Flow fluctuations in Pb+Pb collisions at s 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 μ b − 1 of Pb+Pb collisions at s 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. 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. 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.