Are eccentricity fluctuations able to explain the centrality dependence of $v_4$?

TL;DR

This paper investigates whether fluctuations in eccentricity can explain the centrality dependence of the fourth harmonic flow coefficient $v_4$ in heavy-ion collisions, finding that fluctuations account for most but not all of the observed behavior.

Contribution

The study demonstrates that eccentricity fluctuations largely explain the centrality dependence of $v_4$, but cannot fully account for the large $v_4/(v_2)^2$ ratio in the most central collisions.

Findings

Eccentricity fluctuations significantly impact $v_4$ measurements.

Deviations from local thermal equilibrium are minor corrections.

Fluctuations reproduce data for peripheral and midcentral collisions, but not for the most central ones.

Abstract

The fourth harmonic of the azimuthal distribution of particles $v_4$ has been measured for Au-Au collisions at the Relativistic Heavy Ion Collider (RHIC). The centrality dependence of $v_4$ does not agree with the prediction from hydrodynamics. In particular, the ratio $v_4/(v_2)^2$, where $v_2$ denotes the second harmonic of the azimuthal distribution of particles, is significantly larger than predicted by hydrodynamics. We argue that this discrepancy is mostly due to elliptic flow ($v_2$) fluctuations. We evaluate these fluctuations on the basis of a Monte Carlo Glauber calculation. The effect of deviations from local thermal equilibrium is also studied, but appears to be only a small correction. Combining these two effects allows us to reproduce experimental data for peripheral and midcentral collisions. However, we are unable to explain the large magnitude of $v_4/(v_2)^2$ observed for the most central collisions.