Transition from baryonic to mesonic freeze-out

The recently discovered sharp peak in the K+/π+ ratio in relativistic heavy-ion collisions is discussed in the framework of the statistical model. In this model a rapid change is expected as the hadronic gas undergoes a transition from a baryon-dominated to a meson-dominated gas. The transition occurs at a temperature T=140MeV and baryon chemical potential μB=410MeV corresponding to an incident energy of sNN=8.2GeV. The maximum in the Λ/π ratio is well reproduced by the statistical model, but the change in the K+/π+ ratio is much less pronounced than the one observed by the NA49 Collaboration. The calculated smooth increase of the K−/π− ratio and the shape of the Ξ−/π+ and Ω−/π+ ratios exhibiting maxima at different incident energies is consistent with the presently available experimental data. We conclude that the measured particle ratios with 20-30% deviations agree with a hadronic freeze-out scenario. These deviations seem to occur just in the transition from baryon-dominated to meson-dominated freeze-out.