Logo
ScienceStack
Table of Contents
Fetching ...
Sign In

The Color Glass Condensate and High Energy Scattering in QCD

Edmond Iancu, Raju Venugopalan

TL;DR

The Color Glass Condensate provides a unified weak-coupling framework for high-energy QCD at small x, describing dense gluon systems via classical fields sourced by fast partons. The paper develops the MV model, Wilson-line CGC formalism, and non-linear evolution (BK/JIMWLK) leading to a saturation scale Q_s and geometric scaling, and applies to DIS and nucleus collisions. It discusses predictions for structure functions, dipole scattering, and heavy-ion initial conditions, highlighting both successes (geometric scaling, saturation) and challenges (full thermalization, distinguishing initial vs final state effects). The framework potentially explains unitarization of cross-sections and Froissart-bound-like behavior while informing future eA/ePIC experiments.

Abstract

At very high energies or small values of Bjorken x, the density of partons, per unit transverse area, in hadronic wavefunctions becomes very large leading to a saturation of partonic distributions. When the scale corresponding to the density per unit transverse area, the saturation scale Q_s, becomes large (Q_s\gg Λ_{QCD}), the coupling constant becomes weak (α_S(Q_s)\ll 1) which suggests that the high energy limit of QCD may be studied using weak coupling techniques. This simple idea can be formalized in an effective theory, the Color Glass Condensate (CGC), which describes the behavior of the small x components of the hadronic wavefunction in QCD. The Green functions of the theory satisfy Wilsonian renormalization group equations which reduce to the standard linear QCD evolution equations in the limit of low parton densities. The effective theory has a rich structure that has been explored using analytical and numerical techniques. The CGC can be applied to study a wide range of high energy scattering experiments from Deep Inelastic Scattering at HERA and the proposed Electron Ion Collider (EIC) to proton/deuterium-nucleus and nucleus-nucleus experiments at the RHIC and LHC colliders.

The Color Glass Condensate and High Energy Scattering in QCD

TL;DR

The Color Glass Condensate provides a unified weak-coupling framework for high-energy QCD at small x, describing dense gluon systems via classical fields sourced by fast partons. The paper develops the MV model, Wilson-line CGC formalism, and non-linear evolution (BK/JIMWLK) leading to a saturation scale Q_s and geometric scaling, and applies to DIS and nucleus collisions. It discusses predictions for structure functions, dipole scattering, and heavy-ion initial conditions, highlighting both successes (geometric scaling, saturation) and challenges (full thermalization, distinguishing initial vs final state effects). The framework potentially explains unitarization of cross-sections and Froissart-bound-like behavior while informing future eA/ePIC experiments.

Abstract

At very high energies or small values of Bjorken x, the density of partons, per unit transverse area, in hadronic wavefunctions becomes very large leading to a saturation of partonic distributions. When the scale corresponding to the density per unit transverse area, the saturation scale Q_s, becomes large (Q_s\gg Λ_{QCD}), the coupling constant becomes weak (α_S(Q_s)\ll 1) which suggests that the high energy limit of QCD may be studied using weak coupling techniques. This simple idea can be formalized in an effective theory, the Color Glass Condensate (CGC), which describes the behavior of the small x components of the hadronic wavefunction in QCD. The Green functions of the theory satisfy Wilsonian renormalization group equations which reduce to the standard linear QCD evolution equations in the limit of low parton densities. The effective theory has a rich structure that has been explored using analytical and numerical techniques. The CGC can be applied to study a wide range of high energy scattering experiments from Deep Inelastic Scattering at HERA and the proposed Electron Ion Collider (EIC) to proton/deuterium-nucleus and nucleus-nucleus experiments at the RHIC and LHC colliders.

Paper Structure

This paper contains 36 sections, 180 equations, 31 figures.

Table of Contents

  1. Outstanding Phenomenological Questions in High Energy QCD
  2. Introduction
  3. Light cone kinematics and dynamics
  4. High energy behavior of total cross-sections
  5. Multi-particle production in QCD
  6. Deep Inelastic Scattering
  7. Nucleus-Nucleus and Proton-Nucleus Collisions
  8. Universality of High Energy Scattering
  9. The Effective Theory for the Color Glass Condensate
  10. The hadron wavefunction at small x
  11. The McLerran-Venugopalan model for a large nucleus
  12. The Color Glass
  13. The classical color field
  14. The gluon distribution
  15. Gluon saturation in a large nucleus
  16. ...and 21 more sections

Figures (31)

  • Figure 1: A hadron-hadron collision. The produced particles are shown as circles.
  • Figure 2: The rapidity distribution of particles produced in a hadronic collision.
  • Figure 3: Feynman scaling of rapidity distributions. The two different lines correspond to rapidity distributions at different energies.
  • Figure 4: Deep inelastic scattering of an electron on a hadron.
  • Figure 5: The Zeus data for the gluon structure functions.
  • ...and 26 more figures