Logo
ScienceStack
Table of Contents
Fetching ...
Sign In

The initial state of heavy-ion collisions

Javier L. Albacete, Adrian Dumitru, Cyrille Marquet

TL;DR

This paper surveys the Color Glass Condensate framework as the central theory for the initial state in high-energy heavy-ion collisions, emphasizing nonlinear small-$x$ dynamics and the emergence of a saturation scale $Q_s$ that governs early-time gluon distributions. It covers the BK and JIMWLK evolutions, running-coupling improvements, and practical formalisms such as $k_T$-factorization and the hybrid approach, together with Monte Carlo implementations like IP-Glasma to set initial conditions for hydrodynamics. The review highlights how e+p, p+A, and A+A data constrain initial-state gluon densities, fluctuations, and correlation patterns, including di-hadron correlations and ridge phenomena that originate from the early-time CGC dynamics before final-state interactions dominate. It argues that CGC-based initial conditions are essential for quantitatively describing bulk observables and long-range correlations, and that upcoming p+Pb data at the LHC will be pivotal in sharpening our understanding of high-density QCD and the onset of thermalization.

Abstract

We present a brief review of recent theoretical developments and related phenomenological approaches for understanding the initial state of heavy-ion collisions, with emphasis on the Color Glass Condensate formalism.

The initial state of heavy-ion collisions

TL;DR

This paper surveys the Color Glass Condensate framework as the central theory for the initial state in high-energy heavy-ion collisions, emphasizing nonlinear small- dynamics and the emergence of a saturation scale that governs early-time gluon distributions. It covers the BK and JIMWLK evolutions, running-coupling improvements, and practical formalisms such as -factorization and the hybrid approach, together with Monte Carlo implementations like IP-Glasma to set initial conditions for hydrodynamics. The review highlights how e+p, p+A, and A+A data constrain initial-state gluon densities, fluctuations, and correlation patterns, including di-hadron correlations and ridge phenomena that originate from the early-time CGC dynamics before final-state interactions dominate. It argues that CGC-based initial conditions are essential for quantitatively describing bulk observables and long-range correlations, and that upcoming p+Pb data at the LHC will be pivotal in sharpening our understanding of high-density QCD and the onset of thermalization.

Abstract

We present a brief review of recent theoretical developments and related phenomenological approaches for understanding the initial state of heavy-ion collisions, with emphasis on the Color Glass Condensate formalism.

Paper Structure

This paper contains 15 sections, 19 equations, 10 figures, 1 table.

Table of Contents

  1. Introduction
  2. Color Glass Condensate
  3. A Brief review
  4. Theory Status
  5. e+p to e+A collisions
  6. p+A collisions
  7. Transverse momentum integrated yields
  8. Single inclusive hadron yields and nuclear modification factors
  9. Double inclusive particle production and azimuthal correlations
  10. Bulk features of multiparticle production in A+A, p+A and p+p collisions
  11. Multiplicity distributions in p+p and p+A collisions
  12. The initial state in A+A collisions
  13. Multiplicities and transverse energy
  14. The CGC and its ridge
  15. Conclusions

Figures (10)

  • Figure 1: Left: rcBK fits to HERA data on the reduced cross section in e+p scattering at small-$x$ from Albacete:2010sy. Right: Different nPDF parametrizations for the nuclear gluon distribution Salgado:2011wc at $Q^{2}=1.69$ GeV$^2$.
  • Figure 2: Left: charged particle rapidity distribution for minimum-bias p+Pb collisions at 5 TeV from the KLN gluon saturation model Dumitru:2011wq. Center: rcBK UGD Albacete:2012xq. Right: IP-Sat UGD Tribedy:2011aa. ALICE data from ref. ALICE:2012xs.
  • Figure 3: Left: ALICE preliminary data ALICE:2012mj for the charged hadron nuclear modification factor at mid-rapidity as a function of $p_T$. The theoretical results correspond to the CGC calculations of Tribedy:2011aaAlbacete:2012xqRezaeian:2012ye, the nuclear PDF approach EPS09 Helenius:2012wd, the cold-nuclear matter predictions of Kang:2012kc, and the HIJING Monte Carlo Xu:2012au. Right: LO EPS09 Helenius:2012wd and rcBK-MC Albacete:2012xq forward-rapidity predictions.
  • Figure 4: STAR preliminary data Braidot:2010zh for the Coincident Probability between pairs of hadrons as a function of the relative azimuthal angle in d+Au collisions at RHIC. The theoretical results correspond to two CGC-based calculationsAlbacete:2010pg and Stasto:2011ru and a higher-twist one Kang:2011bp. Figure from Albacete:2012td
  • Figure 5: KNO scaling plot of charged particle multiplicity distributions in non-single diffractive $pp\,/\,p\overline{p}$ collisions at various energies as measured by the UA5 Ansorge:1988kn, ALICE Aamodt:2010ft and CMS Khachatryan:2010nk collaborations, respectively. Note that we restrict to the central rapidity region $|\eta|<0.5$ and to the bulk of the distributions up to 3.5 times the mean multiplicity.
  • ...and 5 more figures