Multi-soft theorems in Gauge Theory from MHV Diagrams

TL;DR

This paper uses MHV diagrams to analyze multi-soft limits in gauge theories, deriving universal behaviors and explicit formulas for various soft particle configurations, including scalars, fermions, and gluons in supersymmetric contexts.

Contribution

It provides a systematic method to compute multi-soft factors in gauge theories using MHV techniques, extending known results to multiple particles and different helicity configurations.

Findings

Derived universal multi-soft theorems for gluons and fermions.

Explicit formulas for multi-soft factors with various helicity and particle configurations.

Identified leading divergence behaviors in double-soft limits.

Abstract

In this work we employ the MHV technique to show that scattering amplitudes with any number of consecutive soft particles behave universally in the multi-soft limit in which all particles go soft simultaneously. After identifying the diagrams which give the leading contribution we give the general rules for writing down compact expressions for the multi-soft factor of m gluons, k of which have negative helicity. We explicitly consider the cases where k equals 1 and 2. In N =4 SYM, the multi-soft factors of 2 scalars or 2 fermions forming a singlet of SU(4) R-symmetry, and m-2 positive helicity gluons are derived. The special case of the double-soft limit gives an amplitude whose leading divergence is 1/\delta^2 and not 1/\delta as in the case of 2 scalars or 2 fermions that do not form a singlet under SU(4). The construction based on the analytic supervertices allows us to obtain simple expressions for the triple-soft limit of 1 scalar and 2 positive helicity fermions, as well as for the quadrapole-soft limit of 4 positive helicity fermions, in a singlet configuration.