Measurement of longitudinal flow decorrelations in Pb+Pb collisions at $$\sqrt{s_{\text {NN}}}=2.76$$ 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 $$|\eta |<2.4$$ 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.