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

TL;DR

This paper measures how the harmonic flow coefficients in Pb+Pb collisions vary with pseudorapidity at two different energies, revealing linear decorrelation increases with eta separation and energy, with insights into flow fluctuation contributions.

Contribution

It provides the first detailed measurement of longitudinal flow decorrelations at two energies, analyzing higher-order moments and separating magnitude and event-plane contributions.

Findings

Flow decorrelation increases linearly with eta separation.

Decorrelations are larger at 2.76 TeV than 5.02 TeV.

Higher-order moments of flow show proportionality to the moment order for v3.

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 $\sqrt{s_{\textrm{NN}}}=2.76$ 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 $k^{\textrm{th}}$-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 $v_2v_3$, respectively.