arXiv:1006.4634v2 [hep-ph] 6 Jul 2010 KUL-TF-10/06 An elementary stringy estimate of transport coefficients of large temperature QCD Francesco Bigazzi and Aldo L. Cotrone Institute for theoretical physics, K.U. Leuven; Celestijnenlaan 200D, B-3001 Leuven, Belgium. francesco.bigazzi@fys.kuleuven.be, aldo.cotrone@fys.kuleuven.be Abstract Modeling QCD at large temperature with a simple holographic five dimensional theory encoding minimal breaking of conformality, allows for the calculation of all the transport coefficients, up to second order, in terms of a single parameter. In particular, the shear and bulk relaxation times are provided. The result follows by deforming the AdS background with a scalar dual to a marginally relevant operator, at leading order in the deformation parameter. 1
1Introduction and results The evolution of the Quark-Gluon Plasma (QGP) produced in heavy ion collisions is fairly well described, after a short period of thermalization and before hadronization, by relativistic hydrodynamics, consistently with the strongly coupled regime of such system [1]. Numerical simulations of the hydrodynamic evolution of the QGP require as input the value of the transport coefficients.While recent simulations indicate that the evolution of the QGP should be quite insensitive to most of the second order coefficients (see for example [2, 3, 4]), it is definitely sensitive to the value of the shear viscosity [5], and can be influenced in a sizable way by the bulk viscosity and possibly the relaxation times [3, 4, 6].Moreover, a complete characterization of the Quark-Gluon Plasma of QCD up to second order still requires the knowledge of the whole set of coefficients. There are currently no first-principle reliable calculations of almost all the second order coefficients for QCD at strong coupling: lattice results give some estimates of the viscosities and the shear relaxation time [7], but they are affected by considerable uncertainties (see for example [8]).In fact, actual simulations, lacking solid data for QCD, make often use, as benchmark values of the transport coefficients, of the ones derived from the gravitational dual ofN= 4 SYM [9] (in some cases together with the bound on the bulk viscosity proposed in [10] and a relation for the relaxation times from [11]). While theN= 4 SYM values for the “shear” coefficients are expected to be in the right ballpark for QCD, they still concern an exactly conformal theory, and in particular the bulk viscosity and many of the second order coefficients are not determined. In order to improve this situation, the first step is to break conformal invariance. Since QCD is approximately conformal in the temperature window 1.5Tc.T.4Tc, the confor- mality breaking effects can be treated perturbatively. In this situation, probably the simplest way of modeling QCD holographically is by a theory where conformality is slightly broken by a marginally relevant operator. The aim of this note is to point out that, in such a scenario, all the transport coefficients up to second order for the uncharged plasma are given in terms of a single parameter (weighting the conformality breaking) by making use of the results in [12, 11]: they are collected in Table 1. In particular, the behavior of the shear and bulk relaxation times is briefly discussed in section 1.2. There are surely more precise ways of modeling holographically QCD (none of which is of course completely correct).Nevertheless, the model considered in this note has the considerable advantage of full calculability, providing one of the few examples in which all the second order transport coefficients are determined.Moreover, the results in Table 1 hold for any theory with gravity dual, where conformality is broken at leading order by a marginally (ir)relevant operator (dual to a scalar with the simplest possible potential (8)), including the cascading plasma [13] and the D3D7 plasmas [14]. 2
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