91 research outputs found
Quark-Gluon-Plasma Formation at SPS Energies?
By colliding ultrarelativistic ions, one achieves presently energy densities
close to the critical value, concerning the formation of a quark-gluon-plasma.
This indicates the importance of fluctuations and the necessity to go beyond
the investigation of average events. Therefore, we introduce a percolation
approach to model the final stage ( fm/c) of ion-ion collisions, the
initial stage being treated by well-established methods, based on strings and
Pomerons. The percolation approach amounts to finding high density domains, and
treating them as quark-matter droplets. In this way, we have a {\bf realistic,
microscopic, and Monte--Carlo based model which allows for the formation of
quark matter.} We find that even at SPS energies large quark-matter droplets
are formed -- at a low rate though. In other words: large quark-matter droplets
are formed due to geometrical fluctuation, but not in the average event.Comment: 7 Pages, HD-TVP-94-6 (1 uuencoded figure
Excitation Function of Energy Density and Partonic Degrees of Freedom in Relativistic Heavy Ion Collisions
We estimate the energy density pile-up at mid-rapidity in central Pb+Pb
collisions from 2 - 200 GeV/nucleon. The energy density is decomposed into
hadronic and partonic contributions. A detailed analysis of the collision
dynamics in the framework of a microscopic transport model shows the importance
of partonic degrees of freedom and rescattering of leading (di)quarks in the
early phase of the reaction for lab-energies > 30 GeV/nucleon. In Pb+Pb
collisions at 160 GeV/nucleon the energy density reaches up to 4 GeV/fm^3, 95%
of which are contained in partonic degrees of freedom.Comment: 10 pages, 4 figure
A stopped Delta-Matter Source in Heavy Ion Collisions at 10 GeV/n
We predict the formation of highly dense baryon-rich resonance matter in
Au+Au collisions at AGS energies. The final pion yields show observable signs
for resonance matter. The Delta(1232) resonance is predicted to be the dominant
source for pions of small transverse momenta. Rescattering effects --
consecutive excitation and deexcitation of Deltas -- lead to a long apparent
lifetime (> 10 fm/c) and rather large volumina (several 100 fm^3) of the
Delta-matter state. Heavier baryon resonances prove to be crucial for reaction
dynamics and particle production at AGS.Comment: 17 pages, 5 postscript figures, uses psfig.sty and revtex.st
Directed and Elliptic Flow in 158 AGeV Pb+Pb Collisions
Directed and elliptic flow of protons and positively charged pions has been
studied in the target fragmentation region using the Plastic Ball detector in
the WA98 experiment. The results exhibit a strong dependence on centrality,
rapidity, and transverse momentum.
The rapidity dependence can be described by a Gaussian distribution. The
model comparisons reveal a large discrepancy of the flow strength obtained from
the data and the simulations.Comment: 4 pages, 4 eps figures, talk at Quark Matter 99, see also
http://qgp.uni-muenster.de/WA98/qm99/flo
Phasespace Correlations of Antideuterons in Heavy Ion Collisions
In the framework of the relativistic quantum molecular dynamics approach
({\small RQMD}) we investigate antideuteron () observables in
Au+Au collisions at 10.7~AGeV. The impact parameter dependence of the formation
ratios and is calculated. In central
collisions, the antideuteron formation ratio is predicted to be two orders of
magnitude lower than the deuteron formation ratio. The yield in
central Au+Au collisions is one order of magnitude lower than in Si+Al
collisions. In semicentral collisions different configuration space
distributions of 's and 's lead to a large
``squeeze--out'' effect for antideuterons, which is not predicted for the
's
On the elliptical flow in asymmetric collisions and nuclear equation of state
We here present the results of elliptical flow for the collision of different
asymmetric nuclei (10Ne20 +13 Al27, 18Ar40 +21 Sc45, 30Zn64 +28 Ni58, 36Kr86
+41 Nb93) by using the Quantum Molecular Dynamics (QMD) model. General features
of elliptical flow are investigated with the help of theoretical simulations.
The simulations are performed at different beam energies between 40 and 105
MeV/nucleon. A significant change can be seen from in-plane to out-of-plane
elliptical flow of different fragments with incident energy. A comparison with
experimental data is also made. Further, we predict, for the first time that,
elliptical flow for different kind of fragments follow power law dependence ?
C(Atot)? for asymmetric systems
Antibaryons in massive heavy ion reactions: Importance of potentials
In the framework of RQMD we investigate antiproton observables in massive
heavy ion collisions at AGS energies and compare to preliminary results of the
E878 collaboration. We focus here on the considerable influence of the *real*
part of an antinucleon--nucleus optical potential on the antiproton momentum
spectra
Nuclear Clusters as a Probe for Expansion Flow in Heavy Ion Reactions at 10-15AGeV
A phase space coalescence description based on the Wigner-function method for
cluster formation in relativistic nucleus-nucleus collisions is presented. The
momentum distributions of nuclear clusters d,t and He are predicted for central
Au(11.6AGeV)Au and Si(14.6AGeV)Si reactions in the framework of the RQMD
transport approach. Transverse expansion leads to a strong shoulder-arm shape
and different inverse slope parameters in the transverse spectra of nuclear
clusters deviating markedly from thermal distributions. A clear ``bounce-off''
event shape is seen: the averaged transverse flow velocities in the reaction
plane are for clusters larger than for protons. The cluster yields
--particularly at low at midrapidities-- and the in-plane (anti)flow of
clusters and pions change if suitably strong baryon potential interactions are
included. This allows to study the transient pressure at high density via the
event shape analysis of nucleons, nucleon clusters and other hadrons.Comment: 38 pages, 9 figures, LaTeX type, eps used, subm. to Phys. Rev.
The directed flow maximum near c_s=0
We investigate the excitation function of quark-gluon plasma formation and of
directed in-plane flow of nucleons in the energy range of the BNL-AGS and for
the E(Lab)=40AGeV Pb+Pb collisions performed recently at the CERN-SPS. We
employ the three-fluid model with dynamical unification of kinetically
equilibrated fluid elements. Within our model with first-order phase transition
at high density, droplets of QGP coexisting with hadronic matter are produced
already at BNL-AGS energies, E(Lab)=10AGeV. A substantial decrease of the
isentropic velocity of sound, however, requires higher energies, E(Lab)=40AGeV.
We show the effect on the flow of nucleons in the reaction plane. According to
our model calculations, kinematic requirements and EoS effects work
hand-in-hand at E(Lab)=40AGeV to allow the observation of the dropping velocity
of sound via an increase of the directed flow around midrapidity as compared to
top BNL-AGS energy.Comment: 10 pages, 4 figures; plot of p(e) at various specific entropies shows
why mixed phase is not soft at AGS energ
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