Abstract
Measurements of anisotropic flow coefficients with two- and multi-particle cumulants for inclusive charged particles in Pb-Pb collisions at sqrt{s_{mathrm{NN}}}=5.02 and 2.76 TeV are reported in the pseudorapidity range |η| < 0.8 and transverse momentum 0.2 < pT < 50 GeV/c. The full data sample collected by the ALICE detector in 2015 (2010), corresponding to an integrated luminosity of 12.7 (2.0) μb−1 in the centrality range 0-80%, is analysed. Flow coefficients up to the sixth flow harmonic (v6) are reported and a detailed comparison among results at the two energies is carried out. The pT dependence of anisotropic flow coefficients and its evolution with respect to centrality and harmonic number n are investigated. An approximate power-law scaling of the form vn(pT) ∼ pTn/3 is observed for all flow harmonics at low pT (0.2 < pT < 3 GeV/c). At the same time, the ratios vn/vmn/ m are observed to be essentially independent of pT for most centralities up to about pT = 10 GeV/c. Analysing the differences among higher-order cumulants of elliptic flow (v2), which have different sensitivities to flow fluctuations, a measurement of the standardised skewness of the event-by-event v2 distribution P(v2) is reported and constraints on its higher moments are provided. The Elliptic Power distribution is used to parametrise P(v2), extracting its parameters from fits to cumulants. The measurements are compared to different model predictions in order to discriminate among initial-state models and to constrain the temperature dependence of the shear viscosity to entropy-density ratio.
Highlights
Mulants for inclusive charged particles in Pb–Pb collisions at sNN = 5.02 and 2.76 TeV are reported in the pseudorapidity range |η| < 0.8 and transverse momentum 0.2 < pT < 50 GeV/c
The measurements are compared to different model predictions in order to discriminate among initial-state models and to constrain the temperature dependence of the shear viscosity to entropy-density ratio
Some theoretical uncertainties in the determination of anisotropic flow coefficients are expected to partially cancel in the ratios of vn coefficients measured at different collision energies, such as those on the choice of initial-state model or on the absolute value of η/s
Summary
The sample of Pb–Pb collisions used for this measurement was recorded with the ALICE detector [41, 42] in November and December 2015 (2010), during the Run 2 (Run 1) of the. These comparisons take into account the correlation between systematic uncertainties of data points in different centrality intervals This observation might indicate little or no temperature dependence of η/s within the temperature range at which anisotropic flow develops at the two center of mass energies. While not all viscous hydrodynamical models shown in figure 6 describe the data up to the highest pT very well, they all do exhibit the same power-law scaling in the ratio of harmonics over the pT range 0.5 < pT < 3 GeV/c, with a precision comparable to the one seen in the data, while they strongly deviate for pT < 0.5 GeV/c. The ratios are shown for one hydrodynamic model [62] in the four most central centrality intervals; it is qualitatively similar in the other centrality intervals and for the other models
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