Elliptic flow at varying energy heavy ion collisions: Partonic vs hadronic dynamics

TL;DR

This paper investigates the causes of elliptic flow behavior in heavy ion collisions, challenging the assumption that quark number scaling breakdown indicates the absence of partonic matter, and explores how hadronic and partonic dynamics influence observed flow patterns.

Contribution

It demonstrates that variations in quark and anti-quark rapidity distributions can obscure quark number scaling, and discusses how different dynamics affect baryon and anti-baryon flow, questioning previous interpretations.

Findings

Quark number scaling can be broken by rapidity distribution effects.

Anti-baryon flow can be greater than baryon flow in partonic coalescence.

Hadronic dynamics can mimic partonic flow patterns.

Abstract

We examine whether the breakdown in elliptic flow quark number scaling observed at the Relativistic Heavy Ion Collider (RHIC) energy scan is related to the turning off of deconfinement by testing the hypothesis that hydrodynamics and parton coalescence always apply, but are obscured, at lower energies, by variations in the widths of quark and anti-quark rapidity distribution. We find that this effect is enough to spoil quark number scaling in elliptic flow. A lack of scaling in data therefore does not signal the absence of partonic degrees of freedom and hadronization by coalescence. In a coalescing partonic fluid, however, elliptic flow of anti-baryons should be the greater than that of baryons, since antibaryons contain a greater admixture of partons from the highly flowing midrapidity region. Intriguingly, purely hadronic dynamics has a similar dependence of baryon-anti-baryons $v_2$ as purely partonic dynamics, again because antibaryons tend to come from regions where the deviation of the system from hydrodynamic behavior is at its smallest. The opposite trend observed in experiment is therefore an indication that we might be misunderstanding the origin of $v_2$. We finish by discussing possible explanations of this, and suggest experimental measurements capable of clarifying the situation.