2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/197641Using heavy-baryon chiral perturbation theory, we have calculated all the diagrams up to two-loop order which contribute to the S-wave pion self-energy in symmetric nuclear matter. Some subtleties related to the definition of pion fields are discussed. The in-medium pion mass is turned out to be increased by only (5 - 6) per cents in normal nuclear matter density, without any off-shell ambiguity.9 pages with 3 eps figuresNuclear Theorytext