Scaling of flow in heavy ion collisions and the low-energy frontier

TL;DR

This paper questions the conventional interpretation of elliptic flow in heavy ion collisions, emphasizing the need for low-energy collider data to clarify the underlying properties of hadronic matter.

Contribution

It critically examines the assumptions behind flow interpretation and highlights the importance of low-energy experiments like NICA for understanding hadronic matter.

Findings

The dependence of $v_2$ on energy, rapidity, and system size complicates the flow interpretation.

Low viscosity and high opacity properties are not clearly established at current energies.

Low-energy collider data is crucial for verifying the flow interpretation.

Abstract

The common interpretation of elliptic flow $v_2$ in heavy ion collisions is that it is produced by hydrodynamic flow at low transverse momentum and by parton energy loss at high transverse momentum. Here, we discuss this interpretation in view of the dependence of $v_2$ with energy, rapidity and system size, and show that it is far from clear how the relevant properties necessary for this interpretation, low viscosity and high opacity, turn on. A low energy collider such as NICA is essential for this interpretation to be verified, understood and related to the fundamental properties of hadronic matter