2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/190548We study pion--nucleon scattering with a chiral lagrangian of pions, nucleons, and $Δ$-isobars. The scattering amplitude is evaluated to one-loop $Q^3$ order, where $Q$ is a generic small momentum, using a new approach which is equivalent to heavy baryon chiral perturbation theory. We obtain a good fit to the experimental phase shifts for pion center-of-mass kinetic energies up to 100 MeV. A sigma term greater than 45 MeV is favored, but the value is not well determined.46 pages, RevTex, 14 figures and 2 tables, minor changes for publicationHigh Energy Physics - PhenomenologyNuclear Theorytext