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The Color Glass Condensate and hadron production in the forward region

Adrian Dumitru, Arata Hayashigaki, Jamal Jalilian-Marian

TL;DR

This paper demonstrates that forward hadron production in high-energy proton-nucleus collisions can be described by collinearly factorized projectile partons evolving via DGLAP scattering off a Color Glass Condensate target, with the target's small-$x$ evolution captured by dipole correlators. The authors compute one-loop corrections including recoil, showing that leading $\log Q^2$ terms organize into DGLAP evolution for projectile PDFs/FFs while the target dependence reduces to two-point Wilson-line dipoles, enabling the use of JIMWLK evolution or phenomenological dipole models. Applying the framework to forward RHIC data ($d+Au$ BRAHMS), they find good agreement once both $Q^2$-evolution and $x$-evolution are included, and they emphasize the substantial recoil effects and the distinct forward region dynamics compared to central LHC kinematics. These results provide strong evidence for the Color Glass Condensate at RHIC and establish a systematic approach to connect CGC physics with standard perturbative QCD evolution, with implications for predictions at higher energies and for electromagnetic probes.

Abstract

We consider one loop corrections to single inclusive particle production in parton-nucleus scattering at high energies, treating the target nucleus as a Color Glass Condensate. We prove by explicit computation that in the leading log Q^2 approximation, these corrections lead to collinear factorization and DGLAP evolution of the projectile parton distribution and hadron fragmentation functions. At leading logarithmic level, and in single-inclusive cross sections, only two-point functions of Wilson lines in the adjoint and fundamental representations (Mueller's dipoles) arise, which can be obtained from the solution of the JIMWLK equations. The application of our results to forward-rapidity production at RHIC energy shows that, in general, recoil effects are large and that the forward rapidity region at RHIC is rather different from the central region at LHC, despite comparable gluon densities in the target. We show that both the quantum x-evolution of the high-density target as well as the DGLAP Q^2-evolution of the parton distribution and fragmentation functions are clearly seen in the BRAHMS data. This provides additional strong evidence for the Color Glass Condensate at RHIC.

The Color Glass Condensate and hadron production in the forward region

TL;DR

This paper demonstrates that forward hadron production in high-energy proton-nucleus collisions can be described by collinearly factorized projectile partons evolving via DGLAP scattering off a Color Glass Condensate target, with the target's small- evolution captured by dipole correlators. The authors compute one-loop corrections including recoil, showing that leading terms organize into DGLAP evolution for projectile PDFs/FFs while the target dependence reduces to two-point Wilson-line dipoles, enabling the use of JIMWLK evolution or phenomenological dipole models. Applying the framework to forward RHIC data ( BRAHMS), they find good agreement once both -evolution and -evolution are included, and they emphasize the substantial recoil effects and the distinct forward region dynamics compared to central LHC kinematics. These results provide strong evidence for the Color Glass Condensate at RHIC and establish a systematic approach to connect CGC physics with standard perturbative QCD evolution, with implications for predictions at higher energies and for electromagnetic probes.

Abstract

We consider one loop corrections to single inclusive particle production in parton-nucleus scattering at high energies, treating the target nucleus as a Color Glass Condensate. We prove by explicit computation that in the leading log Q^2 approximation, these corrections lead to collinear factorization and DGLAP evolution of the projectile parton distribution and hadron fragmentation functions. At leading logarithmic level, and in single-inclusive cross sections, only two-point functions of Wilson lines in the adjoint and fundamental representations (Mueller's dipoles) arise, which can be obtained from the solution of the JIMWLK equations. The application of our results to forward-rapidity production at RHIC energy shows that, in general, recoil effects are large and that the forward rapidity region at RHIC is rather different from the central region at LHC, despite comparable gluon densities in the target. We show that both the quantum x-evolution of the high-density target as well as the DGLAP Q^2-evolution of the parton distribution and fragmentation functions are clearly seen in the BRAHMS data. This provides additional strong evidence for the Color Glass Condensate at RHIC.

Paper Structure

This paper contains 6 sections, 38 equations, 10 figures.

Table of Contents

  1. Introduction
  2. Including recoil in gluon production
  3. Forward hadron production in dA collisions at RHIC
  4. Summary
  5. Radiation from both initial and final state
  6. $2\to1$ kinematics

Figures (10)

  • Figure 1: Diagrams contributing to gluon production in quark-nucleus scattering.
  • Figure 2: $P_t$ dependence of our results compared to BRAHMS minimum bias data.
  • Figure 3: Ratio of the transverse momentum distributions of charged hadrons for $Q^2=(P_t/2)^2$ and $Q^2=P_t^2$.
  • Figure 4: Relative contribution of quarks and gluons from the projectile deuteron.
  • Figure 5: Importance of DGLAP evolution of the proton/hadron distribution/fragmentation functions and of the anomalous dimension of the target gluon distribution.
  • ...and 5 more figures