2,766 research outputs found
Extracting the equation of state from a microscopic non-equilibrium model
We study the thermodynamic properties of infinite nuclear matter with the
Ultrarelativistic Quantum Molecular Dynamics (URQMD), a semiclassical transport
model, running in a box with periodic boundary conditions. It appears that the
energy density rises faster than at high temperatures of ~MeV. This indicates an increase in the number of degrees of freedom.
Moreover, We have calculated direct photon production in Pb+Pb collisions at
160~GeV/u within this model. The direct photon slope from the microscopic
calculation equals that from a hydrodynamical calculation without a phase
transition in the equation of state of the photon source.Comment: Proceedings of the XIV International Conference on Particles and
Nuclei (PANIC'96), 22-28 May 1996, Williamsburg, Virginia, USA, to be
published by World Scientific Publ. Co. (3 pages
Microscopic Analysis of Thermodynamic Parameters from 160 MeV/n - 160 GeV/n
Microscopic calculations of central collisions between heavy nuclei are used
to study fragment production and the creation of collective flow. It is shown
that the final phase space distributions are compatible with the expectations
from a thermally equilibrated source, which in addition exhibits a collective
transverse expansion. However, the microscopic analyses of the transient states
in the reaction stages of highest density and during the expansion show that
the system does not reach global equilibrium. Even if a considerable amount of
equilibration is assumed, the connection of the measurable final state to the
macroscopic parameters, e.g. the temperature, of the transient ''equilibrium''
state remains ambiguous.Comment: 13 pages, Latex, 8 postscript figures, Proceedings of the Winter
Meeting in Nuclear Physics (1997), Bormio (Italy
Microscopic Models for Ultrarelativistic Heavy Ion Collisions
In this paper, the concepts of microscopic transport theory are introduced
and the features and shortcomings of the most commonly used ansatzes are
discussed. In particular, the Ultrarelativistic Quantum Molecular Dynamics
(UrQMD) transport model is described in great detail. Based on the same
principles as QMD and RQMD, it incorporates a vastly extended collision term
with full baryon-antibaryon symmetry, 55 baryon and 32 meson species. Isospin
is explicitly treated for all hadrons. The range of applicability stretches
from GeV/nucleon, allowing for
a consistent calculation of excitation functions from the intermediate energy
domain up to ultrarelativistic energies. The main physics topics under
discussion are stopping, particle production and collective flow.Comment: 129 pages, pagestyle changed using US letter (8.5x11 in) format. The
whole paper (13 Mb ps file) could also be obtained from
ftp://ftp.th.physik.uni-frankfurt.de/pub/urqmd/ppnp2.ps.g
Signatures of dense hadronic matter in ultrarelativistic heavy ion reactions
The behavior of hadronic matter at high baryon densities is studied within
Ultrarelativistic Quantum Molecular Dynamics (URQMD). Baryonic stopping is
observed for Au+Au collisions from SIS up to SPS energies. The excitation
function of flow shows strong sensitivities to the underlying equation of state
(EOS), allowing for systematic studies of the EOS. Dilepton spectra are
calculated with and without shifting the pole. Except for S+Au
collisions our calculations reproduce the CERES data.Comment: Invited talk at RHIC-theory workshop at BNL july 8-1
Transition to resonance-rich matter in heavy ion collisions at RHIC energies
The equilibration of hot and dense nuclear matter produced in the central
region in central Au+Au collisions at AGeV is studied within the
microscopic transport model UrQMD. The pressure here becomes isotropic at fm/c. Within the next 15 fm/c the expansion of the matter proceeds
almost isentropically with the entropy per baryon ratio .
During this period the equation of state in the -plane has a very
simple form, . Comparison with the statistical model (SM) of
an ideal hadron gas reveals that the time of fm/ may be too
short to attain the fully equilibrated state. Particularly, the fractions of
resonances are overpopulated in contrast to the SM values. The creation of such
a long-lived resonance-rich state slows down the relaxation to chemical
equilibrium and can be detected experimentally.Comment: Talk at the conference Strangeness'2000, to be published in J. of
Phys.
Local Thermal and Chemical Equilibration and the Equation of State in Relativistic Heavy Ion Collisions
Thermodynamical variables and their time evolution are studied for central
relativistic heavy ion collisions from 10.7 to 160 AGeV in the microscopic
Ultrarelativistic Quantum Molecular Dynamics model (UrQMD). The UrQMD model
exhibits drastic deviations from equilibrium during the early high density
phase of the collision. Local thermal and chemical equilibration of the
hadronic matter seems to be established only at later stages of the quasi-
isentropic expansion in the central reaction cell with volume 125 fm.
distributions at all collision energies for with a unique
Baryon energy spectra in this cell are approximately reproduced by Boltzmann
rapidly dropping temperature. At these times the equation of state has a simple
form: . At 160 AGeV the strong deviation from
chemical equilibrium is found for mesons, especially for pions, even at the
late stage of the reaction. The final enhancement of pions is supported by
experimental data.Comment: 17 Pages, LaTex, 8 eps figures. Talk given at SQM'98 conference,
20-24 July 1998, Padova, Italy, submitted to J. Phys.
Microscopic calculations of stopping and flow from 160AMeV to 160AGeV
The behavior of hadronic matter at high baryon densities is studied within
Ultrarelativistic Quantum Molecular Dynamics (URQMD). Baryonic stopping is
observed for Au+Au collisions from SIS up to SPS energies. The excitation
function of flow shows strong sensitivities to the underlying equation of state
(EOS), allowing for systematic studies of the EOS. Effects of a density
dependent pole of the -meson propagator on dilepton spectra are studied
for different systems and centralities at CERN energies.Comment: Proceedings of the Quark Matter '96 Conference, Heidelberg, German
Equation of state of resonance-rich matter in the central cell in heavy-ion collisions at =200 AGeV
The equilibration of hot and dense nuclear matter produced in the central
cell of central Au+Au collisions at RHIC ( AGeV) energies is
studied within a microscopic transport model. The pressure in the cell becomes
isotropic at fm/ after beginning of the collision. Within the
next 15 fm/ the expansion of matter in the cell proceeds almost
isentropically with the entropy per baryon ratio , and the
equation of state in the plane has a very simple form,
. Comparison with the statistical model of an ideal hadron gas
indicates that the time fm/c may be too short to reach the fully
equilibrated state. Particularly, the creation of long-lived resonance-rich
matter in the cell decelerates the relaxation to chemical equilibrium. This
resonance-abundant state can be detected experimentally after the thermal
freeze-out of particles.Comment: LATEX, 21 pages incl. 7 figure
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