Particle Production in Heavy Ion Collisions

TL;DR

This work demonstrates that a statistical hadron resonance gas framework, properly implemented with exact conservation laws (canonical vs grand canonical), can describe hadron yields from SIS to RHIC with a unified chemical freeze-out curve. It shows that for large systems and high energies the grand canonical description suffices, while canonical treatments are essential for strangeness in small systems and at lower energies, explaining strangeness suppression and its lifting in heavy ion collisions. The analysis connects chemical freeze-out to the QCD phase boundary, supports deconfinement scenarios at SPS/RHIC energies, and extends to heavy quark production via statistical recombination, predicting notable energy and centrality dependences for charm hadrons. Overall, the paper provides a comprehensive, cross-energy framework for particle production that ties experimental observables to fundamental QCD thermodynamics.

Abstract

The status of thermal model descriptions of particle production in heavy ion collisions is presented. We discuss the formulation of statistical models with different implementation of the conservation laws and indicate their applicability in heavy ion and elementary particle collisions. We analyze experimental data on hadronic abundances obtained in ultrarelativistic heavy ion collisions, in a very broad energy range starting from RHIC/BNL ($\sqrt s=200$ A GeV), SPS/CERN ($\sqrt s\simeq 20$ A GeV) up to AGS/BNL ($\sqrt s\simeq 5$ A GeV) and SIS/GSI ($\sqrt s\simeq 2$ A GeV) to test equilibration of the fireball created in the collision. We argue that the statistical approach provides a very satisfactory description of experimental data covering this wide energy range. Any deviations of the model predictions from the data are indicated. We discuss the unified description of particle chemical freeze--out and the excitation functions of different particle species. At SPS and RHIC energy the relation of freeze--out parameters with the QCD phase boundary is analyzed. Furthermore, the application of the extended statistical model to quantitative understanding of open and hidden charm hadron yields is considered.

Figures (41)

• Figure 1: Schematic space--time view of a heavy ion collision that indicates four basic stages in the evolution of the collision fireball: initial overlap region, pre--equilibrium partonic system, equilibrated quark-gluon plasma and its subsequent hadronization to a hadron gas.
• Figure 2: The pressure $P$ and energy density $\epsilon$ normalized to the temperature to the fourth power, versus temperature normalized to its critical value. The calculationsr23 were performed within LGT for different numbers of flavors. The values of the corresponding ideal gas results are indicated by the arrows.
• Figure 3: The ratio of the total density of positively charged pions that includes all resonance contributions to the density of thermal pions. The calculations are done in the hadron resonance gas model for $\mu_B=$250, 550 MeV and for different temperatures.
• Figure 4: The strange chemical potential $\mu_S$ as a function of baryon--chemical potential for T=120,170 and 200 MeV. The results are obtained by imposing the strangeness neutrality condition in a hadron resonance gas.
• Figure 5: Comparison between thermal model predictions and experimental particle ratios for Pb--Pb collisions at 40 GeV/nucleon. The thermal model calculations are obtained with $T=148$ MeV and $\mu_B=400$ MeV.