. Then, assuming that strict collinear factorisation of massless partons holds also for amplitudes containing massive coloured particles, we derive new constraints on the soft anomalous dimension from multi-collinear limits.