Elliptic Flow from Non-equilibrium Initial Condition with a Saturation Scale

TL;DR

This paper shows that using a non-equilibrium initial condition based on a Color Glass Condensate in kinetic theory models can reconcile experimental elliptic flow data with a low shear viscosity to entropy ratio, challenging previous assumptions.

Contribution

It demonstrates that a Color Glass Condensate initial condition reduces elliptic flow efficiency, aligning data with low shear viscosity estimates in kinetic theory simulations.

Findings

Color Glass Condensate initial condition reduces elliptic flow build-up.

Data compatible with low shear viscosity to entropy ratio ($4\,\pi\eta/s \sim 1$).

Supports consistent description of higher order flow signatures.

Abstract

A current goal of relativistic heavy ion collisions experiments is the search for a Color Glass Condensate as the limiting state of QCD matter at very high density. In viscous hydrodynamics simulations, a standard Glauber initial condition leads to estimate $4\pi \eta/s \sim 1$, while a Color Glass Condensate modeling leads to at least a factor of 2 larger $\eta/s$. Within a kinetic theory approach based on a relativistic Boltzmann-like transport simulation, we point out that the out-of-equilibrium initial distribution proper of a Color Glass Condensate reduces the efficiency in building-up the elliptic flow. Our main result at RHIC energy is that the available data on $v_2$ are in agreement with a $4\pi \eta/s \sim 1$ also for Color Glass Condensate initial conditions, opening the possibility to describe self-consistently also higher order flow, otherwise significantly underestimated, and to pursue further the search for signatures of the Color Glass Condensate.