Pion-nucleon amplitude near threshold: the sigma-term and scattering lengths beyond few loops

Abstract

The pion-nucleon amplitude is considered in the vicinity of the elastic scattering threshold within a relativistic dynamical model dressing the pi N N and pi N Delta vertices self-consistently with an infinite number of meson loops. The dressing is formulated as solution of a system of coupled integral equations incorporating unitarity, crossing symmetry and analyticity constraints. The calculated scattering lengths and the sigma-term agree with recent data analyses. The dressing is important in this model both below and at threshold. The contribution of the Delta resonance is discussed, including effects of the consistent dressing of the pi N Delta vertex. A comparison with the approaches of chiral perturbation theory and the Bethe-Salpeter equation is outlined.