Compton scattering in a unitary approach with causality constraints

Pion-loop corrections for Compton scattering are calculated in a novel appr oach bused on the use of dispersion relations in a formalism obeying unitar ity. The basic framework is presented, including an application to Compton scattering. In the approach the effects of the non-pole contribution arisin g from pion dressing are expressed in terms of (half-off-shell) form factor s and the nucleon self-energy. These quantities are constructed through the application of dispersion integrals to the pole contribution of loop diagr ams, the same as those included in the calculation of the amplitudes throug h a K-matrix formalism. The prescription of minimal substitution is used to restore gauge invariance. The resulting relativistic-covariant model combi nes constraints from unitarity, causality, and crossing symmetry. (C) 2000 Elsevier Science B.V. All rights reserved.