Transition from baryonic to mesonic freeze-out

The recently discovered sharp peak in the K+/pi(+) 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 baryondominated to a meson-dominated gas. The transition occurs at a temperature T = 140 MeV and baryon chemical potential mu(B) = 410 MeV corresponding to an incident energy of root s(NN) = 8.2 GeV. The maximum in the Lambda/pi ratio is well reproduced by the statistical model, but the change in the K+/pi(+) ratio is much less pronounced than the one observed by the NA49 Collaboration. The calculated smooth increase of the K-/pi(-) ratio and the shape of the Xi(-)/pi(+) and Omega(-)/pi(+) 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. (c) 2005 Elsevier B.V. All rights reserved.