Measurement of longitudinal flow de-correlations in Pb+Pb collisions at sqrt(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 |\eta |<2.4 using 7 and 470 \upmu \hbox {b}^{-1} of Pb+Pb collisions at \sqrt{s_{\text {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 \eta intervals does not factorise into the product of single-particle coefficients, and this breaking of factorisation, or flow decorrelation, increases linearly with the \eta 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 k{\text {th}}-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 \eta . 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 v_2^2 and v_2v_3, respectively.