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

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.