Abstract
We present and apply a novel scheme for studying photon– and pion–nucleon scattering beyond the threshold region. Partial-wave amplitudes for the γN and πN states 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. Based on the chiral Lagrangian we recover the empirical s- and p-wave amplitudes up to energies s ≃ 1300 MeV in terms of the parameters relevant at order Q 3 .
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