Exclusion statistics in conformal field theory and the UCPF for WZW models

In this paper we further elaborate on the notion of fractional exclusion statistics, as introduced by Haldane, in two-dimensional conformal field theory, and its connection to the Universal Chiral Partition Function as defined by McCoy and collaborators. We will argue that in general, besides the pseudo-particles introduced recently by Guruswamy and Schoutens, one needs additional 'null quasi-particles' to account for the null-states in the quasi-particle Fock space. We illustrate this in several examples of WZW-models. (C) 2000 Elsevier Science B.V. All rights reserved.