Investigation of hadron multiplicities and hadron yield ratios in heavy ion collisions

TL;DR

This paper critically examines the thermal model for hadron production in heavy ion collisions, proposing modifications to conservation laws, introducing a new freeze-out criterion, and achieving improved data fits.

Contribution

It introduces a revised thermal model with modified conservation laws, a new chemical freeze-out criterion, and considers pion and kaon radii as fit parameters, enhancing data description.

Findings

Modified conservation laws improve multiplicity descriptions.

A new freeze-out criterion of constant entropy per hadron is proposed.

The model accurately fits experimental data including the Strangeness Horn.

Abstract

Here we thoroughly discuss some weak points of the thermal model which is traditionally used to describe the hadron multiplicities measured in the central nucleus-nucleus collisions. In particularly, the role of conservation laws, the values of hard-core radii along with the effects of the Lorentz contraction of hadron eigen volumes and the hadronic surface tension are systematically studied. It is shown that for the adequate description of hadron multiplicities the conservation laws should be modified, whereas for the description of hadron yield ratios the conservation laws are not necessary at all. Also here we analyzed the usual criteria for the chemical freeze-out and found that none of them is robust. A new chemical freeze-out criterion of constant entropy per hadron equals to 7.18 is suggested and a novel effect of adiabatic chemical hadron production is discussed. Additionally, we found that the data for the center of mass energies above 10 GeV lead to the temperature of the nil hadronic surface tension coefficient of about $T_0 = 147 \pm 7$ MeV. This is a very intriguing result since a very close estimate for such a temperature was obtained recently within entirely different approach. We argue that these two independently obtained results evidence that the (tri)critical temperature of the QCD phase diagram is between 140 and 154 MeV. In addition, here we suggest to consider the pion and kaon hard-core radii as new fitting parameters. Such an approach for the first time allows us to simultaneously describe the hadron multiplicities and the Strangeness Horn and get a very high quality fit of the available experimental data.