Compton scattering from chiral dynamics with unitarity and causality

Abstract

Proton Compton scattering is analyzed with the chiral Lagrangian. Partial-wave amplitudes are obtained by an analytic extrapolation of subthreshold reaction amplitudes computed in chiral perturbation theory, where the constraints set by electromagnetic-gauge invariance, causality and unitarity are used to stabilize the extrapolation. We present and discuss predictions for various spin observables and polarizabilties of the proton. While for the transition polarizabilities ${\hat{I}}_{E1M2}^3,{\hat{I}}_{M1E2}^3$ we recover the results of strict chiral perturbation theory, for the diagonal ${\hat{I}}_{E1E1}^3,{\hat{I}}_{M1M1}^3$ elements we find significant effects from rescattering.

18 pages, 3 figures, revised version