Transition from Baryon- to Meson-Dominated Freeze Out -- Early Decoupling around 30 A GeV?

Abstract

The recently discovered sharp peak in the excitation function of the K+/pi+ ratio around 30 A GeV in relativistic heavy-ion collisions is discussed in the framework of the Statistical Model. In this model, the freeze-out of an ideal hadron gas changes from a situation where baryons dominate to one with mainly mesons. This transition occurs at a temperature T = 140 MeV and baryon chemical potential mu(B) = 410 MeV corresponding to an energy of sqrt(s) = 8.2 GeV. The calculated maximum in the K+/pi+ ratio is, however, much less pronounced than the one observed by the NA49 Collaboration. The smooth increase of the K-/pi- ratio with incident energy and the shape of the excitation functions of the Lambda/pi+, Xi-/pi+ and Omega/pi ratios all exhibiting maxima at different incident energies, is consistent with the presently available experimental data. The measured K+/pi+ ratio exceeds the calculated one just at the incident energy when the freeze-out condition is changing. We speculate that at this point freeze-out might occur in a modified way. We discuss a scenario of an early freeze-out which indeed increases K+/pi+ ratio while most other particle ratios remain essentially unchanged. Such an early freeze-out is supported by results from HBT studies.