Saturation of ET/Nch and Freeze-Out Criteria in Heavy-Ion Collisions
Abstract
The pseudorapidity densities of transverse energy, the charged particle multiplicity and their ratios, ET/Nch, are estimated at mid-rapidity, in a statistical-thermal model based on chemical freeze-out criteria, for a wide range of energies from GSI-AGS-SPS to RHIC. It has been observed that in nucleus-nucleus collisions, ET/Nch increases rapidly with beam energy and remains approximately constant at about a value of 800 MeV for beam energies from SPS to RHIC. ET/Nch has been observed to be almost independent of centrality at all measured energies. The statistical-thermal model describes the energy dependence as well as the centrality independence, qualitatively well. The values of ET/Nch are related to the chemical freeze-out criterium, E/N ≈ 1 GeV valid for primordial hadrons. We have studied the variation of the average mass (<MASS>), Ndecays/Nprimordial, Nch/Ndecays and ET/Nch with sNN for all freeze-out criteria discussed in literature. These observables show saturation around SPS and higher sNN, like the chemical freeze-out temperature (Tch).