Collective phenomena in non-central nuclear collisions

TL;DR

The paper surveys azimuthal anisotropic flow in non-central nuclear collisions from AGS to RHIC, focusing on measurement techniques, interpretation, and uncertainties. It compares event-plane, two- and multi-particle correlations, and transform-based methods, detailing how nonflow and flow fluctuations bias different approaches and how higher-order cumulants and LYZ methods isolate true collective flow. The authors synthesize how flow arises from early-stage dynamics, radial expansion, and coalescence, with strong evidence for a low-viscosity, deconfined quark-gluon plasma and significant initial-state fluctuations that influence higher harmonics. They also outline future directions, including beam-energy scans, uranium collisions, and the LHC, emphasizing the need for high-statistics data and refined theoretical modeling to disentangle fluctuations and nonflow from genuine flow signals.

Abstract

Recent developments in the field of anisotropic flow in nuclear collision are reviewed. The results from the top AGS energy to the top RHIC energy are discussed with emphasis on techniques, interpretation, and uncertainties in the measurements.

Figures (54)

• Figure 1: Diagrams of elliptic and directed flow.
• Figure 2: The definitions of the Reaction Plane and Participant Plane coordinate systems.
• Figure 3: The event plane resolution as a function of $v_m \sqrt{M}$. The harmonic number of the correlation $n$ is an integer $k$ times the harmonic number $m$ of the event plane.
• Figure 4: Directed and elliptic flow as a function of rapidity and transverse momentum from minimum bias 158 A GeV Pb+Pb collisions Alt:2003ab.
• Figure 5: Directed flow as a function of rapidity for charged pions from minimum bias 158 A GeV Pb+Pb collisions Borghini:2002mv. Shown are $v_1$ before (squares) and after (circles) correction for momentum conservation.