Elliptic flow in heavy ion collisions at varying energies: Partonic versus hadronic dynamics

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 antiquark 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 antibaryons should be 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-antibaryons elliptic flow 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 elliptic flow. We finish by discussing possible explanations of this and suggest experimental measurements capable of clarifying the situation.