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Central rapidity densities of charged particles at RHIC and LHC

N. Armesto, C. Pajares

TL;DR

This review assesses predictions for central charged-particle rapidity densities at RHIC and LHC across a spectrum of models, highlighting how soft-string, hard-minijet, and thermodynamic frameworks diverge in their estimates. It synthesizes approaches including DPM/DPMJET, SFM with string fusion, RQMD, HIJING, PQCD with saturation and hydrodynamics, and various other transport and percolation-based pictures, explaining how unitarity, energy-momentum conservation, shadowing, and string interactions constrain multiplicities. The work emphasizes that, despite large initial disparities, contemporary predictions cluster in ways tied to the underlying physics (e.g., string fusion reducing central densities; saturation limiting minijet production), and it notes that percolation offers a lower-bound perspective on multiplicities and signals. The practical impact is clear: accurate predictions are essential for detector design and for interpreting QGP signatures, and forthcoming RHIC/LHC data will help discriminate among competing mechanisms governing particle production in ultra-relativistic nuclear collisions.

Abstract

Predictions on central rapidity densities of charged particles at energies of the Relativistic Heavy Ion Collider and the Large Hadron Collider, for central collisions between the largest nuclei that will be available at these accelerators, are reviewed. Differences among the results of the existing models are discussed in relation with their underlying physical basis and with the possibilities to discriminate them.

Central rapidity densities of charged particles at RHIC and LHC

TL;DR

This review assesses predictions for central charged-particle rapidity densities at RHIC and LHC across a spectrum of models, highlighting how soft-string, hard-minijet, and thermodynamic frameworks diverge in their estimates. It synthesizes approaches including DPM/DPMJET, SFM with string fusion, RQMD, HIJING, PQCD with saturation and hydrodynamics, and various other transport and percolation-based pictures, explaining how unitarity, energy-momentum conservation, shadowing, and string interactions constrain multiplicities. The work emphasizes that, despite large initial disparities, contemporary predictions cluster in ways tied to the underlying physics (e.g., string fusion reducing central densities; saturation limiting minijet production), and it notes that percolation offers a lower-bound perspective on multiplicities and signals. The practical impact is clear: accurate predictions are essential for detector design and for interpreting QGP signatures, and forthcoming RHIC/LHC data will help discriminate among competing mechanisms governing particle production in ultra-relativistic nuclear collisions.

Abstract

Predictions on central rapidity densities of charged particles at energies of the Relativistic Heavy Ion Collider and the Large Hadron Collider, for central collisions between the largest nuclei that will be available at these accelerators, are reviewed. Differences among the results of the existing models are discussed in relation with their underlying physical basis and with the possibilities to discriminate them.

Paper Structure

This paper contains 10 sections, 33 equations, 12 figures, 3 tables.

Table of Contents

  1. Introduction
  2. The Dual Parton Model and the DPMJET Monte Carlo code
  3. The String Fusion Model
  4. The Relativistic Quantum Molecular Dynamics Model
  5. The Heavy-Ion Jet Interaction Generator (HIJING)
  6. Perturbative Quantum Chromodynamics and Hydrodynamical models
  7. Other models
  8. Percolation
  9. Cosmic Ray Physics and heavy ion accelerators
  10. Conclusions

Figures (12)

  • Figure 1: Predictions from different models for the charged pseudorapidity density in central ($b\leq 3$ fm) Pb-Pb collisions at the LHC, taken from alice.
  • Figure 2: Lowest order contribution to p-p scattering at high energies in the DPM. Strings are stretched between valence constituents of the protons and hadronize by means of $q\overline{q}$ pair production.
  • Figure 3: First higher order contribution to p-p scattering at high energies in the DPM. Besides the contribution shown in Fig. \ref{['fig2']}, now strings stretched between sea constituents of the protons appear.
  • Figure 4: The popcorn mechanism of diquark breaking.
  • Figure 5: One example of proposed diagram for diquark breaking in nucleon-nucleus collisions.
  • ...and 7 more figures