Measurement of longitudinal flow de-correlations in Pb+Pb collisions at $\sqrt{s_{\mathrm{NN}}}=2.76$ and 5.02 TeV with the ATLAS detector

Abstract

Measurements of longitudinal flow decorrelations are presented for charged particles in the pseudorapidity range $|\eta |<2.4$ using 7 $\mu$b${}^{-1}$ and 470 $\mu$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 slopes for this flow decorrelation are found to be larger at 2.76 TeV than 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 changing with $\eta$. These two contributions are found to be comparable. The longitudinal flow correlations are also measured between $v_n$ of different order in $n$. The longitudinal fluctuations of $v_2$ and $v_3$ are found to be independent of each other, while the longitudinal fluctuations 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.

arXiv comment27 pages plus author list + cover pages (44 pages total), 22 figures, 5 tables, submitted to Eur. Phys. J. C., All figures including auxiliary figures can be found at https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HION-2016-04/

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