- th / 0 40 70 35 v 1 1 2 Ju l 2 00 4 In - medium properties of antikaons at finite temperature : the K − / K + ratio at GSI

The K−/K+ ratio in heavy-ion collisions at GSI is studied including the properties of the participating hadrons in hot and dense matter, paying a particular attention to the in-medium properties of antikaons at finite temperature. The determination of the temperature and chemical potential at freeze-out conditions compatible with the ratio K−/K+ is very delicate, and depends on the approach adopted for the antikaon self-energy. Different approaches for the K− self-energy have been considered: non-interacting K−, on-shell self-energy and single-particle spectral density. We observe that the use of an energy dependent K̄ spectral density lowers considerably the freeze-out temperature with respect to the on-shell approach. We also conclude that the full offshell approach gives rise to the “broad-band equilibration” advocated by Brown, Rho and Song.