CONFORMAL FIELD THEORY ON SURFACES WITH BOUNDARIES AND NONDIAGONAL MODULAR INVARIANTS
Zvi Bern, David C. Dunbar · International Journal of Modern Physics A · 1990
We show that the operator content of a conformal field theory defined on surfaces with boundaries and crosscaps is more restricted when the periodic sector is described by nondiagonal modular invariants than in the case of diagonal modular invariants. By tensoring, the restrictions can be alleviated, leading to a rich structure. Such constructions are useful, for example, in lower-dimensional open superstring models.