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 η/s, near the lower bound predicted for that quantity by AntideSitter 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 η/s. We calculate η/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 η/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 η/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.Ultrarelativistic heavy ion collisions at the Relativistic Heavy Ion Collider (RHIC) are thought to have produced a Quark Gluon Plasma (QGP) with the characteristics of a near ideal fluid [1,2,3,4]. One of the most important current challenges in QGP research is to quantify the transport coefficients of this novel state of matter. Recently, attention in the field has been primarily focused on the shear viscosity to entropy density ratio η/s. Certain supersymmetric gauge theories with gravity duals [5] suggest a lower bound of η min = s/4π for this quantity, often referred to as the KSS bound [6]. Relativistic viscous hydrodynamic calculations require very low values of η/s in order to reproduce the RHIC elliptic flow (v 2 ) data [7,8,9]. However, current calculations assume a fixed value of η/s throughout the entire evolution of the system and neglect its temperature dependence. The exact value of η/s in these calculations is only known within a factor of ≈ 3, due to systematic uncertainties related to the choice of equation of state and initial conditions used [9,10]. A viscous hydrodynamical analysis [9] finds that η/s should lie within a range of 0.08-0.24 depending upon the choice of initial conditions and equation of state. This finding is supported by calculations of η/s for pure gluonic QCD, which yield values close to the KSS bound, and indirect estimates of η/s from calculations of the diffusion of heavy quarks, elliptic flow measurements, and transverse momentum correlations that arrive at roughly comparable values [11,12,13,14,15].Note that the shear viscosity of matter in a relativistic heavy ion collision is a time-dependent quantity. While the partonic phase of such a collision is expected to have a very low value of η/s, after hadronization occurs η/s is expected to rapidly increase. In order to quantitatively constrain the viscosity of the deconfined phase of a relativistic heavy ion collision, a separate calculation of the hadronic viscosity is necessary. Several i...

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Among the key features of hot and dense QCD matter produced in ultrarelativistic heavy-ion collisions at RHIC is its very low shear viscosity, indicative of the properties of a near-ideal fluid, and a large opacity demonstrated by jet energy loss measurements. In this work, we utilize a microscopic transport model based on the Boltzmann equation with quark and gluon degrees of freedom and cross sections calculated from perturbative quantum chromodynamics to simulate an ideal quark-gluon plasma in full thermal and chemical equilibrium. We then use the Kubo formalism to calculate the shear viscosity to entropy-density ratio of the medium as a function of temperature and system composition. One of our key results is that the shear viscosity over entropy-density ratio η/s becomes invariant to the chemical composition of the system when plotted as a function of energy-density instead of temperature.

To better understand the very high thermal conductivity to weight ratio of the graphitic foams recently developed at the Oak Ridge National Laboratory, a Raman spectroscopy study was performed. It was also shown that the Raman scattering can be useful for the characterization of the graphitic foam, being able to evaluate the quality of the samples with respect to the density and location of lattice defects.