Abstract
In relativistic heavy-ion collisions a dense and hot medium is created. It is thought to be the quark-gluon plasma (QGP), a state of matter in which the quarks and gluons, normally confined in hadrons, are (quasi-) free. The QGP is believed to have existed in the first few microseconds after
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the Big Bang and is conjectured to still exist in the neutron star cores. Anisotropic flow is a measure of the momentum anisotropy of particles created in a heavy-ion collision, a consequence of the initial spatial anisotropy of the created medium. This initial anisotropy is largely a consequence of the ellipsoidal shape of the overlap region between the two colliding nuclei. Higher order anisotropy (triangular, rectangular etc.) is the consequence of fluctuations of the shape of the nuclei. Anisotropic flow is expressed by the coefficients of the harmonic decomposition of the anisotropy of the produced particles. Flow is a unique observable, sensitive to the early properties of the created system. The properties of the QGP like the shear viscosity per degree of freedom and expansion parameters are studied using relativistic hydrodynamical models. The integrated elliptic flow (the second harmonic coefficient) of charged particles in Pb-Pb collisions at the LHC with the energy of 2.76 TeV per nucleon pair measured with the ALICE detector is 30% larger than in Au-Au collisions at the relativistic heavy-ion collider (RHIC) with the energy of 200 GeV per nucleon pair, while the flow as function transverse momentum remains virtually the same. This observation is compatible with the predictions of some hydrodynamical models that account for viscous effects. The rise of the value of integrated flow is attributed to a higher value of the mean transverse momentum at the LHC. The measurement of elliptic flow of identified pions, kaons and anti-protons shows a larger mass splitting than in the RHIC measurement. This effect is associated with a higher expansion velocity of the medium (radial flow) and it provides an explanation for the higher mean transverse momentum at the LHC concluded from the rise of integrated flow. The third harmonic coefficient (triangular flow) of identified particles shows a weak centrality dependence conforming with the expectation that higher order anisotropies are caused by fluctuations of the initial conditions. Qualitative similarities with elliptic flow (like mass ordering) support the idea that the mechanism responsible for the higher order momentum anisotropies of the produced particles is similar to that of elliptic flow. The elliptic and triangular flow data seems to favour hydrodynamical models with a very low value of viscosity, close to the conjectured theoretical minimum; a quantitative description needs more theoretical and experimental research. The transverse energy scaling of flow per constituent quark observed at RHIC is broken for elliptic flow en only holds approximately for triangular flow. The idea that quark coalescence is a dominant hadronization mechanism in Pb-Pb collisions at the LHC is difficult to defend; many different processes probably play a role. Also in this case further theory advancements are needed to describe this system quantitatively.
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