Is anisotropic flow really acoustic?

TL;DR

This paper investigates whether anisotropic flow in heavy-ion collisions is driven by sound-like propagation in the quark-gluon plasma, using flow harmonic ratios and scaling patterns to extract viscosity and initial condition constraints.

Contribution

It demonstrates that flow harmonic ratios follow viscous damping scaling consistent with sound propagation, providing new constraints on shear viscosity and initial eccentricity models.

Findings

Flow harmonic ratios exhibit viscous damping scaling.

Scaling patterns constrain shear viscosity to entropy density ratio.

Results are insensitive to initial geometry assumptions.

Abstract

The flow harmonics for charged hadrons ($v_{n}$) and their ratios $(v_n/v_2)_{n\geq 3}$, are studied for a broad range of transverse momenta ($p_T$) and centrality ($\text{cent}$) in Pb+Pb collisions at $\sqrt{s_{NN}}= 2.76$ TeV. They indicate characteristic scaling patterns for viscous damping consistent with the dispersion relation for sound propagation in the plasma produced in the collisions. These scaling properties are not only a unique signature for anisotropic expansion modulated by the specific shear viscosity ($\eta/s$), they provide essential constraints for the relaxation time, a distinction between two of the leading models for initial eccentricity, as well as an extracted $\left< \eta/s \right>$ value which is insensitive to the initial geometry model. These constraints could be important for a more precise determination of $\eta/s$.