In-medium effects on the K−/K+ ratio at GSI

The in-medium modifications on the K−/K+ ratio produced at GSI are studied. Particular attention is paid to the properties of antikaons, which determine the chemical potential and temperature at freeze-out conditions. Different approaches have been considered: non-interacting K−, on-shell self-energy and single-particle spectral density. We observe that the full off-shell approach to the spectral density reproduces the Brown et al. “broad-band equilibration” which is crucial to explain an enhanced K−/K+ ratio.