Heavy quark collisional energy loss in the quark-gluon plasma including finite relaxation time

In this paper, we calculate the soft-collisional energy loss of heavy quarks traversing the viscous quark-gluon plasma including the effects of a finite relaxation time $tau_pi$ on the energy loss. We find that the collisional energy loss depends appreciably on $tau_pi$ . In particular, for typical values of the viscosity-to-entropy ratio, we show that the energy loss obtained using $tau_pi$ = 0 can be $sim$ 10$%$ larger than the one obtained using $tau_pi$ = 0. Moreover, we find that the energy loss obtained using the kinetic theory expression for $tau_pi$ is much larger that the one obtained with the $tau_pi$ derived from the Anti de Sitter/Conformal Field Theory correspondence. Our results may be relevant in the modeling of heavy quark evolution through the quark-gluon plasma.

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.

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 Boltzmanns equation in 0+1 dimensions and show that it tracks kinetic theory better than second order fluid dynamics.

$eta(T)/s (T)$ and collective flow in chiral hydrodynamics

Using relativistic conformal hydrodynamics coupled to the linear $sigma$ model we study the evolution of matter created in heavy--ion collisions. We focus the study on the influence of the dynamics of the chiral fields on the charged-hadron elliptic flow $v_2$ for a temperature--independent as well as for a temperature--dependent $eta/s$ that is calculated from kinetic theory. We find that $v_2$ is not very sensitive to the coupling of chiral fields to the hydrodynamic evolution, but the temperature dependence of $eta/s$ plays a much bigger role on this observable.

Production of thermal photons in a simple chiral-hydrodynamic model

We use a self-consistent chiral-hydrodynamic formalism which combines the linear $sigma$ model with second-order hydrodynamics in 2+1 dimensions to compute the spectrum of thermal photons produced in Au+Au collisions at $sqrt{s_{NN}}=200$ GeV. The temperature-dependent shear viscosity of the model, $eta$, is calculated from the linearized Boltzmann equation. We compare the results obtained in the chiral-hydrodynamic model to those obtained in the second-order theory with a Lattice QCD equation of state and a temperature-independent value of $eta/s$. We find that the thermal photon production is significantly larger in the latter model due to a slower evolution and larger dissipative effects.

Effective dynamics of a nonabelian plasma out of equilibrium

Starting from kinetic theory, we obtain a nonlinear dissipative formalism describing the nonequilibrium evolution of scalar colored particles coupled selfconsistently to nonabelian classical gauge fields. The link between the one-particle distribution function of the kinetic description and the variables of the effective theory is determined by extremizing the entropy production. This method does not rely on the usual gradient expansion in fluid dynamic variables, and therefore the resulting effective theory can handle situations where these gradients (and hence the momentum-space anisotropies) are expected to be large. The formalism presented here, being computationally less demanding than kinetic theory, may be useful as a simplified model of the dynamics of color fields during the early stages of heavy ion collisions and in phenomena related to parton energy loss.

Shear viscosity from thermal fluctuations in relativistic conformal fluid dynamics

Within the framework of relativistic fluctuating hydrodynamics we compute the contribution of thermal fluctuations to the effective infrared shear viscosity of a conformal fluid, focusing on quadratic (in fluctuations), second order (in velocity gradients) terms in the conservation equations. Our approach is based on the separation of hydrodynamic fields in soft and ultrasoft sectors, in which the effective shear viscosity arises due to the action of the soft modes on the evolution of the ultrasoft ones. We find that for a strongly coupled fluid with small shear viscosity--to--entropy ratio $eta/s$ the contribution of thermal fluctuations to the effective shear viscosity is small but significant. Using realistic estimates for the strongly coupled quark--gluon plasma created in heavy ion collisions, we find that for $eta/s$ close to the AdS/CFT lower bound $1/(4pi)$ the correction is positive and at most amounts to 10% in the temperature range 200--300 MeV, whereas for larger values $eta/s sim 2/(4pi)$ the correction is negligible. For weakly coupled theories the correction is very small even for $eta/s=0.08$ and can be neglected.

Dissipative hydrodynamics coupled to chiral fields

Using second--order dissipative hydrodynamics coupled self-consistently to the linear $sigma$ model we study the 2+1 dimensional evolution of the fireball created in Au+Au relativistic collisions. We analyze the influence of the dynamics of the chiral fields on the charged-hadron elliptic flow $v_2$ and on the ratio $v_4/(v_2)^2$ for a temperature-independent as well as for a temperature-dependent viscosity-to-entropy ratio $eta/s$ calculated from the linearized Boltzmann equation in the relaxation time approximation. We find that $v_2$ is not very sensitive to the coupling of chiral sources to the hydrodynamic evolution, but the temperature dependence of $eta/s$ plays a much bigger role on this observable. On the other hand, the ratio $v_4/(v_2)^2$ turns out to be much more sensitive than $v_2$ to both the coupling of the chiral sources and the temperature dependence of $eta/s$.

Thermal photon production in Au+Au collisions: viscous corrections in two different hydrodynamic formalisms

We calculate the spectra of produced thermal photons in Au+Au collisions taking into account the nonequilibrium contribution to photon production due to finite shear viscosity. The evolution of the fireball is modeled by second-order as well as by divergence-type 2+1 dissipative hydrodynamics, both with an ideal equation of state and with one based on Lattice QCD that includes an analytical crossover. The spectrum calculated in the divergence-type theory is considerably enhanced with respect to the one calculated in the second-order theory, the difference being entirely due to differences in the viscous corrections to photon production. Our results show that the differences in hydrodynamic formalisms are an important source of uncertainty in the extraction of the value of $eta/s$ from measured photon spectra. The uncertainty in the value of $eta/s$ associated with different hydrodynamic models used to compute thermal photon spectra is larger than the one occurring in matching hadron elliptic flow to RHIC data.