Abstract

We review topics related to the first moment of azimuthal anisotropy (v1), 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.

Highlights

  • The purpose of relativistic nuclear collision experiments is the creation and study of nuclear matter at high energy densities

  • V1 can probe the very early stages of the collision [19, 20], when the deconfined state of quarks and gluons is expected to dominate the collision dynamics [9, 10]. Both hydrodynamic [21, 22] and transport model [23, 24] calculations indicate that the directed flow of charged particles, especially baryons at midrapidity, is sensitive to the equation of state and can be used to explore the QCD phase diagram

  • In mass-asymmetric collisions like Cu+Au, the well-defined distinction that exists in mass-symmetric collisions between the odd V1(η) component and the even V1(η) component no longer holds

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Summary

Introduction

The purpose of relativistic nuclear collision experiments is the creation and study of nuclear matter at high energy densities. V1 can probe the very early stages of the collision [19, 20], when the deconfined state of quarks and gluons is expected to dominate the collision dynamics [9, 10] Both hydrodynamic [21, 22] and transport model [23, 24] calculations indicate that the directed flow of charged particles, especially baryons at midrapidity, is sensitive to the equation of state and can be used to explore the QCD phase diagram. Various models exhibit this kind of behavior At these energies, both hydrodynamic and nuclear transport calculations predict a negative sign for charged particle dV1/dy near midrapidity, where pions are the dominant particle species.

Differential Measurements of Charged
Directed Flow of Charged Particles in MassAsymmetric Collisions
Differential Measurements of Identified Particle Directed Flow
39 GeV STAR preliminary p
Recent Model Calculations of Directed Flow
Summary and Outlook

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