Transition from baryonic to mesonic freeze-out

Abstract 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 = 140 MeV and baryon chemical potential μ B = 410 MeV corresponding to an incident energy of s N N = 8.2 GeV . 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.