Measurement of longitudinal flow decorrelations in Pb+Pb collisions at sqrt(s)NN=2.76 and 5.02 TeV with the ATLAS detector

Measurements of longitudinal flow correlations are presented for charged particles in the pseudorapidity range | ?| < 2.4 using 7 and 470 ? b - 1 of Pb+Pb collisions at sqrt(s)NN=2.76 and 5.02 TeV, respectively, recorded by the ATLAS detector at the LHC. It is found that the correlation between the harmonic flow coefficients v n measured in two separated ? intervals does not factorise into the product of single-particle coefficients, and this breaking of factorisation, or flow decorrelation, increases linearly with the ? separation between the intervals. The flow decorrelation is stronger at 2.76 TeV than at 5.02 TeV. Higher-order moments of the correlations are also measured, and the corresponding linear coefficients for the kth -moment of the v n are found to be proportional to k for v 3 , but not for v 2 . The decorrelation effect is separated into contributions from the magnitude of v n and the event-plane orientation, each as a function of ?. These two contributions are found to be comparable. The longitudinal flow correlations are also measured between v n of different order in n. The decorrelations of v 2 and v 3 are found to be independent of each other, while the decorrelations of v 4 and v 5 are found to be driven by the nonlinear contribution from v22 and v 2 v 3 , respectively. � 2018, CERN for the benefit of the ATLAS collaboration.