306 research outputs found
Viscosity in the excluded volume hadron gas model
The shear viscosity in the van der Waals excluded volume
hadron-resonance gas model is considered. For the shear viscosity the result of
the non-relativistic gas of hard-core particles is extended to the mixture of
particles with different masses, but equal values of hard-core radius r. The
relativistic corrections to hadron average momenta in thermal equilibrium are
also taken into account. The ratio of the viscosity to the entropy
density s is studied. It monotonously decreases along the chemical freeze-out
line in nucleus-nucleus collisions with increasing collision energy. As a
function of hard-core radius r, a broad minimum of the ratio near fm is found at high collision energies. For the
charge-neutral system at MeV, a minimum of the ratio is reached for fm. To justify a hydrodynamic approach to
nucleus-nucleus collisions within the hadron phase the restriction from below,
fm, on the hard-core hadron radius should be fulfilled in the
excluded volume hadron-resonance gas.Comment: 12 pages, 3 figure
Particle number fluctuations in nuclear collisions within excluded volume hadron gas model
The multiplicity fluctuations are studied in the van der Waals excluded
volume hadron-resonance gas model. The calculations are done in the grand
canonical ensemble within the Boltzmann statistics approximation. The scaled
variances for positive, negative and all charged hadrons are calculated along
the chemical freeze-out line of nucleus-nucleus collisions at different
collision energies. The multiplicity fluctuations are found to be suppressed in
the van der Waals gas. The numerical calculations are presented for two values
of hard-core hadron radius, fm and 0.5 fm, as well as for the upper
limit of the excluded volume suppression effects.Comment: 19 pages, 4 figure
Particle Number Fluctuations in Canonical Ensemble
Fluctuations of charged particle number are studied in the canonical
ensemble. In the infinite volume limit the fluctuations in the canonical
ensemble are different from the fluctuations in the grand canonical one. Thus,
the well-known equivalence of both ensembles for the average quantities does
not extend for the fluctuations. In view of a possible relevance of the results
for the analysis of fluctuations in nuclear collisions at high energies, a role
of the limited kinematical acceptance is studied.Comment: 13 pages, 9 figures, LaTe
Dynamical equilibration of strongly interacting "infinite" parton matter within the parton-hadron-string dynamics transport approach
We study the kinetic and chemical equilibration in "infinite" parton matter
within the parton-hadron-string dynamics off-shell transport approach, which is
based on a dynamical quasiparticle model (DQPM) for partons matched to
reproduce lattice QCD results-including the partonic equation of state-in
thermodynamic equilibrium. The "infinite" parton matter is simulated by a
system of quarks and gluons within a cubic box with periodic boundary
conditions, at different energy densities, initialized slightly out of kinetic
and chemical equilibrium. We investigate the approach of the system to
equilibrium and the time scales for the equilibration of different observables.
We, furthermore, study particle distributions in the strongly interacting
quark-gluon plasma (sQGP) including partonic spectral functions, momentum
distributions, abundances of the different parton species and their
fluctuations (scaled variance, skewness, and kurtosis) in equilibrium. We also
compare the results of the microscopic calculations with the ansatz of the
DQPM. It is found that the results of the transport calculations are in
equilibrium well matched by the DQPM for quarks and antiquarks, while the gluon
spectral function shows a slightly different shape due to the explicit
interaction of partons. The time scales for the relaxation of fluctuation
observables are found to be shorter than those for the average values.
Furthermore, in the local subsystem, a strong change of the fluctuation
observables with the size of the local volume is observed. These fluctuations
no longer correspond to those of the full system and are reduced to Poissonian
distributions when the volume of the local subsystem becomes much smaller than
the total volume.Comment: 21 pages, 20 figure
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