Celestial current algebra from Low’s subleading soft theorem

The leading soft photon theorem implies that four-dimensional scattering amplitudes are controlled by a two-dimensional (2D) U(1) Kac-Moody symmetry that acts on the celestial sphere at null infinity (I). This celestial U(1) current is realized by components of the electromagnetic vector potential on the boundaries of I. Here, we develop a parallel story for Low's subleading soft photon theorem. It gives rise to a second celestial current, which is realized by vector potential components that are subleading in the large radius expansion about the boundaries of I. The subleading soft photon theorem is reexpressed as a celestial Ward identity for this second current, which involves novel shifts by one unit in the conformal dimension of charged operators.