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

Measurements of longitudinal flow correlations are presented for charged particles in the pseudorapidity range $$ using 7 and 470 $$ of Pb+Pb collisions at $$ and 5.02 TeV, respectively, recorded by the ATLAS detector at the LHC. It is found that the correlation between the harmonic flow coefficients $$ 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 $$ -moment of the $$ are found to be proportional to k for $$ , but not for $$ . The decorrelation effect is separated into contributions from the magnitude of $$ and the event-plane orientation, each as a function of $$ . These two contributions are found to be comparable. The longitudinal flow correlations are also measured between $$ of different order in n. The decorrelations of $$ and $$ are found to be independent of each other, while the decorrelations of $$ and $$ are found to be driven by the nonlinear contribution from $$ and $$ , respectively.