FUSION IN CONFORMAL FIELD-THEORY AS THE TENSOR PRODUCT OF THE SYMMETRY ALGEBRA

Following a recent proposal of Richard Borcherds to regard fusion as the ringlike tensor product of modules of a quantum ring, a generalization of rings and vertex algebras, we define fusion as a certain quotient of the (vector space) tensor product of representations of the symmetry algebra A. We prove that this tensor product is associative and symmetric up to equivalence. We also determine explicitly the action of A on it, under which the central extension is preserved. Having defined fusion in this way, determining the fusion rules is then the algebraic problem of decomposing the tensor product into irreducible representations. We demonstrate how to solve this for the case of the WZW and the minimal models and recover thereby the well-known fusion rules. The action of the symmetry algebra on the tensor product is given in terms of a comultiplication. We calculate the R matrixs of this comultiplication and find that it is triangular. This seems to shed some new light on the possible role of the quantum group in conformal field theory.