Abstract
Measurements of longitudinal flow correlations are presented for charged particles in the pseudorapidity range |eta |<2.4 using 7 and 470 upmu hbox {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_2v_3, respectively.
Highlights
Heavy-ion collisions at RHIC and the LHC create hot, dense matter whose space-time evolution is well described by relativistic viscous hydrodynamics [1,2]
Owing to strong eventby-event (EbyE) density fluctuations in the initial state, the space-time evolution of the produced matter fluctuates event by event. These fluctuations lead to correlations of particle multiplicity in momentum space in both the transverse and longitudinal directions with respect to the collision axis
Measurement of the longitudinal flow dynamics requires the calculation of the flow vector qn via Eq (1) in the inner detector (ID) and the forward calorimeters (FCal)
Summary
Heavy-ion collisions at RHIC and the LHC create hot, dense matter whose space-time evolution is well described by relativistic viscous hydrodynamics [1,2]. Owing to strong eventby-event (EbyE) density fluctuations in the initial state, the space-time evolution of the produced matter fluctuates event by event These fluctuations lead to correlations of particle multiplicity in momentum space in both the transverse and longitudinal directions with respect to the collision axis. Recent model calculations pred√ict an increase of longitudinal flow fluctuations at lower sNN [24] Measurements of these observables at two collision energies can provide new insights into the initial condition along the longitudinal direction and should help in the development of full three-dimensional viscous hydrodynamic models. Using these new observables, this paper improves the study of the longitudinal dynamics of collective flow in three ways. This paper presents a meaususirnemg Penbt+oPfbflcoowllisdieocnosrraetl√atisoNnNin=vo2l.v7i6nganvd2,5v.032, v4 and TeV
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