Twisted scattering from Chern-Simons theory

Abstract

We consider boundary conditions of two-dimensional Dirac fermions which preserve a non-anomalous continuous symmetry which acts differently on the left- and right-movers. Such boundaries are equivalent to topological defects of Weyl fermions preserving the symmetry. We give a general construction of such defects for conformally-embedded symmetries using Chern-Simons theory. They always exist and can be realised as half-space gauging defects for some finite categorical symmetry. Using this technology, we compute the twist lines to which fermions become attached upon scattering through the defect, the boundary central charge of the corresponding folded boundary state, and its mod-2 valued SPT class. As applications, we first consider abelian symmetries and find agreement with known results in the literature. We then study a novel example where an $SU(2) \times SU(4) \times U(1)^2$ symmetry is preserved. In contrast to the abelian case, the twist lines are intrinsically non-invertible and belong to either a $\text{TY}(\mathbb{Z}_3 \times \mathbb{Z}_3)$ or $\text{Rep}(\mathbb{S}_3) \times \mathbb{Z}_3$ symmetry depending on whether scattering is to the left or the right.