Ribbon tableaux and q-analogues of fusion rules in WZW conformal field theories

Starting from known $q$-analogues of ordinary SU(n) tensor products multiplicities, we introduce $q$-analogues of the fusion coefficients of the WZW conformal field theories associated with SU(n). We conjecture combinatorial interpretations of these polynomials, which can be proved in special cases. This allows us to derive in a simple way various kinds of branching functions, the simplest ones being the characters of the minimal unitary series of the Virasoro algebra. We also obtain $q$-analogues of the dimensions of spaces of nonabelian theta functions.