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

Abstract

With the new viscous hydrodynamic + hadron cascade hybrid code VISHNU, a rather precise (O(25%)) extraction of the QGP shear viscosity (η/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 (η/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 (η/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.Prologue -how to measure (η/s) QGP : Hydrodynamics converts the initial spatial deformation of the fireball created in relativistic heavy-ion collisions into final state momentum anisotropies. Viscosity degrades the conversion efficiency ε x = y 2 −x 2 y 2 +x 2

Dissipative hydrodynamics coupled to chiral fields

“…in viscous hydrodynamic simulations, which may provide further crucial constraints on η/s and on 044904-6 models used to calculate QGP initial conditions (see, e.g., Refs. [75,[82][83][84][85][86][87]). …”

The shear channel spectral function in hot Yang-Mills theory

“…This work has been largely motivated by the experimental heavy ion programs taking place at RHIC and the LHC [1,2], which have highlighted the importance of not only understanding the bulk equilibrium properties of the produced matter but also its hydrodynamic behavior. One particularly prominent observation in this direction has been the discovery of the sizable effect that the small, yet finite value of the shear viscosity η has on the flow properties of the plasma [3,4]. Unfortunately, the first principles determination of transport coefficients in an interacting quantum field theory is a notoriously hard problem.…”

On the infrared behavior of the shear spectral function in hot Yang-Mills theory

“…Considering also the fact that our present HTL result shows perfect numerical agreement with the unresummed one down to frequencies ω ≈ 0.1T , we find it not worth the effort to undertake the challenge of evaluating the resummed HTL vertex contribution to the shear spectral function. 4 In fact, due to the unambiguity (and the more bening small-ω behavior) of the unresummed result, we will choose to use it in the evaluation of the imaginary time correlator below.…”

“…[1][2][3][4][5]). Despite the strong experimental and phenomenological motivation, a nonperturbative first principles tool to predict its value is, however, still lacking, even though an extensive amount of work has been devoted to the topic in the weak coupling [6,7], lattice [8][9][10] and gauge/gravity frontiers [11][12][13].…”