Unified Description of Freeze-Out Parameters in Relativistic Heavy Ion Collisions

Abstract

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 per hadron in the local rest frame of the system, independent of the beam energy and of the target and beam particles.

Resonances from a hadronic fireball

“…Thermodynamical parameters of this stage could be obtained by fitting the final total hadron multiplicities. Typical temperature is found to be T chem ∼ 160 ± 10 MeV [11,12]. During the second stage of collision, elastic scattering processes change momentum distributions of hadrons in accord with a decreasing temperature until they cease and distributions freeze in.…”

“…In our study we consider a two-stage picture of the hadronic phase of heavy-ion collision at SPS energies which can be characterized by the notions of chemical and thermal freeze-outs [11,12,13]. At the initial stage of the hadronic phase we assume the temperature T to be close to the QCD phase transition temperature T c ≈ 170±10 MeV.…”

Hadron production in central nucleus–nucleus collisions at chemical freeze-out

“…An earlier proposed freeze-out criterion corresponds to an average energy per average number of hadrons of approximately 1 GeV [11]. It has also been argued recently that the freezeout points can be described by a constant entropy density divided by T 3 [23].…”

“…This makes the model a unique tool in the attempt to quantify from the experimental side the features of the phase diagram of hadronic matter [17,18,19]. Recent analyses over a broad energy range [20,21] have contributed in the on-going efforts to understand the chemical freeze-out criteria [1,10,11,22,23,24] in the phase diagram. It has been pointed out within a thermal model analysis that scanning the energy one encounters a transition from baryon-to meson-dominated freeze-out [25], with its associated fingerprint on the characteristics of hadron yields, also evidenced earlier [26].…”

“…An analysis of the energy dependence of the thermal parameters extracted from fits of the experimental data, temperature (T ) and baryo-chemical potential (µ b ), established the "line of chemical freeze-out" [10]. These data were subsequently interpreted in terms of an universal condition for chemical freezeout [11]. Remarkably, it appears that at the top SPS energy the (T ,µ b ) values reach the phase boundary between the hadronic world and the quark-gluon plasma (QGP) [1,10], as calculated solving Quantum Chromo-Dynamics (QCD) on the lattice [12,13,14].…”