Flow fluctuations in Pb+Pb collisions at $\sqrt{s_\text{NN}}=5.02$ TeV with the ATLAS detector

Measurements of four-particle cumulants $c_n\{4\}$ for $n=1,2,3,4$ are presented using 470 $\mu\text{b}^{-1}$ of Pb+Pb collisions at $\sqrt{s_\text{NN}}=5.02$ TeV with the ATLAS detector at the LHC. These cumulants provide information on the event-by-event fluctuations of single harmonics $p(v_n)$. For the first time, a negative $c_1\{4\}$ is observed. The $c_4\{4\}$ is found to be negative in central collisions but changes sign around $20-25\%$ centrality and increases for more peripheral collisions. This behavior is consistent with a nonlinear contribution to $v_4$ that is proportional to $v_2^2$. $c_2\{4\}$ and $c_3\{\{4\}\}$ are calculated using two reference event classes in order to investigate the influence of volume fluctuations on the flow fluctuations. Over most of the centrality range, $c_2\{4\}$ and $c_3\{4\}$ are found to be negative, while in the ultra-central collisions, $c_2\{4\}$ changes sign and becomes positive, suggesting a deviation from Gaussian behavior in the event-by-event fluctuation of $v_2$. The magnitudes of the sign change are also found to be dependent of the event class definition.