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The origin of transverse flow at the SPS (1998)

Marcus Bleicher Christian Spieles Christoph Ernst Lars Gerland Sven Soff Horst Stöcker Walter Greiner Steffen A. Bass

We study the transverse expansion in central Pb+Pb collisions at the CERN SPS. Strong collective motion of hadrons can be created. This flow is mainly due to meson baryon rescattering. It allows to study the angular distribution of intermediate mass meson baryon interactions.

Intermediate mass dileptons from secondary Drell-Yan processes (1998)

Christian Spieles Lars Gerland Nils Hammon Marcus Bleicher Steffen A. Bass Horst Stöcker Walter Greiner C. Lourenco Ramona Vogt

Recent reports on enhancements of intermediate and hight mass muon pairs producedin heavy ion collisions have attracted much attention.

Fluctuations and inhomogenities of energy density and isospin in Pb + Pb at the SPS (1998)

Marcus Bleicher Lars Gerland Christian Spieles Adrian Dumitru Steffen A. Bass Mohamed Belkacem Mathias Brandstetter Christoph Ernst Ludwig Neise Sven Soff Henning Weber Horst Stöcker Walter Greiner

The main goal of heavy ion physics in the last fifteen years has been the search for the quark-gluon-plasma(QGP). Until now, unambigous experimental evidence for the QGP is missing.

Can momentum correlations proof kinetic equilibration in heavy ion collisions at 160/A-GeV? (1998)

Marcus Bleicher Mohamed Belkacem Christoph Ernst Henning Weber Lars Gerland Christian Spieles Steffen A. Bass Horst Stöcker Walter Greiner

We perform an event-by-event analysis of the transverse momentum distribution of final state particles in central Pb(160AGeV)+Pb collisions within a microscopic non-equilibrium transport model (UrQMD). Strong influence of rescattering is found. The extracted momentum distributions show less fluctuations in A+A collisions than in p+p reactions. This is in contrast to simplified p+p extrapolations and random walk models.

Excitation function of energy density and partonic degrees of freedom in relativistic heavy ion collisions (1998)

H. Weber Christoph Ernst Marcus Bleicher Larissa V. Bravina Horst Stöcker Walter Greiner Christian Spieles Steffen A. Bass

We estimate the energy density epsilon pile-up at mid-rapidity in central Pb+Pb collisions from 2 200 GeV/nucleon. epsilon 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 Elab 30 GeV/nucleon. In Pb+Pb collisions at 160 GeV/nucleon the energy density reaches up to 4 GeV/fm3, 95% of which are contained in partonic degrees of freedom.

Dissociation rates of J / psi's with comoving mesons : thermal versus nonequilibrium scenario. (1998)

Christian Spieles Ramona Vogt Lars Gerland Steffen A. Bass Marcus Bleicher Horst Stöcker Walter Greiner

We study J/psi dissociation processes in hadronic environments. The validity of a thermal meson gas ansatz is tested by confronting it with an alternative, nonequilibrium scenario. Heavy ion collisions are simulated in the frame- work of the microscopic transport model UrQMD, taking into account the production of charmonium states through hard parton-parton interactions and subsequent rescattering with hadrons. The thermal gas and microscopic transport scenarios are shown to be very dissimilar. Estimates of J/psi survival probabilities based on thermal models of comover interactions in heavy ion collisions are therefore not reliable.

J/psi suppression in heavy ion collisions - interplay of hard and soft QCD processes (1998)

Christian Spieles Ramona Vogt Lars Gerland Steffen A. Bass Marcus Bleicher Leonid Frankfurt Mark Strikman Horst Stöcker Walter Greiner

We study J/psi suppression in AB collisions assuming that the charmonium states evolve from small, color transparent configurations. Their interaction with nucleons and nonequilibrated, secondary hadrons is simulated us- ing the microscopic model UrQMD. The Drell-Yan lepton pair yield and the J/psi /Drell-Yan ratio are calculated as a function of the neutral transverse en- ergy in Pb+Pb collisions at 160 GeV and found to be in reasonable agreement with existing data.

Distinguishing hadronic cascades from hydrodynamic models in Pb(160 AGeV)+Pb reactions by impact parameter variation (1998)

Marcus Bleicher Manuel Reiter Adrian Dumitru Jörg Brachmann Christian Spieles Steffen A. Bass Horst Stöcker Walter Greiner

We propose to study the impact parameter dependence of the anti-Lambda/anti-Proton ratio in Pb(160AGeV)+Pb reactions. The anti-Lambda/anti-Proton ratio is a sensible tool to distinguish between hadronic cascade models and hydrodynamical models, which incorporate a QGP phase transition.

Local thermal and chemical equilibration and the equation of state in relativistic heavy ion collisions (1998)

Larissa V. Bravina Mathias Brandstetter Mark I. Gorenstein Eugene E. Zabrodin Mohamed Belkacem Marcus Bleicher Steffen A. Bass Christoph Ernst M. Hofmann Sven Soff Horst Stöcker Walter Greiner

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 3. Baryon energy spectra in this cell are reproduced by Boltzmann distributions at all collision energies for t > 10 fm/c with a unique rapidly dropping temperature. At these times the equation of state has a simple form: P = (0.12 - 0.15) Epsilon. At SPS energies 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.

Reaction dynamics in Pb+Pb at the CERN/SPS: from partonic degrees of freedom to freeze-out (1998)

Steffen A. Bass Henning Weber Christoph Ernst Marcus Bleicher Mohamed Belkacem Larissa V. Bravina Sven Soff Horst Stöcker Walter Greiner Christian Spieles

We analyze the reaction dynamics of central Pb+Pb collisions at 160 GeV/nucleon. First we estimate the energy density pile-up at mid-rapidity and calculate its excitation function: 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 E >= 30 GeV/nucleon. The energy density reaches up to 4 GeV/fm 3, 95% of which are contained in partonic degrees of freedom. It is shown that cells of hadronic matter, after the early reaction phase, can be viewed as nearly chemically equilibrated. This matter never exceeds energy densities of 0.4 GeV/fm 3, i.e. a density above which the notion of separated hadrons loses its meaning. The final reaction stage is analyzed in terms of hadron ratios, freeze-out distributions and a source analysis for final state pions.

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