Unitary and causal dynamics based on the chiral Lagrangian

Abstract

Pion-nucleon scattering, pion photoproduction, and nucleon Compton scattering are analyzed within a scheme based on 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. Experimental data are reproduced up to energies s≃1300 MeV in terms of the parameters relevant at order Q3. A striking puzzle caused by an old photon asymmetry measurement close to the pion production threshold is discussed.