Azimuthal Anisotropy Distributions in High-Energy Collisions

TL;DR

This paper investigates how flow fluctuations in high-energy collisions can distinguish initial state anisotropies from hydrodynamic response, revealing insights into the medium's viscosity and system size effects.

Contribution

It introduces a method to disentangle initial anisotropy from response using non-Gaussian flow fluctuations in small systems.

Findings

Flow response coefficient decreases mildly with smaller systems.

The results suggest a low viscosity-to-entropy ratio.

Flow fluctuations help separate initial conditions from hydrodynamic response.

Abstract

Elliptic flow in ultrarelativistic heavy-ion collisions results from the hydrodynamic response to the spatial anisotropy of the initial density profile. A long-standing problem in the interpretation of flow data is that uncertainties in the initial anisotropy are mingled with uncertainties in the response. We argue that the non-Gaussianity of flow fluctuations in small systems can be used to disentangle the initial state from the response. We apply this method to recent measurements of anisotropic flow in Pb+Pb and p+Pb collisions at the LHC. The response coefficient is found to decrease mildly as the system becomes smaller. This mild decrease is consistent with a low value of the ratio of viscosity over entropy.