Aaboud, M.; Aad, G.; Abbott, B.; Abbott, D. C.; Abdinov, O.; Abed Abud, A.; Abhayasinghe, D. K.; Abidi, S. H.; AbouZeid, O. S.; Abraham, N. L.
(2020):
Fluctuations of anisotropic flow in Pb plus Pb collisions at root s(NN)=5.02 TeV with the ATLAS detector.
In: Journal of High Energy Physics, No. 1, 51

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Abstract
Multiparticle azimuthal cumulants are measured as a function of centrality and transverse momentum using 470 mu b(1) of Pb+Pb collisions at root s(NN) = 5.02TeV with the ATLAS detector at the LHC. These cumulants provide information on the eventbyevent fluctuations of harmonic flow coefficients v(n) and correlated fluctuations between two harmonics v(n) and v(m). For the first time, a nonzero fourparticle cumulant is observed for dipolar flow, v(1). The fourparticle cumulants for elliptic flow, v(2), and triangular flow, v(3), exhibit a strong centrality dependence and change sign in ultracentral collisions. This sign change is consistent with significant nonGaussian fluctuations in v(2) and v(3). The fourparticle cumulant for quadrangular flow, v(4), is found to change sign in midcentral collisions. Correlations between two harmonics are studied with three and fourparticle mixedharmonic cumulants, which indicate an anticorrelation between v(2) and v(3), and a positive correlation between v(2) and v(4). These correlations decrease in strength towards central collisions and either approach zero or change sign in ultracentral collisions. To investigate the possible flow fluctuations arising from intrinsic centrality or volume fluctuations, the results are compared between two different event classes used for centrality definitions. In peripheral and midcentral collisions where the cumulant signals are large, only small differences are observed. In ultracentral collisions, the differences are much larger and transverse momentum dependent. These results provide new information to disentangle flow fluctuations from the initial and final states, as well as new insights on the influence of centrality fluctuations.