OPUS 4 | Universitätspublikationen

"Antiflow" of antiprotons in heavy ion collisions (1994)

André Jahns Christian Spieles Heinz Sorge Horst Stöcker Walter Greiner

In the framework of the relativistic quantum dynamics approach we investigate antiproton observables in Au-Au collisions at 10.7A GeV. The rapidity dependence of the in-plane directed transverse momentum p(y) of p's shows the opposite sigh of the nucleon flow, which has indeed recently been discovered at 10.7A GeV by the E877 group. The "antiflow" of p's is also predicted at 2A GeV and at 160 A GeV and appears at all energies also for pi's and K's. These predicted p anticorrelations are a direct proof of strong p annihilation in massive heavy ion reactions.

Nucleus-nucleus collisions at highest energies (1996)

Marcus Bleicher N. Amelin Steffen A. Bass Mathias Brandstetter Adrian Dumitru Christoph Ernst Lars Gerland Jens Konopka Christian Spieles Henning Weber L. A. Winckelmann Horst Stöcker Walter Greiner

The microscopic phasespace approach URQMD is used to investigate the stopping power and particle production in heavy systems at SPS and RHIC energies. We find no gap in the baryon rapidity distribution even at RHIC. For CERN energies URQMD shows a pile up of baryons and a supression of multi-nucleon clusters at midrapidity.

A Microscopic calculation of secondary Drell-Yan production in heavy ion collisions (1997)

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

A study of secondary Drell-Yan production in nuclear collisions is presented for SPS energies. In addition to the lepton pairs produced in the initial collisions of the projectile and target nucleons, we consider the potentially high dilepton yield from hard valence antiquarks in produced mesons and antibaryons. We calculate the secondary Drell-Yan contributions taking the collision spectrum of hadrons from the microscopic model URQMD. The con- tributions from meson-baryon interactions, small in hadron-nucleus interac- tions, are found to be substantial in nucleus-nucleus collisions at low dilepton masses. Preresonance collisions of partons may further increase the yields.

Direct photons in Pb+Pb at CERN-SPS from microscopic transport theory (1997)

Adrian Dumitru Marcus Bleicher Steffen A. Bass Christian Spieles Ludwig Neise Horst Stöcker Walter Greiner

Direct photon production in central Pb+Pb collisions at CERN-SPS energy is calculated within the relativistic microscopic transport model UrQMD, and within distinctly di erent versions of relativistic hydrodynamics. We find that in UrQMD the local momentum distributions of the secondaries are strongly elongated along the beam axis initially. Therefore, the preequilibrium contribution dominates the photon spectrum at transverse momenta above H 1.5 GeV. The hydrodynamics prediction of a strong correlation between the temperature and radial expansion velocities on the one hand and the slope of the transverse momentum distribution of direct photons on the other hand thus is not recovered in UrQMD. The rapidity distribution of direct photons in UrQMD reveals that the initial conditions for the longitudinal expansion of the photon source (the meson fluid ) resemble rather boostinvariance than Landau-like flow.

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.

Equilibrium and nonequilibrium effects in nucleus nucleus collisions (1999)

Larissa V. Bravina Mark I. Gorenstein Eugene E. Zabrodin Steffen A. Bass Mohamed Belkacem Marcus Bleicher Mathias Brandstetter Markus Hofmann Sven Soff Christian Spieles H. Weber Horst Stöcker Walter Greiner

Abstract: Local thermal and chemical equilibration is studied for central AqA collisions at 10.7 160 AGeV in the Ultrarelativis- . tic Quantum Molecular Dynamics model UrQMD . The UrQMD model exhibits strong deviations from local equilibrium at the high density hadron string phase formed during the early stage of the collision. Equilibration of the hadron resonance matter is established in the central cell of volume Vs125 fm3 at later stages, tG10 fmrc, of the resulting quasi-isentropic expansion. The thermodynamical functions in the cell and their time evolution are presented. Deviations of the UrQMD quasi-equilibrium state from the statistical mechanics equilibrium are found. They increase with energy per baryon and lead to a strong enhancement of the pion number density as compared to statistical mechanics estimates at SPS energies. PACS: 25.75.-q; 24.10.Lx; 24.10.Pa; 64.30.qt

Dissociation of expanding c anti-c states in heavy ion collisions (1999)

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 using 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 energy in Pb+Pb collisions at 160 GeV and found to be in reasonable agreement with existing data.

"Pressure equilibration" in ultrarelativistic heavy ion collisions (1997)

Jörg Brachmann Adrian Dumitru Christian Spieles Joachim A. Maruhn Horst Stöcker Walter Greiner

We study the time scale for pressure equilibration in heavy ion collisions at AGS energies within the three-fluid hydrodynamical model and a microscopic cascade model (UrQMD). We find that kinetic equilibrium is reached in both models after a time of 5 fm/c (center-of-mass time). Thus, observables which are sensitive to the early stage of the reaction differ considerably from the expectations within the instant thermalization scenario (one-fluid hydrodynamical model).

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