598 research outputs found
Violation of energy-per-hadron scaling in a resonance matter
Yields of hadrons, their average masses and energies per hadron at the stage
of chemical freeze-out in (ultra)relativistic heavy-ion collisions are analyzed
within the statistical model. The violation of the scaling / = 1 GeV
observed in Au+Au collisions at = 130 AGeV is linked to the
formation of resonance-rich matter with a considerable fraction of baryons and
antibaryons. The rise of the energy-per-hadron ratio in baryon-dominated matter
is discussed. A violation of the scaling condition is predicted for a very
central zone of heavy-ion collisions at energies around 40 AGeV.Comment: 5 pages incl. 3 figures and 2 tables, to be published in Phys. Rev.
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.
Transverse momentum dependence of directed particle flow at 160 AGeV
The transverse momentum () dependence of hadron flow at SPS energies is
studied. In particular, the nucleon and pion flow in S+S and Pb+Pb collisions
at 160 AGeV is investigated. For simulations the microscopic quark-gluon string
model (QGSM) is applied. It is found that the directed flow of pions changes sign from a negative slope in the low- region to a
positive slope at GeV/c as recently also observed
experimentally. The change of the flow behaviour can be explained by early
emission times for high- pions. We further found that a substantial amount
of high- pions are produced in the very first primary NN collisions at the
surface region of the touching nuclei. Thus, at SPS energies high-
nucleons seem to be a better probe for the hot and dense early phase of nuclear
collisions than high- pions. Both, in the light and in the heavy system
the pion directed flow exhibits large negative values when
the transverse momentum approaches zero, as also seen experimentally in Pb+Pb
collisions. It is found that this effect is caused by nuclear shadowing. The
proton flow, in contrary, shows the typical linear increase with rising .Comment: REVTEX, 20 pages incl. 6 figures, revised and extended versio
Dynamical vs geometric anisotropy in relativistic heavy-ion collisions: which one prevails?
We study the influence of geometric and dynamical anisotropies on the
development of flow harmonics and, simultaneously, on the second- and
third-order oscillations of femtoscopy radii. The analysis is done within the
Monte Carlo event generator HYDJET++, which was extended to dynamical
triangular deformations. It is shown that the merely geometric anisotropy
provides the results which anticorrelate with the experimental observations of
either (or ) or second-order (or third-order) oscillations of the
femtoscopy radii. Decays of resonances significantly increase the emitting
areas but do not change the phases of the radii oscillations. In contrast to
the spatial deformations, the dynamical anisotropy alone provides the correct
qualitative description of the flow and the femtoscopy observables
simultaneously. However, one needs both types of the anisotropy to match
quantitatively the experimental data.Comment: minor corrections, published versio
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
Roto-flexoelectric coupling impact on the phase diagrams and pyroelectricity of thin SrTiO3 films
The influence of the flexoelectric and rotostriction coupling on the phase
diagrams of ferroelastic-quantum paraelectric SrTiO3 films was studied using
Landau-Ginzburg-Devonshire (LGD) theory. The phase diagrams in coordinates
temperature - film thickness were calculated for different epitaxial misfit
strains. Tensile misfit strains stimulate appearance of the spontaneous
out-of-plane structural order parameter (displacement vector of an appropriate
oxygen atom from its cubic position) in the structural phase. Compressive
misfit strains stimulate appearance of the spontaneous in-plane structural
order parameter. Gradients of the structural order parameter components, which
inevitably exist in the vicinity of film surfaces due to the termination and
symmetry breaking, induce improper polarization and pyroelectric response via
the flexoelectric and rotostriction coupling mechanism. Flexoelectric and
rotostriction coupling results in the roto-flexoelectric field that is
antisymmetric inside the film, small in the central part of the film, where the
gradients of the structural parameter are small, and maximal near the surfaces,
where the gradients of the structural parameter are highest. The field induces
improper polarization and pyroelectric response. Penetration depths of the
improper phases (both polar and structural) can reach several nm from the film
surfaces. An improper pyroelectric response of thin films is high enough to be
registered with planar-type electrode configurations by conventional
pyroelectric methods.Comment: 35 pages, 9 figures, 1 appendix, 1 tabl
Baryon stopping and strange baryon/antibaryon production at SPS energies
The amount of proton stopping in central Pb+Pb collisions from 20-160 AGeV as
well as hyperon and antihyperon rapidity distributions are calculated within
the UrQMD model in comparison to experimental data at 40, 80 and 160 AGeV taken
recently from the NA49 collaboration. Furthermore, the amount of baryon
stopping at 160 AGeV for Pb+Pb collisions is studied as a function of
centrality in comparison to the NA49 data. We find that the strange baryon
yield is reasonably described for central collisions, however, the rapidity
distributions are somewhat more narrow than the data. Moreover, the
experimental antihyperon rapidity distributions at 40, 80 and 160 AGeV are
underestimated by up to factors of 3 - depending on the annihilation cross
section employed - which might be addressed to missing multi-meson fusion
channels in the UrQMD model.Comment: 18 pages, including 7 eps figures, to be published in Phys. Rev.
Anisotropic flow fluctuations in hydro-inspired freeze-out model for relativistic heavy ion collisions
The LHC data on event-by-event harmonic flow coefficients measured in PbPb
collisions at center-of-mass energy 2.76 TeV per nucleon pair are analyzed and
interpreted within the HYDJET++ model. To compare the model results with the
experimental data the unfolding procedure is employed. The essentially
dynamical origin of the flow fluctuations in hydro-inspired freeze-out approach
has been established. It is shown that the simple modification of the model via
introducing the distribution over spatial anisotropy parameters permits
HYDJET++ to reproduce both elliptic and triangular flow fluctuations and
related to it eccentricity fluctuations of the initial state at the LHC energy.Comment: 12 pages including 9 figures as EPS-files; prepared using LaTeX
package for publication in the European Physical Journal
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
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