Measurement of longitudinal flow decorrelations in Pb+Pb collisions at s NN = 2.76 and 5.02 TeV with the ATLAS detector

Abstract

Measurements of longitudinal flow correlations are presented for charged particles in the pseudorapidity range $\left|\eta \right|<2.4$ using 7 and 470 $\mu {\text{b}}^{-1}$ 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 $v_2{v}_3$ , respectively.