2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/48099A brief discussion of the elastic pion-pion scattering amplitude to two loops in chiral perturbation theory is given. Some technical details of the evaluation of two-loop diagrams are addressed, and results relevant for the pionium decay are presented.8 pages, 2 figures, Proc. Int. Workshop Hadronic Atoms and Positronium in the Standard Model, Dubna, Russia, May 26-31, 1998High Energy Physics - Phenomenologytext