Kac and new determinants for fractional superconformal algebras

We derive the Kac and new determinant formulas for an arbitrary (integer) level K fractional superconformal algebra using the BRST cohomology techniques developed in conformal field theory. In particular, we reproduce the Kac determinants for the Virasoro (K=1) and superconformal (K=2) algebras. For K≥3 there always exist modules where the Kac determinant factorizes into a product of more fundamental new determinants. Using our results for general K, we sketch the nonunitarity proof for the SU(2) minimal series; as expected, the only unitary models are those already known from the coset construction. We apply the Kac determinant formulas for the spin-4/3 parafermion current algebra (i.e., the K=4 fractional superconformal algebra) to the recently constructed three-dimensional flat Minkowski space-time representation of the spin-4/3 fractional superstring.