The Horn and the Thermal Model

The recently discovered sharp peak in the K + /π + ratio in relativistic heavy-ion collisions is discussed in the framework of the thermal 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 µB = 410 MeV corresponding to an incident energy of √ sNN = 8.2 GeV. The thermal model has been extremely successful in bringing order to a very large number of experimental results on particle yields in relativistic heavy-ion collisions. The results for the temperature and baryon chemical potential have been found to be consistent with having E/N = 1 GeV from the lowest beam energies up to the highest ones. This is illustrated in Fig. 1 which combines results from SIS up to RHIC. The NA49 Collaboration has recently performed a series of measurements of Pb-Pb collisions at 20, 30, 40, 80 and 158 AGeV beam energies. When these results are combined with measurements at lower beam energies from the AGS and SIS they reveal an unusually sharp variation with beam energy in the Λ/ � π� ,w ith� π �≡ 3/2(π + + π � ), and in the K + /π + ratios. Such a strong variation with energy does not occur in pp collisions and therefore indicates a major difference in heavy-ion collisions. This transition has been referred to in Ref. (1) as the "horn". A strong variation with energy of the Λ/ � π� ratio has been predicted on the basis of arguments put forward in (2). In (3) another, less spectacular, possibility for the origin of the sharp maximum, namely as being due to the transition from a baryon-dominated to a meson-dominated hadronic gas has been suggested; the distinction being based on whether the entropy of the hadronic gas is dominated by baryons or by mesons. For this purpose various quantities along the freeze-out