31 research outputs found
Relativistic Hadron-Hadron Collisions in the Ultra-Relativistic Quantum Molecular Dynamics Model (UrQMD)
Hadron-hadron collisions at high energies are investigated in the
Ultra-relativistic-Quantum-Molecular-Dynamics approach (UrQMD). This
microscopic transport model is designed to study pp, pA and A+A collisions. It
describes the phenomenology of hadronic interactions at low and intermediate
energies ( GeV) in terms of interactions between known hadrons and
their resonances. At high energies, GeV, the excitation of color
strings and their subsequent fragmentation into hadrons dominates the multiple
production of particles in the UrQMD model. The model shows a fair overall
agreement with a large body of experimental h-h data over a wide range of h-h
center-of-mass energies. Hadronic reaction data with higher precision would be
useful to support the use of the UrQMD model for relativistic heavy ion
collisions.Comment: 66 pages, Download the UrQMD model from
http://www.th.physik.uni-frankfurt.de/~urqmd/urqmd.htm
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
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
Nonequilibrium models of relativistic heavy-ion collisions
To be published in J. Phys. G - Proceedings of SQM 2004 : We review the results from the various hydrodynamical and transport models on the collective flow observables from AGS to RHIC energies. A critical discussion of the present status of the CERN experiments on hadron collective flow is given. We emphasize the importance of the flow excitation function from 1 to 50 A.GeV: here the hydrodynamic model has predicted the collapse of the v2-flow ~ 10 A.GeV; at 40 A.GeV it has been recently observed by the NA49 collaboration. Since hadronic rescattering models predict much larger flow than observed at this energy we interpret this observation as evidence for a first order phase transition at high baryon density r b. Moreover, the connection of the elliptic flow v2 to jet suppression is examined. It is proven experimentally that the collective flow is not faked by minijet fragmentation. Additionally, detailed transport studies show that the away-side jet suppression can only partially (< 50%) be due to hadronic rescattering. Furthermore, the change in sign of v1, v2 closer to beam rapidity is related to the occurence of a high density first order phase transition in the RHIC data at 62.5, 130 and 200 A.GeV
Equilibrium and non-equilibrium effects in relativistic heavy ion collisions
The hypothesis of local equilibrium (LE) in relativistic heavy ion collisions at energies from AGS to RHIC is checked in the microscopic transport model. We find that kinetic, thermal, and chemical equilibration of the expanding hadronic matter is nearly reached in central collisions at AGS energy for t >_ fm/c in a central cell. At these times the equation of state may be approximated by a simple dependence P ~= (0.12-0.15) epsilon. Increasing deviations of the yields and the energy spectra of hadrons from statistical model values are observed for increasing bombarding energies. The origin of these deviations is traced to the irreversible multiparticle decays of strings and many-body (N >_ 3) decays of resonances. The violations of LE indicate that the matter in the cell reaches a steady state instead of idealized equilibrium. The entropy density in the cell is only about 6% smaller than that of the equilibrium state
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
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.
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
Strangeness enhancement from strong color fields at RHIC
In ultra-relativistic heavy ion collisions, early stage multiple scatterings
may lead to an increase of the color electric field strength. Consequently,
particle production - especially heavy quark (and di-quark) production - is
greatly enhanced according to the Schwinger mechanism. We test this idea via
the Ultra-relativistic Quantum Molecular Dynamics model (UrQMD) for Au+Au
collisions at the full RHIC energy ( AGeV). Relative to p+p
collisions, a factor of 60, 20 and 7 enhancement respectively, for
(), (), and , () is predicted for a model
with increased color electric field strength
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.