Macroscopic approximation to relativistic kinetic theory from a nonlinear closure

We use a macroscopic description of a system of relativistic particles based on adding a nonequilibrium tensor to the usual hydrodynamic variables. The nonequilibrium tensor is linked to relativistic kinetic theory through a nonlinear closure suggested by the Entropy Production Principle; the evolution equation is obtained by the method of moments, and together with energy-momentum conservation closes the system. Transport coefficients are chosen to reproduce second order fluid dynamics if gradients are small. We compare the resulting formalism to exact solutions of Boltzmann's equation in 0+1 dimensions and show that it tracks kinetic theory better than second order fluid dynamics.Comment: v2: 6 two-column pages, 2 figures. Corrected typos and a numerical error, and added reference

A hydrodynamic approach to QGP instabilities

We show that the usual linear analysis of QGP Weibel instabilities based on the Maxwell-Boltzmann equation may be reproduced in a purely hydrodynamic model. The latter is derived by the Entropy Production Variational Method from a transport equation including collisions, and can describe highly nonequilibrium flow. We find that, as expected, collisions slow down the growth of Weibel instabilities. Finally, we discuss the strong momentum anisotropy limit.Comment: 11 pages, no figures. v2: minor changes, added references. Accepted in Phys. Rev.

Production of thermal photons in viscous fluid dynamics with temperature-dependent shear viscosity

We compute the spectrum of thermal photons created in Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV, taking into account dissipative corrections in production processes corresponding to the quark--gluon plasma and hadronic phases. To describe the evolution of the fireball we use a viscous fluid dynamic model with different parametrizations for the temperature--dependence of $\eta/s$. We find that the spectrum significantly depends on the values of $\eta/s$ in the QGP phase, and is almost insensitive to the values in the hadronic phase. We also compare the influence of the temperature--dependence of $\eta/s$ on the spectrum of thermal photons to that of using different equations of state in the fluid dynamic simulations, finding that both effects are of the same order of magnitude.Comment: 16 pages, 4 figures. Accepted for publication in Mod. Phys. Lett.