2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/192102We evaluate the S-wave pion--nucleon scattering length $a^-$ in the framework of heavy baryon chiral perturbation theory up--to--and--including terms of order $M_π^4$. We show that the order $M_π^4$ piece of the isovector amplitude at threshold, $T^-_{\rm thr}$, vanishes exactly. We predict for the isovector scattering length, $0.088 \, M_{π^+}^{-1} \le a^- \le 0.096 \, M_{π^+}^{-1}$.5 pp, LaTeX file, 2 figures (appended as separate compressed tar file, amin.uu)High Energy Physics - PhenomenologyNuclear Theorytext