An elementary stringy estimate of transport coefficients of large temperature QCD

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.Comment: 11 pages; v2: comments and references adde

Dissipative corrections to particle spectra and anisotropic flow from a saddle-point approximation to kinetic freeze out

Lang C, Borghini N. Dissipative corrections to particle spectra and anisotropic flow from a saddle-point approximation to kinetic freeze out. The European Physical Journal C. 2014;74(7): 2955.A significant fraction of the changes in momentum distributions induced bydissipative phenomena in the description of the fluid fireball created inultrarelativistic heavy ion collisions are actually taking place when the fluidturns into individual particles. We study these corrections in the limit of alow freeze-out temperature of the flowing medium, and show that they mostlyaffect particles with a higher velocity than the fluid. For these, we deriverelations between different flow harmonics, from which the functional form ofthe dissipative corrections could ultimately be reconstructed from experimentaldata

Thermal photons in QGP and non-ideal effects

We investigate the thermal photon production-rates using one dimensional boost-invariant second order relativistic hydrodynamics to find proper time evolution of the energy density and the temperature. The effect of bulk-viscosity and non-ideal equation of state are taken into account in a manner consistent with recent lattice QCD estimates. It is shown that the \textit{non-ideal} gas equation of state i.e $\epsilon-3\,P\,\neq 0$ behaviour of the expanding plasma, which is important near the phase-transition point, can significantly slow down the hydrodynamic expansion and thereby increase the photon production-rates. Inclusion of the bulk viscosity may also have similar effect on the hydrodynamic evolution. However the effect of bulk viscosity is shown to be significantly lower than the \textit{non-ideal} gas equation of state. We also analyze the interesting phenomenon of bulk viscosity induced cavitation making the hydrodynamical description invalid. We include the viscous corrections to the distribution functions while calculating the photon spectra. It is shown that ignoring the cavitation phenomenon can lead to erroneous estimation of the photon flux.Comment: 11 pages, 13 figures; accepted for publication in JHE