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The shear viscosity η of a quark–gluon plasma in equilibrium can be calculated analytically using multiple methods or numerically using the Green–Kubo relation. It has been realized, which we confirm here, that the Chapman–Enskog method agrees well with the Green–Kubo result for both isotropic and anisotropic two-body scatterings. We then apply the Chapman–Enskog method to study the shear viscosity of the parton matter from a multi-phase transport model. In particular, we study the parton matter in the center cell of central and midcentral Au + Au collisions at 200A GeV and Pb + Pb collisions at 2760A GeV, which is assumed to be a plasma in thermal equilibrium but partial chemical equilibrium. As a result of using a constant Debye mass or cross section σ for parton scatterings, the η / s ratio increases with time (as the effective temperature decreases), contrary to the trend preferred by Bayesian analysis of the experimental data or pQCD results that use temperature-dependent Debye masses. At σ = 3 mb that enables the transport model to approximately reproduce the elliptic flow data of the bulk matter, the average η / s of the parton matter in partial equilibrium is found to be very small, between one to two times 1 / (4 π) . [ABSTRACT FROM AUTHOR]

It is commonly believed that azimuthal anisotropies in relativistic heavy ion collisions are generated by hydrodynamic evolution of the strongly interacting quark-gluon plasma. Here we use transport models to study how azimuthal anisotropies depend on the number of collisions that each parton suffers. We find that the majority of ν 2 comes from the anisotropic escape of partons, not from the parton collective flow, for semi-central Au+Au collisions at 200A GeV. As expected, the fraction of ν 2 from the anisotropic particle escape is even higher for smaller systems such as d+Au. Our transport model results also confirm that azimuthal anisotropies would be dominated by hydrodynamic flow at unrealistically-high parton cross sections. Our finding thus naturally explains the similarity of azimuthal anisotropies in small and large systems; however, it presents a challenge to the paradigm of anisotropic flow. [ABSTRACT FROM AUTHOR]

The rapidity-odd directed flows (v 1) of identified hadrons are expected to follow the coalescence sum rule when the created matter is initially in parton degrees of freedom and then hadronizes through quark coalescence. A recent study has considered the v 1 of produced hadrons that do not contain u or d constituent quarks. It has constructed multiple hadron sets with a small mass difference but given difference in electric charge Δ q and strangeness Δ S between the two sides, where a nonzero and increasing Δ v 1 with Δ q has been proposed to be a consequence of electromagnetic fields. In this study, we examine the consequence of coalescence sum rule on the Δ v 1 of the hadron sets in the absence of electromagnetic fields. We find that in general Δ v 1 ≠ 0 for a hadron set with nonzero Δ q and/or Δ S due to potential v 1 differences between u ¯ and d ¯ and between s and s ¯ quarks. We further propose methods to extract the coefficients for the Δ q - and Δ S -dependences of the direct flow difference, where a nonzero constant term would indicate the breaking of the coalescence sum rule. The extraction methods are then demonstrated with transport model results. [ABSTRACT FROM AUTHOR]

We trace the development of azimuthal anisotropy ( v n , n = 2 , 3 ) via parton–parton collision history in two transport models. The parton v n is studied as a function of the number of collisions of each parton in Au + Au and d + Au collisions at s NN = 200 GeV . It is found that the majority of v n comes from the anisotropic escape probability of partons, with no fundamental difference at low and high transverse momenta. The contribution to v n from hydrodynamic-type collective flow is found to be small. Only when the parton–parton cross-section is set unrealistically large does this contribution start to take over. Our findings challenge the current paradigm emerged from hydrodynamic comparisons to anisotropy data. [ABSTRACT FROM AUTHOR]