Superconvergence and Duality

Superconvergence relations for the transverse gauge field propagator can be used in order to show that the corresponding gauge quanta are not elements of the physical state space, as defined by the BRST algebra. With a given gauge group, these relations are valid for a limited region in the number of matter fields, indicating a phase transition at the boundary. In the case of SUSY gauge theories with matter fields in the fundamental representation, the results predicted by superconvergence can be compared directly with those obtained on the basis of duality and the conformal algebra. There is exact agreement.