Transition from baryon-to meson-dominated freeze-out—early decoupling around 30 A GeV?

The recently discovered sharp peak in the excitation function of the K+/?+ 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 a baryon chemical potential ?B = 410 MeV corresponding to an energy of . The calculated maximum in the K+/?+ ratio is, however, much less pronounced than that observed by the NA49 Collaboration. The smooth increase of the K?/?? ratio with incident energy and the shape of the excitation functions of the ?/?+, ??/?+ and ??/?+ ratios all exhibiting maxima at different incident energies is consistent with the presently available experimental data. The measured K+/?+ 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 the K+/?+ ratio while most other particle ratios remain essentially unchanged. Such an early freeze-out is supported by results from HBT studies.