Rapidity scaling of multiplicity and flow in weakly and strongly interacting systems

TL;DR

This paper investigates how multiplicity and elliptic flow scale with rapidity in different systems, highlighting the challenges of hydrodynamic models and the naturalness of weakly coupled systems in explaining observed flow patterns.

Contribution

It provides a comparative analysis of rapidity scaling in weakly and strongly coupled systems, emphasizing the limitations of hydrodynamics and proposing experimental tests for system characterization.

Findings

Multiplicity scaling is straightforward and insensitive to transport properties.

Elliptic flow scaling is difficult for hydrodynamic models with low Knudsen number.

Weakly coupled systems naturally reproduce observed elliptic flow scaling.

Abstract

We examine the "naturalness" of the scaling of multiplicity and elliptic flow $v_2$ with rapidity in weakly and strongly coupled systems. We show that multiplicity scaling is relatively straight-forward to incorporate in existing ansatze with no unnatural assumptions, and argue that this scaling is relatively insensitive to the transport properties of the system. On the other hand, we argue that the observed scaling of elliptic flow observed is problematic to describe within a hydrodynamic model (the Knudsen number $K \ll 1$), but arises more naturally within weakly coupled systems (where the Knudsen number $\sim 1$). We conclude by an overview of ways proposed to make weakly coupled systems compatible with the absolute value of elliptic flow, and by indicating experimental probes which could clarify these issues.