On local fields invariant under the action of topological defects

Abstract

In the context of rational conformal field theories (RCFT) we look into the problem of constructing and classifying pairs consisting of a local operator and a topological defect which commutes or anticommutes with it. We discuss the bulk and boundary versions of the problem. In the latter one considers a conformal boundary condition, a boundary operator on it and a junction with a topological defect. In the case of the charge conjugation modular invariant commuting configurations in each problem can be obtained when a certain restriction on the fusion rules in realised. We study the corresponding fusion rule problems in detail. While in the bulk case it reduces to realising the $a\times b = c$ fusion rule which was studied in arXiv:2012.14689 [hep-th], in the boundary it leads to a new type of problem. We obtain a full solution to this problem for the $\mathrm{SU(3)}$ WZW theory, thus constructing a class of commuting boundary operators and junctions in that theory, and suggest an approach to general WZW theories.