This talk focuses on two aspects of chiral symmetry, taken from both cold and hot chiral symmetry breaking.
(a) At zero temperature, we focus on the charged pion polarizability, ap, for which there is a large spread in both the theoretical and experimental values. We are able to give a unified and comparative approach for the various theoretical models via the Das-Mathur-Okubo sum rule. From the experimental point of view, more experiments are required for clarification.
(b) For hot systems, we present a non-equilibrium evolution equation for quarks and mesons based on a chirally invariant LaGrange density. Several general features of phase transitions, such as critical scattering at the phase transition, can be observed in quark-anti-quark cross sections. First toy simulations of collisions are shown.
