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Hadron production in nuclear collisions at RHIC and high density QCD

D. Kharzeev, M. Nardi

TL;DR

The paper compares conventional eikonal Glauber calculations to high density QCD saturation predictions for hadron production in Au+Au collisions at RHIC. It quantifies the fraction of multiplicity from hard processes, finding F grows from ~22% to ~37% between 56 and 130 GeV, and discusses centrality, energy, and A-dependence predictions. In the saturation picture, a self-consistent saturation scale Q_s^2 ~ 2 GeV^2 implies a high initial energy density (~18 GeV/fm^3); notably, centrality trends predicted by both approaches are similar, indicating the need for complementary observables such as p_t distributions to distinguish the underlying dynamics. The work highlights the onset of high parton density at collider energies and outlines how future measurements could discriminate between frameworks.

Abstract

We analyze the first results on charged particle multiplicity at RHIC in the conventional eikonal approach and in the framework of high density QCD. We extract the fraction $F$ of the hadron multiplicity originating from ``hard'' (i.e. proportional to the number of binary collisions) processes; we find a strong growth of this fraction with energy: $F(\sqrt{s}=56 {\rm GeV}) \simeq 22%$, while $F(\sqrt{s}=130 {\rm GeV}) \simeq 37 %$. This indicates a rapid increase in the density of the produced particles. We outline the predictions of high density QCD for the centrality, energy, and atomic number dependence of hadron production. Surprisingly, the predictions of the conventional eikonal approach and of high density QCD for centrality dependence of hadron multiplicity at $\sqrt{s}=130 {\rm GeV}$ appear very similar.

Hadron production in nuclear collisions at RHIC and high density QCD

TL;DR

The paper compares conventional eikonal Glauber calculations to high density QCD saturation predictions for hadron production in Au+Au collisions at RHIC. It quantifies the fraction of multiplicity from hard processes, finding F grows from ~22% to ~37% between 56 and 130 GeV, and discusses centrality, energy, and A-dependence predictions. In the saturation picture, a self-consistent saturation scale Q_s^2 ~ 2 GeV^2 implies a high initial energy density (~18 GeV/fm^3); notably, centrality trends predicted by both approaches are similar, indicating the need for complementary observables such as p_t distributions to distinguish the underlying dynamics. The work highlights the onset of high parton density at collider energies and outlines how future measurements could discriminate between frameworks.

Abstract

We analyze the first results on charged particle multiplicity at RHIC in the conventional eikonal approach and in the framework of high density QCD. We extract the fraction of the hadron multiplicity originating from ``hard'' (i.e. proportional to the number of binary collisions) processes; we find a strong growth of this fraction with energy: , while . This indicates a rapid increase in the density of the produced particles. We outline the predictions of high density QCD for the centrality, energy, and atomic number dependence of hadron production. Surprisingly, the predictions of the conventional eikonal approach and of high density QCD for centrality dependence of hadron multiplicity at appear very similar.

Paper Structure

This paper contains 4 sections, 20 equations, 3 figures.

Table of Contents

  1. Introduction
  2. The eikonal approach to hadron production
  3. High density QCD and hadron production in nuclear collisions
  4. Summary and discussion

Figures (3)

  • Figure 1: Charged multiplicity distribution at $\sqrt{s}=130$ A GeV; solid line (histogram) -- PHOBOS result Phobos; dashed line -- distribution corresponding to participant scaling ($x=0$); dotted line -- distribution corresponding to the $37 \%$ admixture of "hard" component in the multiplicity ($x=0.09$); see text for details.
  • Figure 2: Correlation between the charged multiplicity near $\eta = 0$ and the forward energy; dash--dotted line -- the correlation corresponding to the participant scaling; solid line -- the correlation containing $37\%$ admixture of "hard" component in the multiplicity; dashed line is the participant scaling correlation normalized to the measured central multiplicity.
  • Figure 3: Centrality dependence of the charged multiplicity per participant pair near $\eta = 0$ at $\sqrt{s} = 130$ A GeV; the curves represent the prediction based on the conventional eikonal approach, while the diamonds correspond to the high density QCD prediction (see text). The square indicates the $pp$ multiplicity.