Elliptic Flow from Minijet Production in Heavy Ion Collisions

TL;DR

This paper investigates how non-flow minijet production contributes to elliptic flow (v2) in ultrarelativistic heavy-ion collisions within a saturation framework. It develops a two-particle correlation approach using k_T-factorization with unintegrated gluon distributions characterized by a saturation scale Q_s, deriving a concrete v2(pT) from minijet-induced correlations and exploring its small- and large-pT limits as well as centrality dependence. The authors fit STAR v2(pT) data with Q_s around 1 GeV^2 and moderate α_s, arguing that minijet correlations can account for much of the high-pT elliptic flow and can contaminate standard flow analyses. They emphasize the need for flow analysis methods that suppress minijet contributions to isolate genuine collective QGP effects and outline future improvements involving rapidity correlations and evolution effects.

Abstract

We calculate the contribution to the elliptic flow observable v2 from two-particle correlations in minijet production in ultrarelativistic heavy ion collisions. We use a minijet production cross section derived in a model inspired by saturation approach to high energy scattering. Resulting differential elliptic flow v2(pT) is an increasing function of pT for transverse momenta below the saturation scale Qs. At higher transverse momenta (pT > Qs) differential flow stops growing and becomes approximately constant, reproducing the elliptic flow saturation data reported by STAR. The centrality dependence of the minijet contribution to v2 is also in good agreement with the data.

Figures (4)

• Figure 1: (a) One- and (b) two-gluon production amplitudes. Thick dots denote Lipatov vertices.
• Figure 2: Nuclear collision in the transverse plane.
• Figure 3: Differential elliptic flow data from STAR starsn versus the predictions of our minijet model. Different lines correspond to our predictions with different values of the saturation scale: $Q_s= 1.0$ GeV (solid line), $Q_s=1.1$ GeV (dashed line) and $Q_s=0.9$ GeV (dash-dotted line). We used the following parameters $\Lambda$=0.15 GeV, $\alpha_s$=0.3, $A$=197 (Gold).
• Figure 4: Centrality dependence of elliptic flow given by the STAR data (black dots) and our fit (empty crosses). We used $\Lambda$=0.15 GeV, $\alpha_s$=0.3, $A$=197, $Q_s$=1 GeV and $N_\mathrm{part} (B=0)$ = 344 dn.