Rise of azimuthal anisotropies as a signature of the Quark-Gluon-Plasma
  in relativistic heavy-ion collisions

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Abstract

The azimuthal anisotropies of the collective transverse flow of hadrons are investigated in a large range of heavy-ion collision energy within the Parton-Hadron-String Dynamics (PHSD) microscopic transport approach which incorporates explicit partonic degrees of freedom in terms of strongly interacting quasiparticles (quarks and gluons) in line with an equation-of-state from lattice QCD as well as dynamical hadronization and hadronic dynamics in the final reaction phase. The experimentally observed increase of the elliptic flow \(v_2\) with bombarding energy is successfully described in terms of the PHSD approach in contrast to a variety of other kinetic models based on hadronic interactions. The analysis of higher-order harmonics \(v_3\) and \(v_4\) shows a similar tendency of growing deviations between partonic and purely hadronic models with increasing bombarding energy. This signals that the excitation functions of azimuthal anisotropies provide a sensitive probe for the underling degrees of freedom excited in heavy-ion collisions.

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Shear-Viscosity to Entropy Density Ratio of a Relativistic Hadron Gas

Steffen A. Bass, Nasser Demir (2008)

Ultrarelativistic heavy-ion collisions at the Relativistic Heavy-Ion Collider (RHIC) are thought to have produced a state of matter called the Quark-Gluon-Plasma, characterized by a very small shear viscosity to entropy density ratio \(\eta/s\), near the lower bound predicted for that quantity by Anti-deSitter space/Conformal Field Theory (AdS/CFT) methods. As the produced matter expands and cools, it evolves through a phase described by a hadron gas with rapidly increasing \(\eta/s\). We calculate \(\eta/s\) as a function of temperature in this phase and find that its value poses a challenge for viscous relativistic hydrodynamics, which requires small values of \(\eta/s\) throughout the entire evolution of the reaction in order to successfully describe the collective flow observables at RHIC. We show that the inclusion of non-unit fugacities will reduce \(\eta/s\) in the hadronic phase, yet not sufficiently to be compatible with viscous hydrodynamics. We therefore conclude that the origin of the low viscosity matter at RHIC must be in the partonic phase of the reaction.

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The hot non-perturbative gluon plasma is an almost ideal colored liquid

W. Cassing, A. Peshier (2005)

We study properties of a gluon plasma above the critical temperature \(T_c\) in a generalized quasi-particle approach with a Lorentz spectral function. The model parameters are determined by a fit of the entropy \(s\) to lattice QCD data. The effective degrees of freedom are found to be rather heavy and of a sizeable width. With the spectral width being closely related to the interaction rate, we find a large effective cross section, which is comparable to the typical distance squared of the quasiparticles. This suggests that the system should be viewed as a liquid as also indicated by an estimate of the plasma parameter \(\Gamma\). Furthermore, within the quasiparticle approach we find a very low viscosity to entropy ratio, \(\eta/s \sim 0.2\) for \(T > 1.05 T_c\), supporting the recent conjecture of an almost ideal quark-gluon liquid seen at RHIC.