Flow fluctuations in Pb+Pb collisions at √ sNN = 5.02 TeV with the ATLAS detector

Abstract

Measurements of four-particle cumulants cn{4} for n = 1, 2, 3, 4 are presented using 470 μb−1 of Pb+Pb collisions at √sNN = 5.02 TeV with the ATLAS detector at the LHC. These cumulants provide information on the event-by-event fluctuations of single harmonics p(vn). For the first time, a negative c1{4} is observed. The c4{4} is found to be negative in central collisions but changes sign around 20 − 25% centrality. This behavior is consistent with a nonlinear contribution to v4 that is proportional to v22 . c2{4} and c3{4} are calculated using two reference event classes in order to investigate the influence of volume fluctuations. Over most of the centrality range, c2{4} and c3{4} are found to be negative, while in the ultra-central collisions, c2{4} changes sign and becomes positive, suggesting a deviation from Gaussian behavior in the event-by-event fluctuation of v2. The magnitudes of the sign change are also found to be dependent of the event class definition.