The fluctuations of quadrangular flow

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.