Compton scattering from chiral dynamics with unitarity and causality

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 $γ_{E1M2}, γ_{M1E2}$ we recover the results of strict chiral perturbation theory, for the diagonal $γ_{E1E1}, γ_{M1M1}$ elements we find significant effects from rescattering.

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