Anisotropic flow in Run 2

18 March 2016

Exploiting the data collected during November 2015 with Pb–Pb collisions at the record-breaking energy of √sNN = 5.02 TeV, ALICE measured for the first time the anisotropic flow of charged particles at this energy.

Relativistic heavy-ion collisions are the tool of choice to investigate quark–gluon plasma (QGP) – a state of matter where quarks and gluons move freely over distances that are large in comparison to the typical size of a hadron. Anisotropic flow, which measures the momentum anisotropy of final-state particles, is sensitive on the one hand to the initial density and to the initial geometry fluctuations of the overlap region, and on the other hand to the transport properties of the QGP. Flow is quantified by the Fourier coefficients, νn, of the azimuthal distribution of the final-state charge particles. The dominant flow coefficient, ν2, referred to as elliptic flow, is related to the initial geometric anisotropy. Higher coefficients, such as triangular flow (ν3) and quadrangular flow (ν4), can be related primarily to the response of the produced QGP to fluctuations of the initial energy density profile of the participating nucleons.

Figure 1 shows the centrality dependence of flow coefficients, both for 2.76 and 5.02 TeV Pb–Pb collisions. Compared with the lower-energy results, the anisotropic flows ν2, ν3 and ν4 increase at the newly measured energy by (3.0±0.6)%, (4.3±1.4)% and (10.2±3.8)%, respectively, in the centrality range 0–50%.

The transport properties of the created matter are investigated by comparing the experimental results with hydrodynamic model calculations, where the shear-viscosity to entropy density ratio, η/s, is the dominant parameter. Previous studies demonstrated that anisotropic flow measurements are best described by calculations using a value of η/s close to 1/4π, which corresponds to the lowest limits for a quantum fluid. It is observed in figure 1 that the magnitude and the increase of anisotropic flow measured at the higher energy remain compatible with hydrodynamic predictions, favouring a constant value for η/s going from √sNN = 2.76 to 5.02 TeV Pb–Pb collisions.

It is also observed that the results of the pT-differential flow are comparable for both energies. This observation indicates that the increase measured in the integrated flow (figure 1) reflects the increase of the mean transverse momentum. Further comparisons of differential-flow measurements and theoretical calculations will provide a unique opportunity to test the validity of the hydrodynamic picture, and the power to further discriminate between various possibilities for the temperature dependence of the shear-viscosity to entropy density ratio of the produced matter in heavy-ion collisions at highest energies.

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