2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/202602We present the pion-pion elastic scattering amplitude at finite temperature to one-loop in Chiral Perturbation Theory. The thermal scattering amplitude properly defined allows to generalize the perturbative unitarity relation to the $T\neq 0$ case. Our result provides a model independent prediction of an enhanced pion-pion low-energy phase shift with the temperature and it has physical applications within the context of Relativistic Heavy Ion Collisions.11 pages, 2 figures. Some references and clarifying comments added and new figures included. Final version to appear in Physics Letters BHigh Energy Physics - Phenomenologytext