Measurement of longitudinal flow correlations in Pb+Pb collisions at $\sqrt{s_{NN}}=2.76$ 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 Pb+Pb collisions at $\sqrt{s_{NN}}=2.76$ and $5.02$ TeV, respectively, with the ATLAS detector at the LHC. It is found that the correlation between the harmonic flow coefficients $\bf{v}_n$ measured in two separated $\eta$ intervals does not factorize into the product of single-particle flow harmonic $\bf{v}_n$, and this breaking of factorization, or flow decorrelation, increases linearly with the $\eta$ separation between the intervals. The linear coefficients of the flow decorrelation are found to be larger at 2.76 TeV than 5.02 TeV. The longitudinal flow correlations are also measured between $\bf{v}_n$ of different order in n. The longitudinal fluctuations of $\bf{v}_4$ are found to be driven by the non-linear contribution from $\bf{v}_2^2$ respectively.