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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.

Azimuthal correlations of pions in relativistic heavy ion collisions at 1 GeV/nucl. (1995)

Steffen A. Bass Christoph Hartnack Horst Stöcker Walter Greiner

Triple differential cross sections of pions in heavy ion collisions at 1 GeV/nucl. are studied with the IQMD model. After discussing general properties of resonance and pion production we focus on azimuthal correlations: At projectile- and target-rapidities we observe an anticorrelation in the in-plane transverse momentum between pions and protons. At c.m.-rapidity, however, we find that high pt pions are being preferentially emitted perpendicular to the event-plane. We investigate the causes of those correlations and their sensitivity on the density and momentum dependence of the real and imaginary part of the nucleon and pion optical potential.

Neural networks for impact parameter determination (1996)

Steffen A. Bass Arnd Bischoff Joachim Maruhn Horst Stöcker Walter Greiner

Abstract: An accurate impact parameter determination in a heavy ion collision is crucial for almost all further analysis. The capabilities of an artificial neural network are investigated to that respect. A novel input generation for the network is proposed, namely the transverse and longitudinal momentum distribution of all outgoing (or actually detectable) particles. The neural network approach yields an improvement in performance of a factor of two as compared to classical techniques. To achieve this improvement simple network architectures and a 5 × 5 input grid in (pt, pz) space are suffcient.

Nucleus-nucleus collisions at highest energies (1996)

Marcus Bleicher Nikolai Amelin Steffen A. Bass Mathias Brandstetter Adrian Dumitru Christoph Ernst Lars Gerland Jens Konopka Christian Spieles Henning Weber Luke Andreas 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.

Modelling the many-body dynamics of heavy ion collisions (1997)

Christoph Hartnack Rajeev Kumar Puri Jörg Aichelin Jens Konopka Steffen A. Bass Horst Stöcker Walter Greiner

Basic problems of the semiclassical microscopic modelling of strongly interacting systems are discussed within the framework of Quantum Molecular Dynamics (QMD). This model allows to study the influence of several types of nucleonic interactions on a large variety of observables and phenomena occur- ring in heavy ion collisions at relativistic energies. It is shown that the same predictions can be obtained with several numerically completely di erent and independently written programs as far as the same model parameters are employed and the same basic approximations are made. Many observ- ables are robust against variations of the details of the model assumptions used. Some of the physical results, however, depend also on rather technical parameters like the preparation of the initial configuration in phase space. This crucial problem is connected with the description of the ground state of single nuclei, which di ers among the various approaches. An outlook to an improved molecular dynamics scheme for heavy ion collisions is given.

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.

Intermediate mass excess of dilepton production in heavy ion collisions at BEVALAC energies (1998)

Christoph Ernst Steffen A. Bass Mohamed Belkacem Horst Stöcker Walter Greiner

Dielectron mass spectra are examined for various nuclear reactions recently measured by the DLS collaboration. A detailed description is given of all dilepton channels included in the transport model UrQMD 1.0, i.e. Dalitz decays of π, η, ω, ή mesons and of the (1232) resonance, direct decays of vector mesons and pn bremsstrahlung. The microscopic calculations reproduce data for light systems fairly well, but tend to underestimate the data in pp at high energies and in pd at low energies. These conventional sources, however, cannot explain the recently reported enhancement for nucleus-nucleus collisions in the mass region 0.15GeV ≤ Me+e- ≤ 0.6GeV. Chiral scaling and ω meson broadening in the medium are investigated as a source of this mass excess. They also cannot explain the recent DLS data.

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.

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