The kinematics of the coupling of Regge poles to photons

Abstract

We examine two problems which arise in understanding Regge-pole exchange in amplitudes which include photons; namely how there can be a pion-exchange pole in pion photoproduction, and a finite pomeron coupling in forward Compton scattering. We try to resolve these problems by noting that the usual sense-nonsense zeros, which might seem to make the couplings vanish, can in fact be replaced in the partialwave amplitudes by zeros in the kinematical factors. Since the zeros are absent from the total amplitudes, we are left with finite couplings. This replacement is a peculiarity of photon couplings. It has the disadvantage of changing the threshold behaviour of the partial-wave amplitudes, but seems to be the only plausible way out of the dilemma. There are no other special requirements of gauge invariance, so that these photon-induced processes may be Reggeized in exactly the same way as purely hadronic reactions.