Low's Subleading Soft Theorem as a Symmetry of QED

Understand

It was shown by F.

• Low in the 1950s that the subleading terms of soft photon S-matrix elements obey a universal linear relation.
• In this paper we give a new interpretation to this old relation, for the case of massless QED, as an infinitesimal symmetry of the S-matrix.
• The symmetry is shown to be locally generated by a vector field on the conformal sphere at null infinity.