The fluctuations of quadrangular flow

TL;DR

This paper reports the first measurement of the fourth cumulant of quadrangular flow in heavy-ion collisions, revealing a sign change at high centrality, explained by hydrodynamic models.

Contribution

It provides the first experimental measurement of $v_4igrace{4}$ and demonstrates its sign change, supported by hydrodynamic predictions.

Findings

Fourth cumulant $v_4igrace{4}$ changes sign above 30 ext{% centrality.

Hydrodynamic models predict the sign change due to nonlinear response.

Elliptic flow fluctuations influence quadrangular flow behavior.

Abstract

The ATLAS Collaboration has measured for the first time the fourth cumulant of quadrangular flow, $v_4\{4\}^4$. Unlike the fourth cumulants of elliptic and triangular flows, it presents a change of sign above 30\% centrality. We show that this change of sign is predicted by event-by-event hydrodynamics. We argue that it results from the combined effects of a nonlinear hydrodynamic response, which couples quadrangular flow to elliptic flow, and elliptic flow fluctuations.