Infinite Soft Theorems from Gauge Symmetry

TL;DR

This paper demonstrates that the soft behavior of photons and gravitons can be fully determined to infinite order using gauge invariance, reproducing recent results from large gauge transformation Ward identities.

Contribution

It shows that infinite soft theorems can be derived from gauge invariance through a specific projection, including modifications from higher-dimensional operators.

Findings

Infinite soft theorems derived from gauge invariance.

Homogeneous terms do not contribute under a particular soft-limit projection.

Soft theorems are modified by higher-dimensional operators.

Abstract

In this letter we show that the soft behaviour of photons and graviton amplitudes, after projection, can be determined to infinite order in soft expansion via ordinary on-shell gauge invariance. In particular, as one of the particle's momenta becomes soft, gauge invariance relates the non-singular diagrams of an n-point amplitude to that of the singular ones up to possible homogeneous terms. We demonstrate that with a particular projection of the soft-limit, the homogeneous terms do not contribute, and one arrives at an infinite soft theorem. This reproduces the result recently derived from the Ward identity of large gauge transformations. We also discuss the modification of these soft theorems due to the presence of higher-dimensional operators.