Measurement of longitudinal flow decorrelations in Pb+Pb collisions at √s̅N̅N̅ = 2.76 and 5.02 TeV with the ATLAS detector

Measurements of longitudinal flow correlations are presented for charged particles in the pseudorapidity range |η|<2.4 using 7 and 470 μb⁻¹ of Pb+Pb collisions at √sNN = 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 ν_n measured in two separated η intervals does not factorise into the product of single-particle coefficients, and this breaking of factorisation, or flow decorrelation, increases linearly with the η 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 ν_n are found to be proportional to k for ν₃, but not for ν₂. The decorrelation effect is separated into contributions from the magnitude of ν_n and the event-plane orientation, each as a function of η. These two contributions are found to be comparable. The longitudinal flow correlations are also measured between vn of different order in n. The decorrelations of ν₂ and ν₃ are found to be independent of each other, while the decorrelations of ν₄ and ν₅ are found to be driven by the nonlinear contribution from v²₂ and ν₂ν₃, respectively