- th / 9 90 60 65 v 1 2 2 Ju n 19 99 1 Particle Ratios at CERN , BNL and GSI : Unified description of Freeze - Out Parameters

It is shown that the chemical freeze-out parameters obtained at CERN/SPS, BNL/AGS and GSI/SIS energies all correspond to a unique value of 1 GeV for the average energy per hadron. Information about the chemical freeze-out parameters, the temperature, T ch , and baryon chemical potential µ ch B can be obtained from ratios of integrated particle yields [1,2]. This is because various effects like transverse flow or particle production from a superposition of fireballs cancel out in such ratios provided the freeze-out parameters are unique. We will discuss in succession particle ratios at GSI/SIS, BNL/AGS and SPS/CERN. We will then discuss common properties, in particular, the observation that all of them correspond to an average energy of 1 GeV per hadron independent of the beam energy [3]. A systematic study of the particle ratios measured at GSI/SIS energies has been done recently [4,5]. In figure 1 we show the results obtained in [5] for Ni-Ni at 1.0 AGeV. The particle ratios are consistent with the interpretation that the hadronic composition of the final state is fixed at a unique temperature and baryon chemical potential. Since the temperature is low, the number of particles created is small and it is therefore necessary to take into account the exact conservation of strangeness because strange particles are always produced in pairs and it is more difficult to create two particles in a small cold system than it is to create one particle. There is no necessity to introduce any other parameters and a good fit can be achieved with full chemical equilibrium including strange particles, i.e. with γ s = 1. In view of the fact that kaons are produced below threshold at SIS this is remarkable.