K^-/K^+ ratio at GSI in hot and dense matter

The $K^{-}\mathrm{/}{K}^{+}$ 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 $K^{-}\mathrm{/}{K}^{+}$ 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 $\bar{K}$ spectral density, including both s- and p-wave components of the $\bar{K}N$ interaction, lowers considerably the freeze-out temperature and gives rise to the “broad-band equilibration” advocated by Brown, Rho and Song.