Orientation matters for NIMreps

TL;DR

This paper explores how orientation influences NIMreps in conformal field theory, revealing new structures and clarifying the interpretation of boundary states on non-orientable surfaces, with applications to WZW models.

Contribution

It introduces the role of orientation in NIMreps, providing a detailed analysis and complete classification for various modular invariants, and clarifies the formalism for oriented versus unoriented cases.

Findings

Orientation information disentangles complex NIMrep structures.

Some diagonal invariants lack NIMreps, indicating no corresponding CFT.

Complete NIMreps and coefficients are obtained for many WZW invariants.

Abstract

The problem of finding boundary states in CFT, often rephrased in terms of "NIMreps" of the fusion algebra, has a natural extension to CFT on non-orientable surfaces. This provides extra information that turns out to be quite useful to give the proper interpretation to a NIMrep. We illustrate this with several examples. This includes a rather detailed discussion of the interesting case of the simple current extension of A_2 level 9, which is already known to have a rich structure. This structure can be disentangled completely using orientation information. In particular we find here and in other cases examples of diagonal modular invariants that do not admit a NIMrep, suggesting that there does not exist a corresponding CFT. We obtain the complete set of NIMreps (plus Moebius and Klein bottle coefficients) for many exceptional modular invariants of WZW models, and find an explanation for the occurrence of more than one NIMrep in certain cases. We also (re)consider the underlying formalism, emphasizing the distinction between oriented and unoriented string annulus amplitudes, and the origin of orientation-dependent degeneracy matrices in the latter.