Large mass Q-Qbar production from the Color Glass Condensate

TL;DR

This work develops a leading-order calculation of heavy quark pair production in the Color Glass Condensate framework for high-energy heavy-ion collisions, treating the two nuclei as classical color sources. By solving the classical Yang-Mills field to order ρ1ρ2 and using external-field propagators, the authors derive the pair-production amplitude and show its equivalence to kt-factorization in the high-k⊥ region, with the cross-section expressible in terms of unintegrated gluon distributions. They demonstrate that kt-factorization is broken beyond leading order due to multiple scattering and density effects, and they discuss the energy dependence entering through the saturation scale Q_s and quantum evolution. The paper also outlines the procedures required to go beyond leading order, including numerical approaches to solving the Yang-Mills equations and higher-point correlators, highlighting the theoretical foundation for understanding heavy quark production in dense QCD matter. Overall, the results clarify factorization properties in the CGC and set the stage for quantitative studies of heavy quark production at small x in heavy-ion collisions.

Abstract

We compute quark-antiquark pair production in the context of the Color Glass Condensate model for central heavy-ion collisions. The calculation is performed analytically to leading order in the density of hard sources present in the projectiles, and is applicable to quarks with a mass large compared to the saturation momentum. The formulas derived in this paper are compared to expressions derived in the framework of collinearly factorized perturbative QCD and in kt factorization models. We comment on the breaking of kt factorization which occurs beyond leading order in our approach.

Figures (3)

• Figure 1: The leading contributions to the pair production amplitude in terms of $A^\mu_1$, $A^\mu_2$ and $A^\mu_{12}$. The gluon line terminated by a cross represents an insertion $-igA^\mu(x)$ of the external field.
• Figure 2: The Feynman diagrams that can contribute to pair production at the order ${\cal O}(\rho_1\rho_2)$. The bold lines represent the classical sources $\rho_1$ and $\rho_2$.
• Figure 3: Contributions to the pair production amplitude at order ${\cal O}(\rho_1^2 \rho_2)$. The diagrams involving bremsstrahlung have been omitted. The bold lines represent the hard color sources.