Soft charges and zero modes at null boundaries

Abstract

Asymptotic symmetries at null boundaries provide a powerful window into the soft sector of field theories, revealing conserved charges and infinite-dimensional symmetry algebras beyond the standard bulk dynamics. While these structures are usually associated with residual gauge transformations, null boundaries also exhibit a characteristic global zero-mode ambiguity, whose physical consequences are less explored. We discuss how this zero mode modifies the canonical structure of field theories at null boundaries. In particular, we show that it gives rise to a new quasilocal edge observable, with consequences to the charge algebra. It can arise on any kind of null boundaries, both at the null infinity and the finite distance such as a black hole horizon.