Constraints from the Drell-Hearn-Gerasimov sum rule in chiral models of composite fermions

We analyze a dispersion relation for a spin-flip Compton amplitude in a chiral model of composite quarks and leptons. We discuss the asymptotic behavior of the amplitude and show under what conditions the Drell-Hearn-Gerasimov sum rule holds. Using a small-mass expansion we derive a consistency condition that relates the photoproduction cross sections for different helicities. We show that this condition suggests the existence of higher-spin excited fermions, as well as relations among a priori independent photoproduction amplitudes.