  The soft photon theorem in U(1) gauge theories with only massless charged
particles has recently been shown to be the Ward identity of an
infinite-dimensional asymptotic symmetry group. This symmetry group is
comprised of gauge transformations which approach angle-dependent constants at
null infinity. In this paper, we extend the analysis to all U(1) theories,
including those with massive charged particles such as QED.

High Energy Physics - Theory;High Energy Physics - Phenomenology