J ul 2 00 3 K − / K + ratio at GSI in hot and dense matter

The K − /K + ratio in heavy-ion collisions at GSI energies is studied including the properties of the participating hadrons in hot and dense matter. 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. Three approaches for the K − self-energy are considered: non-interacting K − , on-shell self-energy and single-particle spectral density. With respect to the on-shell approach, the use of an energy dependent ¯ K spectral density, including both sand p-wave components of the ¯ KN interaction, lowers considerably the freeze-out temperature and gives rise to the " broad-band equilibration " advocated by Brown, Rho and Song.