2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/192153We study pion-nucleon scattering at tree level with a chiral lagrangian of pions, nucleons, and $Δ$-isobars using a K-matrix unitarization procedure. Evaluating the scattering amplitude to order $Q^2$, where $Q$ is a generic small momentum scale, we obtain a good fit to the experimental phase shifts for pion center-of-mass kinetic energies up to $50 $MeV. The fit can be extended to 150 MeV when we include the order-$Q^3$ contributions. Our results are independent of the off-shell $Δ$ parameter.13 pages, RevTex, Two tables and two figures, Minor changes, To appear in PRCHigh Energy Physics - PhenomenologyNuclear Theorytext