Asymptotic Symmetries of Massless QED in Even Dimensions

TL;DR

This paper extends the connection between soft photon theorems and asymptotic symmetries to massless QED in even-dimensional spacetimes, revealing an infinite set of large gauge symmetries linked to soft photons.

Contribution

It generalizes the relationship between soft theorems and asymptotic symmetries to higher even dimensions, identifying new large gauge transformations and their role in massless QED.

Findings

Soft photon theorems are recast as Ward identities in higher dimensions.

Infinite-dimensional asymptotic symmetry groups are identified in even-dimensional spacetimes.

Soft photons act as Goldstone bosons for spontaneously broken symmetries.

Abstract

We consider the scattering of massless particles coupled to an abelian gauge field in 2n-dimensional Minkowski spacetime. Weinberg's soft photon theorem is recast as Ward identities for infinitely many new nontrivial symmetries of the massless QED S-matrix, with one such identity arising for each propagation direction of the soft photon. These symmetries are identified as large gauge transformations with angle-dependent gauge parameters that are constant along the null generators of null infinity. Almost all of the symmetries are spontaneously broken in the standard vacuum and the soft photons are the corresponding Goldstone bosons. Our result establishes a relationship between soft theorems and asymptotic symmetry groups in any even dimension.