ABSORPTION AND DISPERSION OF PIONS AT FINITE TEMPERATURE

We consider the pole shift in the propagator and analyse both absorptive and dispersive properties of pions at finite temperature. The leading-order term in an expansion in powers of the density is determined by the pi-pi forward scattering amplitude. We evaluate this amplitude in a semi-phenomenological manner, using chiral perturbation theory at threshold and relying on experimental information at higher energies. In particular we analyse the contributions generated by rho-exchange and show that they substantially modify the lowest-order prediction of chiral perturbation theory.