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

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