2,503 research outputs found
The onset of fluid-dynamical behavior in relativistic kinetic theory
In this proceedings we discuss recent findings regarding the large order
behavior of the Chapman-Enskog expansion in relativistic kinetic theory. It is
shown that this series in powers of the Knudsen number has zero radius of
convergence in the case of a Bjorken expanding fluid described by the Boltzmann
equation in the relaxation time approximation. This divergence stems from the
presence of non-hydrodynamic modes, which give non-perturbative contributions
to the Knudsen series.Comment: 4 pages, 1 figure, proceedings for Quark Matter 201
Analytical attractor and the divergence of the slow-roll expansion in relativistic hydrodynamics
We find the general analytical solution of the viscous relativistic
hydrodynamic equations (in the absence of bulk viscosity and chemical
potential) for a Bjorken expanding fluid with a constant shear viscosity
relaxation time. We analytically determine the hydrodynamic attractor of this
fluid and discuss its properties. We show for the first time that the slow-roll
expansion, a commonly used approach to characterize the attractor, diverges.
This is shown to hold also in a conformal plasma. The gradient expansion is
found to converge in an example where causality and stability are violated.Comment: 22 pages, 7 figure
Perfect fluidity of a dissipative system: Analytical solution for the Boltzmann equation in
In this paper we obtain an analytical solution of the relativistic Boltzmann
equation under the relaxation time approximation that describes the
out-of-equilibrium dynamics of a radially expanding massless gas. This solution
is found by mapping this expanding system in flat spacetime to a static flow in
the curved spacetime . We further
derive explicit analytic expressions for the momentum dependence of the single
particle distribution function as well as for the spatial dependence of its
moments. We find that this dissipative system has the ability to flow as a
perfect fluid even though its entropy density does not match the equilibrium
form. The non-equilibrium contribution to the entropy density is shown to be
due to higher order scalar moments (which possess no hydrodynamical
interpretation) of the Boltzmann equation that can remain out of equilibrium
but do not couple to the energy-momentum tensor of the system. Thus, in this
system the slowly moving hydrodynamic degrees of freedom can exhibit true
perfect fluidity while being totally decoupled from the fast moving,
non-hydrodynamical microscopic degrees of freedom that lead to entropy
production.Comment: 17 pages, 2 figures; added reference, version accepted for
publication in Phys. Rev.
Debye screening mass near deconfinement from holography
In this paper the smallest thermal screening mass associated with the
correlator of the -odd operator, , is determined in strongly coupled non-Abelian
gauge plasmas which are holographically dual to non-conformal, bottom-up
Einstein+scalar gravity theories. These holographic models are constructed to
describe the thermodynamical properties of plasmas near deconfinement
at large and we identify this thermal mass with the Debye screening mass
. In this class of non-conformal models with a first order deconfinement
transition at , displays the same behavior found for the
expectation value of the Polyakov loop (which we also compute) jumping from
zero below to a nonzero value just above the transition. In the case of a
crossover phase transition, has a minimum similar to that found for the
speed of sound squared . This holographic framework is also used to
evaluate as a function of in a strongly coupled conformal gauge
plasma dual to Gauss-Bonnet gravity. In this case, decreases with
increasing in accordance with extrapolations from weak coupling
calculations.Comment: 48 pages, 16 figures, updated reference
Transient Fluid Dynamics of the Quark-Gluon Plasma According to AdS/CFT
We argue, using the AdS/CFT correspondence, that the transient dynamics of
the shear stress tensor in a strongly coupled SYM plasma is not
described by relaxation-type, fluid dynamical equations: at long times the
equations of motion should contain a \textit{second-order} comoving derivative
of the shear stress tensor. This occurs because in this strongly-coupled system
the lowest "non-hydrodynamical" quasinormal modes associated with shear stress
possess a nonzero real part at zero wavenumber. We use Weyl invariance to
obtain the most general equations of motion containing 2 comoving derivatives
of the shear stress tensor in the transient regime that are compatible with the
symmetries. We show that the asymptotic solution of this theory valid at times
much larger than the timescale associated with the "non-hydrodynamical" modes
reproduces the well-known results previously obtained directly from the AdS/CFT
correspondence. If the QGP formed in heavy ion collisions can be at least
qualitatively understood in terms of strongly-coupled SYM
theory, the second time derivative present in the equations of motion of the
fluid may lead to an unexpected dependence on the initial conditions for the
shear stress tensor needed in numerical hydrodynamic simulations.Comment: 11 pages, 1 figure: major changes in the text were made in order to
clarify the discussio
Bulk Viscosity Effects in Event-by-Event Relativistic Hydrodynamics
Bulk viscosity effects on the collective flow harmonics in heavy ion
collisions are investigated, on an event by event basis, using a newly
developed 2+1 Lagrangian hydrodynamic code named v-USPhydro which implements
the Smoothed Particle Hydrodynamics (SPH) algorithm for viscous hydrodynamics.
A new formula for the bulk viscous corrections present in the distribution
function at freeze-out is derived starting from the Boltzmann equation for
multi-hadron species. Bulk viscosity is shown to enhance the collective flow
Fourier coefficients from to when GeV
even when the bulk viscosity to entropy density ratio, , is
significantly smaller than .Comment: 23 pages, 12 figures, corrected typos, one reference include
Elliptic Flow Suppression due to Hadron Mass Spectrum
Hadron resonance gas models provide a good description of the equation of
state of quantum chromodynamics determined by lattice QCD calculations at
temperatures MeV. In this paper we investigate the effects of
an exponentially increasing hadron mass spectrum (Hagedorn spectrum) on the
azimuthal anisotropy of the rapidly expanding matter formed in
ultrarelativistic heavy ion collisions. If the temperature at which the
conversion from fluid degrees of freedom to hadrons is sufficiently close to
the Hagedorn temperature, the production of Hagedorn resonances suppresses the
differential elliptic flow of all hadron species.Comment: 6 pages, 3 figures, improved discussion, added references, to appear
in Physical Review
Large Nc Deconfinement Transition in the Presence of a Magnetic Field
We investigate the effect of a homogeneous magnetic field on the thermal
deconfinement transition of QCD in the large limit. First we discuss how
the critical temperature decreases due to the inclusion of
flavors of massless quarks. Then we study the equivalent correction in the
presence of an external Abelian magnetic field. To leading order in
, the fact that the deconfinement critical temperature decreases
with the magnetic field depends solely on quarks behaving paramagnetically.
Finally, we discuss the effects from a finite quark mass and its competition
with magnetic effects.Comment: 5 pages, 1 figure; v2: version accepted for publication in Phys. Rev.
Causality and existence of solutions of relativistic viscous fluid dynamics with gravity
A new approach is described to help improve the foundations of relativistic
viscous fluid dynamics and its coupling to general relativity. Focusing on
neutral conformal fluids constructed solely in terms of hydrodynamic variables,
we derive the most general viscous energy-momentum tensor yielding equations of
motion of second order in the derivatives, which is shown to provide a novel
type of generalization of the relativistic Navier-Stokes equations for which
causality holds. We show how this energy-momentum tensor may be derived from
conformal kinetic theory. We rigorously prove existence, uniqueness, and
causality of solutions of this theory (in the full nonlinear regime) both in a
Minkowski background and also when the fluid is dynamically coupled to
Einstein's equations. Linearized disturbances around equilibrium in Minkowski
spacetime are stable in this causal theory. A numerical study reveals the
presence of an out-of-equilibrium hydrodynamic attractor for a rapidly
expanding fluid. Further properties are also studied and a brief discussion of
how this approach can be generalized to non-conformal fluids is presented.Comment: 33 pages, 5 figures. Substantial improvements were made: the new
conformal tensor is now derived from kinetic theory; the causality and
well-posedness theorems now hold under more general conditions on the
transport coefficients; further discussion and applications have also been
included; new references were adde
On the fluid behavior of a baryon rich hadron resonance gas
We investigate the effects of finite baryon chemical potential on the
transport properties of a hadron resonance gas. We find that a hadron resonance
gas with large baryon number density is closer to the ideal fluid limit than
the corresponding gas with zero baryon number. This suggests that the system
created at the Relativistic Heavy Ion Collider (RHIC) at lower collision
energies may behave as a fluid, with an effective fluidity close to the one
found at RHIC's highest energy near phase transition. This might explain why
the differential elliptic flow coefficient measured at lower collisional
energies at RHIC is similar to the one observed at high energies.Comment: 8 pages, 8 figure
- β¦