Saturation Physics and Deuteron--Gold Collisions at RHIC

TL;DR

This paper reviews saturation and Color Glass Condensate theory as a universal description of high-energy hadronic and nuclear interactions, emphasizing both classical field descriptions and quantum small-x evolution. It derives and analyzes particle production observables in proton-nucleus collisions, presenting gluon, valence quark, and electromagnetic probe results within the CGC framework and highlighting the role of the BK and JIMWLK equations. The authors compare CGC predictions to RHIC d+Au data, finding qualitative and quantitative agreement, which they argue supports the possible discovery of CGC at RHIC, while outlining further measurements to test this interpretation. The work also discusses two-particle correlations and the breakdown of certain factorization schemes in the CGC setting, illustrating rich nonlinear dynamics that shape particle production across rapidity and transverse momentum.

Abstract

We present a review of parton saturation/Color Glass Condensate physics in the context of deuteron-gold ($d+Au$) collisions at RHIC. Color Glass Condensate physics is a universal description of all high energy hadronic and nuclear interactions. It comprises classical (McLerran-Venugopalan model and Glauber-Mueller rescatterings) and quantum evolution (JIMWLK and BK equations) effects both in small-$x$ hadronic and nuclear wave functions and in the high energy scattering processes. Proton-nucleus (or $d+A$) collisions present a unique opportunity to study Color Glass Condensate predictions, since many relevant observables in proton-nucleus collisions are reasonably well-understood theoretically in the Color Glass Condensate approach. In this article we review the basics of saturation/Color Glass Condensate physics and reproduce derivations of many important observables in proton (deuteron)--nucleus collisions. We compare the predictions of Color Glass physics to the data generated by $d+Au$ experiments at RHIC and observe an agreement between the data and the theory, indicating that Color Glass Condensate has probably been discovered at RHIC. We point out further experimental measurements which need to be carried out to test the discovery.

Figures (65)

• Figure 1: Gluon cascade leading to the BFKL evolution. All gluon emissions are ordered in light cone "+" momenta, and, correspondingly, in light cone time.
• Figure 2: Scattering of two onia with the interaction mediated by the BFKL evolution.
• Figure 3: Schematic representation of a single rung of the BFKL evolution.
• Figure 4: BFKL evolution as a ladder diagram.
• Figure 5: The diagrams contributing to effective Lipatov vertex, which is denoted by a very thick dot.