Quark pair production in high energy pA collisions: General features

TL;DR

The paper develops and applies the Color Glass Condensate framework to quark-pPair production in high-energy pA collisions, combining multiple scattering (MV model) with small-x quantum evolution (BK equation). It shows how pair observables emerge from Wilson-line correlators, reveals systematic kT-factorization breaking at moderate transverse momenta, and quantifies nuclear-size and quark-mass dependencies. By solving BK evolution for correlators, the work elucidates rapidity-dependent shadowing and Cronin effects, predicting suppression patterns in R_pA that vary with P_T and Y, with qualitative distinctions between RHIC and LHC kinematics. The results establish a bridge between all-twist CGC dynamics and conventional collinear/pQCD expectations, providing guidance for interpreting heavy-flavor pair production in future collider data.

Abstract

A consistent treatment of both multiple scattering and small x quantum evolution effects on pair production in high energy pA collisions is feasible in the framework of the Color Glass Condensate (hep-ph/0402257). We first discuss the properties of quark pair production in the classical effective theory where only multiple scattering effects are included. Explicit results are given for pair production as a function of the invariant mass of pairs, the pair momenta, the atomic mass number A and the quark mass. We relate the logarithms that appear in our formulation of pair production to logarithms that appear in the limit of collinear factorization in QCD. Violations of kT factorization and medium modifications, as represented by the Cronin effect, are also investigated. We next consider how small x quantum evolution (shadowing) effects modify the results for pair production. In particular, we provide results for the rapidity distribution of pairs and the dependence of the Cronin effect on rapidity. We discuss the dependence of our results on the initial conditions for small x evolution and comment on its implications for pair production at RHIC and the LHC.

Figures (14)

• Figure 1: Cartoon representing gluon production in proton-nucleus collisions in the Color Glass Condensate framework.
• Figure 2: The two types of terms that enter in the $Q\overline{Q}$ production amplitude in pA collisions. The gluon emitted by the proton can either emit the pair after the collision with the nucleus (left diagram) or before colliding with the nucleus (diagram on the right). The black blob denotes multiple scatterings resummed via Wilson lines.
• Figure 3: Pair cross-section (in arbitrary units) in the MV model, as a function of ${\boldsymbol P}_\perp$ and $M$.
• Figure 4: Diagrams in the framework of collinear factorization obtained when one (top), or both (bottom), of the transverse momentum transfer to the produced pair from the proton and nucleus becomes small.
• Figure 5: Pair cross-section as a function of ${\boldsymbol P}_\perp$ for fixed $M$ (left). Pair cross-section as a function of $M$ for fixed ${\boldsymbol P}_\perp$ (right). The results in the ${\boldsymbol k}_\perp$-factorized approximation are shown with thin lines.