The shear viscosity of a gluon gas is calculated using the Green-Kubo relation. Time correlations of the energy-momentum tensor in thermal equilibrium are extracted from microscopic simulations using a parton cascade solving various Boltzmann collision processes. We find that the pQCD based gluon bremsstrahlung described by Gunion-Bertsch processes significantly lowers the shear viscosity by a factor of 3 − 8 compared to elastic scatterings. The shear viscosity scales with the coupling as η ∼ 1/(α 2 s log(1/α s )). For constant α s the shear viscosity to entropy density ratio η/s has no dependence on temperature. Replacing the pQCD-based collision angle distribution of binary scatterings by an isotropic form decreases the shear viscosity by a factor of 3.
The quark gluon plasma produced in ultra-relativistic heavy-ion collisions exhibits remarkable features. It behaves like a nearly perfect liquid with a small shear viscosity to entropy density ratio and leads to the quenching of highly energetic particles. We show that both effects can be understood for the first time within one common framework. Employing the parton cascade Boltzmann Approach to Multi-Parton Scatterings (BAMPS), the microscopic interactions and the space-time evolution of the quark gluon plasma are calculated by solving the relativistic Boltzmann equation. Based on cross sections obtained from perturbative QCD with explicitly taking the running coupling into account, we calculate the nuclear modification factor and elliptic flow in ultra-relativistic heavy-ion collisions. With only one single parameter associated with coherence effects of medium-induced gluon radiation, the experimental data of both observables can be understood on a microscopic level. Furthermore, we show that perturbative QCD interactions with a running coupling lead to a sufficiently small shear viscosity to entropy density ratio of the quark gluon plasma, which provides a microscopic explanation for the observations stated by hydrodynamic calculations.In ultra-relativistic heavy-ion collisions at the Relativistic Heavy-Ion Collider (RHIC) at BNL and the Large Hadron Collider (LHC) at CERN a hot and dense medium is created that consists of quarks and gluons. Experimental data shows that this quark gluon plasma (QGP) possesses a strong collective behavior and that high-energy partons deposit a sizeable amount of their energy in this medium [1,2].The collective behavior is often quantified by the elliptic flow coefficient v 2 , which is the second harmonic of the Fourier decomposition of the azimuthal angle distribution of particle yields. Comparisons to hydrodynamic calculations reveal that the QGP behaves like a nearly perfect liquid with a small shear viscosity to entropy density ratio [3]. However, the microscopic reason for this small ratio is currently not understood.Experimental data of the nuclear modification factor R AA , which is defined as the yield in heavy-ion (A+A) collisions divided by the yield in proton-proton (p+p) collisions scaled with the number of binary collisions,and the momentum imbalance of fully reconstructed jets indicate that high-energy particles are quenched by the created medium and lose lots of their energy [1, 2]. Several calculations based on perturbative QCD (pQCD) energy loss in the QGP can describe the experimental data [4][5][6][7][8][9][10][11]. A simultaneous understanding of collective bulk phenomena and jet quenching on the microscopic level remains a challenge, although several partonic transport models [12][13][14][15][16][17] have been developed to address this issue. In this paper we will present new results on both observables obtained with the partonic transport model Boltzmann Approach to Multi-Parton Scatterings (BAMPS). Based on cross sections calculated in pQCD, soft and ha...
Starting from a classical picture of shear viscosity we construct a stationary velocity gradient in a microscopic parton cascade. Employing the Navier-Stokes ansatz we extract the shear viscosity coefficient η. For elastic isotropic scatterings we find an excellent agreement with the analytic values. This confirms the applicability of this method. Furthermore, for both elastic and inelastic scatterings with pQCD based cross sections we extract the shear viscosity coefficient η for a pure gluonic system and find a good agreement with already published calculations.
Abstract. Second-order dissipative hydrodynamic equations for each component of a multi-component system are derived using the entropy principle. Comparison of the solutions with kinetic transport results demonstrates validity of the obtained equations. We demonstrate how the shear viscosity of the total system can be calculated in terms of the involved cross sections and partial densities. Presence of the interspecies interactions leads to a characteristic time-dependence of the shear viscosity of the mixture, which also means that the shear viscosity of a mixture cannot be calculated using the Green-Kubo formalism the way it has been done recently. This finding is of interest for understanding of the shear viscosity of a quark-gluon-plasme extracted from comparisons of hydrodynamic simulations with experimental results from RHIC and LHC.
An overview is presented of transverse momentum distributions of particles at the LHC using the Tsallis distribution. The use of a thermodynamically consistent form of this distribution leads to an excellent description of charged and identified particles. The values of the Tsallis parameter q are truly remarkably consistent.
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