Elliptic Anisotropy $v_2$ May Be Dominated by Particle Escape instead of Hydrodynamic Flow

TL;DR

This study shows that in relativistic heavy ion collisions, azimuthal anisotropies are mainly caused by particle escape rather than hydrodynamic flow, challenging traditional interpretations of these phenomena.

Contribution

The paper demonstrates through transport models that particle escape dominates azimuthal anisotropy, even in large systems, questioning the hydrodynamic flow paradigm.

Findings

Majority of $v_2$ arises from particle escape, not collective flow.

Azimuthal anisotropies are similar in small and large systems.

Hydrodynamic flow dominates only at unrealistically high cross sections.

Abstract

It is commonly believed that azimuthal anisotropies in relativistic heavy ion collisions are generated by hydrodynamic evolution of the strongly interacting quark-gluon plasma. Here we use transport models to study how azimuthal anisotropies depend on the number of collisions that each parton suffers. We find that the majority of $v_2$ comes from the anisotropic escape of partons, not from the parton collective flow, for semi-central Au+Au collisions at 200A GeV. As expected, the fraction of $v_2$ from the anisotropic particle escape is even higher for smaller systems such as d+Au. Our transport model results also confirm that azimuthal anisotropies would be dominated by hydrodynamic flow at unrealistically-high parton cross sections. Our finding thus naturally explains the similarity of azimuthal anisotropies in small and large systems; however, it presents a challenge to the paradigm of anisotropic flow.