Measurement of longitudinal flow decorrelations in Pb+Pb collisions at √sNN=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 μ b ^{- 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 v_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 v_n are found to be proportional to k for v₃, but not for v₂. The decorrelation effect is separated into contributions from the magnitude of v_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 v_n of different order in n. The decorrelations of v₂ and v₃ are found to be independent of each other, while the decorrelations of v₄ and v₅ are found to be driven by the nonlinear contribution from v22 and v₂v₃, respectively.