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≠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.