Systematic studies of correlations between different order flow harmonics in Pb-Pb collisions at sNN=2.76 TeV

Abstract

The correlations between event-by-event fluctuations of anisotropic flow harmonic amplitudes have been measured in Pb-Pb collisions at √ s NN = 2.76 TeV with the ALICE detector at the Large Hadron Collider.The results are reported in terms of multiparticle correlation observables dubbed symmetric cumulants. These observables are robust against biases originating from nonflow effects. The centrality dependence of correlations between the higher order harmonics (the quadrangular v 4 and pentagonal v 5 flow) and the lower order harmonics (the elliptic v 2 and triangular v 3 flow) is presented. The transverse momentum dependences of correlations between v 3 and v 2 and between v 4 and v 2 are also reported. The results are compared to calculations from viscous hydrodynamics and a multiphase transport (AMPT) model calculations. The comparisons to viscous hydrodynamic models demonstrate that the different order harmonic correlations respond differently to the initial conditions and the temperature dependence of the ratio of shear viscosity to entropy density (η/s). A small average value of η/s is favored independent of the specific choice of initial conditions in the models. The calculations with the AMPT initial conditions yield results closest to the measurements. Correlations among the magnitudes of v 2 , v 3 , and v 4 show moderate p T dependence in midcentral collisions. This might be an indication of possible viscous corrections to the equilibrium distribution at hadronic freeze-out, which might help to understand the possible contribution of bulk viscosity in the hadronic phase of the system. Together with existing measurements of individual flow harmonics, the presented results provide further constraints on the initial conditions and the transport properties of the system produced in heavy-ion collisions.

Higher harmonic non-linear flow modes of charged hadrons in Pb-Pb collisions at $$ \sqrt{s_{\mathrm{NN}}} $$ = 5.02 TeV

“…Consequently, the early constraints placed the value of η/s between 0.08 to 0.16 [33][34][35]. However, the limited sensitivity of the elliptic flow to η/s and the large uncertainty in the initial state anisotropy inhibit a precise determination of the value of η/s [34,[36][37][38], and its temperature dependence, which was recently shown to be explorable during the second run of LHC [39,40]. In addition, part of the anisotropic flow can also originate from the hadronic phase [41][42][43].…”

“…Flow harmonics of order n ≥ 3 reveal finer details of initial conditions [6,8,10,11,13], enabling to constrain η/s better [39,40,48,49]. Higher flow harmonics n > 3 do not exhibit a linear response to the initial anisotropy [26] as a finite contribution is induced by the initial state anisotropy of the lower orders [50,51].…”

Collective Effects in Nuclear Collisions: Experimental Overview

“…Besides, it was found that although hydrodynamic calculations using EKRT initial condition and various parameterizations could nicely describe measurements of v n , none of the hydrodynamic calculations could quantitatively fit the sizes of S C(3, 2) and S C(4, 2) simultaneously. This study was further extended to correlations between higher order flow coefficients by ALICE [39] and compared to more available theoretical calculations [25], which again suggest S C(m, n) as a tight constraint on theoretical models. Recently, results are also reported in various collision systems, including Pb-Pb, Au-Au, Xe-Xe, p-Pb and pp collisions [40,41].…”

Measurement of two-particle correlations with respect to second- and third-order event planes in Au + Au collisions at sNN=200 GeV

“…We assume that the v n measured through the event-plane method are the same as those in the correlation functions. Event-by-event v n fluctuations [54], v n -v m correlations between different orders [55], normalized symmetric cumulants (v 2 n -v 2 m correlations) [56], and rapidity-dependent event-plane decorrelations [57] are not included in this background model. To take into account the…”