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Register a new userInfluence of the shear viscosity of the quark-gluon plasma on elliptic flow in ultrarelativistic heavy-ion collisions
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We investigate the influence of a temperature-dependent shear viscosity over entropy density ratio eta/s on the transverse momentum spectra and elliptic flow of hadrons in ultrarelativistic heavy-ion collisions. We find that the elliptic flow in sqrt(s_NN) = 200 GeV Au+Au collisions at RHIC is dominated by the viscosity in the hadronic phase and in the phase transition region, but largely insensitive to the viscosity of the quark-gluon plasma (QGP). At the highest LHC energy, the elliptic flow becomes sensitive to the QGP viscosity and insensitive to the hadronic viscosity.
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We explore the influence of a temperature-dependent shear viscosity over entropy density ratio $eta/s$ on the azimuthal anisotropies v_2 and v_4 of hadrons at various rapidities. We find that in Au+Au collisions at full RHIC energy, $sqrt{s_{NN}}=200$ GeV, the flow anisotropies are dominated by hadronic viscosity at all rapidities, whereas in Pb+Pb collisions at the LHC energy, $sqrt{s_{NN}}=2760$ GeV, the flow coefficients are affected by the viscosity both in the plasma and hadronic phases at midrapidity, but the further away from midrapidity, the more dominant the hadronic viscosity is. We find that the centrality and rapidity dependence of the elliptic and quadrangular flows can help to distinguish different parametrizations of $(eta/s)(T)$. We also find that at midrapidity the flow harmonics are almost independent of the decoupling criterion, but show some sensitivity to the criterion at back- and forward rapidities.
We study the evolution of the quark-gluon composition of the plasma created in ultra-Relativistic Heavy-Ion Collisions (uRHICs) employing a partonic transport theory that includes both elastic and inelastic collisions plus a mean fields dynamics associated to the widely used quasi-particle model. The latter, able to describe lattice QCD thermodynamics, implies a chemical equilibrium ratio between quarks and gluons strongly increasing as $Trightarrow T_c$, the phase transition temperature. Accordingly we see in realistic simulations of uRHICs a rapid evolution from a gluon dominated initial state to a quark dominated plasma close to $T_c$. The quark to gluon ratio can be modified by about a factor of $sim 20$ in the bulk of the system and appears to be large also in the high $p_T$ region. We discuss how this aspect, often overflown, can be important for a quantitative study of several key issues in the QGP physics: shear viscosity, jet quenching, quarkonia suppression. Furthermore a bulk plasma made by more than $80%$ of quarks plus antiquarks provides a theoretical basis for hadronization via quark coalescence.
Averaged over ensemble of initial conditions kinetic transport equations of weakly coupled systems of quarks and gluons are derived. These equations account for the correlators of fluctuations of particles and classical gluon fields. The isotropization of particle momenta by field fluctuations at the early prethermal stage of matter evolution in ultrarelativistic heavy ion collisions is discussed. Our results can be useful for understanding under what conditions isotropization of the quark-gluon plasma in ultrarelativistic heavy ion collisions can be reached within phenomenologically observed time scales.
Recent classical-statistical numerical simulations have established the bottom-up thermalization scenario of Baier et al. as the correct weak coupling effective theory for thermalization in ultrarelativistic heavy-ion collisions. We perform a parametric study of photon production in the various stages of this bottom-up framework to ascertain the relative contribution of the off-equilibrium Glasma relative to that of a thermalized Quark-Gluon Plasma. Taking into account the constraints imposed by the measured charged hadron multiplicities at RHIC and the LHC, we find that Glasma contributions are important especially for large values of the saturation scale at both energies. These non-equilibrium effects should therefore be taken into account in studies where weak coupling methods are employed to compute photon yields.
We argue that an expanding quark-gluon plasma has an anomalous viscosity, which arises from interactions with dynamically generated colour fields. The anomalous viscosity dominates over the collisional viscosity for large velocity gradients or weak coupling. This effect may provide an explanation for the apparent near perfect liquidity of the matter produced in nuclear collisions at RHIC without the assumption that it is a strongly coupled state.
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