The QGP shear viscosity -- elusive goal or just around the corner?

Journal of Physics G Nuclear and Particle Physics (Impact Factor: 5.33). 06/2011; 38(12). DOI: 10.1088/0954-3899/38/12/124045
Source: arXiv

ABSTRACT With the new viscous hydrodynamic + hadron cascade hybrid code VISHNU, a
rather precise (O(25%)) extraction of the QGP shear viscosity (eta/s)_QGP from
heavy-ion elliptic flow data is possible if the initial eccentricity of the
collision fireball is known with <5% accuracy. At this point, eccentricities
from initial state models differ by up to 20%, leading to an O(100%)
uncertainty for (eta/s)_QGP. It is shown that a simultaneous comparison of
elliptic and triangular flow, v_2 and v_3, puts strong constraints on initial
state models and can largely eliminate the present uncertainty in (eta/s)_QGP.
The variation of the differential elliptic flow v_2(p_T) for identified hadrons
between RHIC and LHC energies provides additional tests of the evolution model.

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    ABSTRACT: Posterior distributions for physical parameters describing relativistic heavy-ion collisions, such as the viscosity of the quark-gluon plasma, are extracted through a comparison of hydrodynamic-based transport models to experimental results from 100$A$ GeV + 100$A$ GeV Au+Au collisions at the Relativistic Heavy Ion Collider (RHIC). By simultaneously varying six parameters and by evaluating several classes of observables, we are able to explore the complex intertwined dependencies of observables to model parameters. We obtain a full multi-dimensional posterior distribution for the model output given a large set of experimental observations, the methods developed here provide a range of acceptable values for each parameter, and reveal correlations between them. The breadth of observables and the number of parameters considered here go far beyond previous studies in this field. The statistical tools, which are based upon Gaussian Process emulators, are tested in detail and should be extendable to larger data sets and a higher number of parameters.
    03/2013; 89(3).
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    ABSTRACT: Time evolution of a “little bang” created in heavy-ion collisions can be divided into two phases, the pre-equilibrium and hydrodynamic. At what moment does the evolution become hydrodynamic and is there any universality in the hydrodynamic flow? To answer these questions we briefly discuss various versions of hydrodynamics and their applicability conditions. In particular, we elaborate on the idea of “universal” (all-order resumed) hydrodynamics and propose a simple new model for it. The model is motivated by results obtained recently via the Anti–de Sitter and conformal field correspondence. Finally, charged hadron multiplicities in heavy-ion collisions at the Relativistic Heavy Ion Collider and Large Hadron Collider are discussed. At the freeze-out, the multiplicities can be related to total entropy produced in the collision. Assuming the universal hydrodynamics to hold, we calculate the entropy production in the hydro stage of the collision. We end up speculating about a connection between the multiplicity growth and the temperature dependence of the quark-gluon plasma viscosity.
    Physical Review C 08/2011; 84(6). · 3.72 Impact Factor
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    ABSTRACT: In this talk, we discuss a number of recent calculations aimed at determining the spectral functions corresponding to various components of the energy momentum tensor in high-temperature SU(N) Yang-Mills theory. The computations reviewed include applications of both weak coupling and gauge/gravity techniques, and thus enable one to access different limits of the quantities. The motivation for the work is twofold: On one hand, the results are hoped to aid the eventual nonperturbative extraction of the bulk and shear viscosities from lattice data, while on the other hand they also enable an immediate comparison of the lattice, perturbative and holographic predictions for certain Euclidean correlators.


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