Cronin Effect and High-p_T Suppression in pA Collisions

TL;DR

The paper analyzes gluon production in pA collisions within the Color Glass Condensate framework, starting from a quasi-classical McLerran-Venugopalan baseline that yields Cronin enhancement due to multiple scatterings. It then incorporates nonlinear small-x evolution, showing that evolution suppresses gluon production at all p_T, flattening the Cronin peak and driving R^{pA} below unity with a strong centrality dependence. The work develops leading and higher-twist analyses, examines geometric scaling regions, and uses a toy model to illustrate the robust suppression pattern at high energies, with clear implications for interpreting RHIC and LHC data and distinguishing initial-state CGC effects from final-state interactions.

Abstract

We review the predictions of the theory of Color Glass Condensate for gluon production cross section in p(d)A collisions. We demonstrate that at moderate energies, when the gluon production cross section can be calculated in the framework of McLerran-Venugopalan model, it has only partonic level Cronin effect in it. At higher energies/rapidities corresponding to smaller values of Bjorken x quantum evolution becomes important. The effect of quantum evolution at higher energies/rapidities is to introduce suppression of high-p_T gluons slightly decreasing the Cronin enhancement. At still higher energies/rapidities quantum evolution leads to suppression of produced gluons at all values of p_T.

Figures (8)

• Figure 1: "Conventional" definition of unintegrated gluon distribution relating it to the gluon dipole cross section. The exchanged gluon lines can connect to either gluon in the dipole.
• Figure 2: Definition of unintegrated gluon distribution in McLerran-Venugopalan model.
• Figure 3: The ratio $R_A$ of unintegrated gluon distributions in the nucleus and in the nucleon. The thin line represents the Weizsäcker-Williams gluon distribution [Eq. (\ref{['wwglue']})] while the thick line correspond to the more conventional one inspired by $k_T$-factorization [Eq. (\ref{['ktglue']})].
• Figure 4: The ratio $R^{pA}$ for gluons plotted as a function of $k_T/Q_{s0}$ in the quasi-classical McLerran-Venugopalan model as found in KM. The cutoff is $\Lambda = 0.2 \, Q_s$.
• Figure 5: Gluon production in pA collisions as seen in the transverse plane. To make the picture easier to read the gluon is placed far away from the proton which is highly unlikely to happen in real life.