Equation of State of an Interacting Pion Gas with Realistic π – π Interactions

Within the finite-temperature Greens-function formalism we study the equation of state of a hot interacting pion gas at zero chemical potential. Employing realistic ππ meson-exchange interactions we selfconsistently calculate the in-medium single-pion selfenergy and the ππ scattering amplitude in the quasiparticle approximation. These quantities are then used to evaluate the thermodynamic potential, Ω π (T), from which the state variables: pressure-, entropy-and energy-density can be derived. In contrast to earlier calculations based on the low-energy Weinberg Lagrangian we find an overall increase as compared to the free-gas results. We also consider the possibility of a dropping ρ-meson mass as suggested by the 'Brown-Rho Scaling' law.