Effects of flow fluctuations and partial thermalization on $v_4$

TL;DR

This paper investigates the factors influencing the fourth harmonic $v_4$ in heavy-ion collisions, finding that flow fluctuations mainly explain the large $v_4/(v_2)^2$ ratio, with partial thermalization having a minor effect.

Contribution

It demonstrates that elliptic flow fluctuations are the primary cause of the large $v_4/(v_2)^2$ ratio, challenging existing models of eccentricity fluctuations.

Findings

Flow fluctuations explain most of the large $v_4/(v_2)^2$ ratio.

Partial thermalization has a small effect on $v_4/(v_2)^2$.

Standard eccentricity fluctuation models cannot fully account for the data in central collisions.

Abstract

The second and fourth Fourier harmonic of the azimuthal distribution of particles, $v_2$ and $v_4$, have been measured in Au+Au collisions at the Relativistic Heavy Ion Collider (RHIC). The harmonic $v_4$ is mainly induced from $v_2$ as a higher-order effect. However, the ratio $v_4/(v_2)^2$ is significantly larger than predicted by hydrodynamics. Effects of partial thermalization are estimated on the basis of a transport calculation, and are shown to increase $v_4/(v_2)^2$ by a small amount. We argue that the large value of $v_4/(v_2)^2$ seen experimentally is mostly due to elliptic flow fluctations. However, the standard model of eccentricity fluctuations is unable to explain the large magnitude of $v_4/(v_2)^2$ in central collisions.