Finite Temperature Pion Scattering to one-loop in Chiral Perturbation Theory

We 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 $T\mathrm{\ne }0$ 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.