The first moment of azimuthal anisotropy in nuclear collisions from AGS to LHC energies

TL;DR

This paper reviews the measurement and theoretical understanding of the first azimuthal anisotropy moment ($v_1$) in heavy-ion collisions across a wide energy range, from AGS to LHC, to explore QGP and QCD phase diagram.

Contribution

It provides a comprehensive overview of experimental data and theoretical models of $v_1$ from various energies, highlighting recent progress and challenges.

Findings

$v_1$ measurements vary with collision energy and particle type.

Theoretical models show diverse predictions for $v_1$ behavior.

Insights into the QCD phase diagram are discussed.

Abstract

We review topics related to the first moment of azimuthal anisotropy ($v_1$), commonly known as directed flow, focusing on both charged particles and identified particles from heavy-ion collisions. Beam energies from the highest available, at the CERN LHC, down to projectile kinetic energies per nucleon of a few GeV per nucleon, as studied in experiments at the Brookhaven AGS, fall within our scope. We focus on experimental measurements and on theoretical work where direct comparisons with experiment have been emphasized. The physics addressed or potentially addressed by this review topic includes the study of Quark Gluon Plasma, and more generally, investigation of the Quantum Chromodynamics phase diagram and the equation of state describing the accessible phases.