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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]

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]

The near-side depression in two-proton or two-antiproton azimuthal correlations in pp collisions at s = 7 TeV has been observed experimentally and then qualitatively reproduced in our earlier studies with a multi-phase transport model. In this study, we further investigate the origin of the depression feature in two-baryon correlations in small collision systems. We find that the initial parton-level spatial correlation, a finite expansion in the parton stage, and quark coalescence are important ingredients leading to the near-side depression. In particular, we find that a finite expansion of the parton system leads to a finite space-momentum correlation at hadronization, which then converts the near-side depression in the coordinate space to that in the momentum space. These results suggest that a partonic matter with a finite lifetime is formed in the pp collisions. Further studies are needed to determine whether the partonic matter is near local equilibrium and can thus be called a QGP or far away from local equilibrium. [ABSTRACT FROM AUTHOR]