Saturation of $E_T/N_{ch}$ and Freeze-Out Criteria in Heavy-Ion Collisions

TL;DR

This paper investigates the saturation of the ratio of transverse energy to charged particle multiplicity ($E_T/N_{ch}$) in heavy-ion collisions across various energies, using a statistical-thermal model to relate it to freeze-out criteria and observed saturation phenomena.

Contribution

It demonstrates that the $E_T/N_{ch}$ ratio saturates at high energies and relates this to chemical freeze-out conditions, providing a comprehensive analysis across different energies and collision centralities.

Findings

$E_T/N_{ch}$ saturates around 800 MeV at SPS and RHIC energies.

The ratio is nearly independent of collision centrality.

The saturation correlates with the chemical freeze-out temperature and criteria.

Abstract

The pseudorapidity densities of transverse energy, the charged particle multiplicity and their ratios, $E_T/N_{ch}$, 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, $E_T/N_{ch}$ increases rapidly with beam energy and remains approximately constant at about a value of 800 MeV for beam energies from SPS to RHIC. $E_T/N_{ch}$ 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 $E_T/N_{ch}$ are related to the chemical freeze-out criterium, $E/N \approx 1 GeV$ valid for primordial hadrons. We have studied the variation of the average mass $(<MASS>), N_{decays}/N_{primordial}, N_{ch}/N_{decays}$ and $E_T/N_{ch}$ with $\sqrt{s_{NN}}$ for all freeze-out criteria discussed in literature. These observables show saturation around SPS and higher $\sqrt{s_{NN}}$, like the chemical freeze-out temperature ($T_{ch}$).