150 research outputs found
Elliptic flow of the dilute Fermi gas: From kinetics to hydrodynamics
We use the Boltzmann equation in the relaxation time approximation to study
the expansion of a dilute Fermi gas at unitarity. We focus, in particular, on
the approach to the hydrodynamic limit. Our main finding are: i) In the regime
that has been studied experimentally hydrodynamic effects beyond the
Navier-Stokes approximation are small, ii) mean field corrections to the
Boltzmann equation are not important, iii) experimental data imply that
freezeout occurs very late, that means that the relaxation time remains smaller
than the expansion time during the entire evolution of the system, iv) the
experimental results also imply that the bulk viscosity is significantly
smaller than the shear viscosity of the system.Comment: 18 pages, 6 figure
Thermal Dimuon Yields at NA60
Dilepton emission rates from a hadronic gas at finite temperature and baryon
density are completely constrained by broken chiral symmetry in a density
expansion. The rates can be expressed in terms of vacuum correlations which are
measured in annihilation, decays and photo-reactions on
nucleons and nuclei. In this paper, the theoretical results are summarized and
the total dimuon yield is calculated by integrating the dimuon rates over the
space-time history of a fireball based on hydrodynamic calculations with CERN
SPS conditions. The resulting dimuon yield is in good agreement with the recent
measurements reported by NA60.Comment: 9 pages, 8 figure
Toward parton equilibration with improved parton interaction matrix elements
The Quark-Gluon Plasma can be produced in high energy heavy ion collisions
and how it equilibrates is important for the extraction of the properties of
strongly interacting matter. A radiative transport model can be used to reveal
interesting characteristics of Quark-Gluon Plasma thermalization. For example,
screened parton interactions always lead to partial pressure isotropization.
Systems with different initial pressure anisotropies evolve toward the same
asymptotic evolution. In particular, radiative processes are crucial for the
chemical equilibration of the system. Matrix elements under the soft and
collinear approximation for these processes, as first derived by Gunion and
Bertsch, are widely used. A different approach is to start with the exact
matrix elements for the two to three and its inverse processes. General
features of this approach will be reviewed and the results will be compared
with the Gunion-Bertsch results. We will comment on the possible implications
of the exact matrix element approach on Quark-Gluon Plasma thermalization.Comment: Presented at the 11th International Conference on Nucleus-Nucleus
Collisions (NN2012), San Antonio, Texas, USA, 27 May-1 June 201
Production of Photons and Dileptons in the Glasma
We study the production of photons and dileptons during the pre-equilibrium
Glasma stage in heavy ion collisions and discuss the implications in light of
the PHENIX data. We find that the measured distributions of such
electromagnetic emissions, while having some features not well understood if
hypothesized to entirely arise from a thermalized Quark-Gluon Plasma, have some
qualitative features that might be described after including effects from a
thermalizing Glasma. The shape and centrality dependence of the transverse
momentum spectra of the so-called "thermal photons" are well described. The
mass and transverse momentum dependence of intermediate mass dileptons also
agree with our estimates. The low transverse momenta from which the excessive
dileptons (in low to intermediate mass region) arise is suggestive of emissions
from a Bose condensate. We also predict the centrality dependence of dilepton
production. Uncertainties in the current approach and improvements in the
future are discussed.Comment: 24 pages, 7 figures, new results and references added in v
Quarkonium above deconfinement as an open quantum system
Quarkonium at temperatures above deconfinement is modeled as an open quantum
system, whose dynamics is determined not just by a potential energy and mass,
but also by a drag coefficient which characterizes its interaction with the
medium. The reduced density matrix for a heavy particle experiencing
dissipative forces is expressed as an integral over paths in imaginary time and
evaluated numerically. We demonstrate that dissipation could affect the
Euclidean heavy-heavy correlators calculated in lattice simulations at
temperatures just above deconfinement.Comment: 13 pages, 1 figur
A PNJL model in 0+1 Dimensions
We formulate the Polyakov-Nambu-Jona-Lasinio (PNJL) model in 0+1 dimensions.
The thermodynamics captured by the partition function yields a bulk pressure,
as well as quark susceptibilities versus temperature that are similar to the
ones in 3+1 dimensions. Around the transition temperature the behavior in the
pressure and quark susceptibilities follows from the interplay between the
lowest Matsubara frequency and the Polyakov line. The reduction to the lowest
Matsubara frequency yields a matrix Model. In the presence of the Polyakov line
the UV part of the Dirac spectrum features oscillations when close to the
transition temperature.Comment: 18 pages, 13 figure
Ekpyrosis and inflationary dynamics in heavy ion collisions: the role of quantum fluctuations
We summarize recent significant progress in the development of a
first-principles formalism to describe the formation and evolution of matter in
very high energy heavy ion collisions. The key role of quantum fluctuations
both before and after a collision is emphasized. Systematic computations are
now feasible to address early time dynamics essential to quantifying properties
of strongly interacting quark-gluon matter.Comment: Talk by R.V. at Quark Matter 2011, Annecy, France, May 23-28, 2011.
LaTex, 4 pages; v2, final version to appear in J. Phys.
Low mass dilepton radiation at RHIC
In this work we discuss the emission of low mass dilepton radiation from a
hydrodynamic evolution model of Au-Au collisions and make comparisons with
recent PHENIX measurements. The dilepton emission rates from the hadronic phase
are treated at finite temperature and baryon density and are completely
constrained by broken chiral symmetry in a density expansion. The rates are
expressed in terms of vacuum correlators which are measured in
annihilation, decays and photo-reactions on nucleons and nuclei. We
consider two possibilities for the hadronic phase: A chemical equilibrated an
off equilibrium hadronic gas. We find that while chemical off-equilibrium helps
explain part of the low mass (0.15 M GeV 0.7) enhancement seen in
the data there is still a large discrepancy.Comment: 8 pages, 12 figure
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