Latest documents http://publikationen.ub.uni-frankfurt.de/index/index/ Mon, 23 Jan 2006 10:08:06 +0100 Mon, 23 Jan 2006 10:08:06 +0100 http://publikationen.ub.uni-frankfurt.de/frontdoor/index/index/docId/2991 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. 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 preprint http://publikationen.ub.uni-frankfurt.de/frontdoor/index/index/docId/2991 Mon, 23 Jan 2006 10:08:06 +0100 http://publikationen.ub.uni-frankfurt.de/frontdoor/index/index/docId/3009 Abstract: We study transverse expansion and directed flow in Au(11AGeV)Au reactions within a multi-fluid dynamical model. Although we do not employ an equation of state (EoS) with a first order phase transition, we find a slow increase of the transverse velocities of the nucleons with time. A similar behaviour can be observed for the directed nucleon flow. This is due to non-equilibrium e ects which also lead to less and slower conversion of longitudinal into transverse momentum. We also show that the proton rapidity distribution at CERN energies, as calculated within this model, agrees well with the preliminary NA44-data. Jörg Brachmann; Adrian Dumitru; Marcus Bleicher; Joachim A. Maruhn; Horst Stöcker; Walter Greiner preprint http://publikationen.ub.uni-frankfurt.de/frontdoor/index/index/docId/3009 Tue, 17 Jan 2006 10:01:46 +0100 http://publikationen.ub.uni-frankfurt.de/frontdoor/index/index/docId/3022 We discuss the early evolution of ultrarelativistic heavy-ion collisions within a multi- fluid dynamical model. In particular, we show that due to the finite mean-free path of the particles compression shock waves are smeared out considerably as compared to the one-fluid limit. Also, the maximal energy density of the baryons is much lower. We discuss the time scale of kinetic equilibration of the baryons in the central region and its relevance for directed flow. Finally, thermal emission of direct photons from the fluid of produced particles is calculated within the three-fluid model and two other simple expansion models. It is shown that the transverse momentum and rapidity spectra of photons give clue to the cooling law and the early rapidity distribution of the photon source. Adrian Dumitru; Jörg Brachmann; Marcus Bleicher; Joachim A. Maruhn; Horst Stöcker; Walter Greiner preprint http://publikationen.ub.uni-frankfurt.de/frontdoor/index/index/docId/3022 Mon, 16 Jan 2006 10:45:00 +0100 http://publikationen.ub.uni-frankfurt.de/frontdoor/index/index/docId/3040 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. Christian Spieles; Lars Gerland; Nils Hammon; Marcus Bleicher; Steffen A. Bass; Horst Stöcker; Walter Greiner; Carlos Lourenco; Ramona Vogt preprint http://publikationen.ub.uni-frankfurt.de/frontdoor/index/index/docId/3040 Thu, 05 Jan 2006 09:42:26 +0100 http://publikationen.ub.uni-frankfurt.de/frontdoor/index/index/docId/3075 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. Adrian Dumitru; Marcus Bleicher; Steffen A. Bass; Christian Spieles; Ludwig Neise; Horst Stöcker; Walter Greiner preprint http://publikationen.ub.uni-frankfurt.de/frontdoor/index/index/docId/3075 Thu, 15 Dec 2005 11:52:03 +0100 http://publikationen.ub.uni-frankfurt.de/frontdoor/index/index/docId/3080 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. Marcus Bleicher; Mohamed Belkacem; Christoph Ernst; Henning Weber; Lars Gerland; Christian Spieles; Steffen A. Bass; Horst Stöcker; Walter Greiner preprint http://publikationen.ub.uni-frankfurt.de/frontdoor/index/index/docId/3080 Wed, 14 Dec 2005 13:32:36 +0100 http://publikationen.ub.uni-frankfurt.de/frontdoor/index/index/docId/3092 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. H. Weber; Christoph Ernst; Marcus Bleicher; Larissa V. Bravina; Horst Stöcker; Walter Greiner; Christian Spieles; Steffen A. Bass preprint http://publikationen.ub.uni-frankfurt.de/frontdoor/index/index/docId/3092 Fri, 09 Dec 2005 10:24:09 +0100 http://publikationen.ub.uni-frankfurt.de/frontdoor/index/index/docId/3108 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 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 article http://publikationen.ub.uni-frankfurt.de/frontdoor/index/index/docId/3108 Wed, 07 Dec 2005 13:22:18 +0100 http://publikationen.ub.uni-frankfurt.de/frontdoor/index/index/docId/3123 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. Christian Spieles; Ramona Vogt; Lars Gerland; Steffen A. Bass; Marcus Bleicher; Leonid Frankfurt; Mark Strikman; Horst Stöcker; Walter Greiner article http://publikationen.ub.uni-frankfurt.de/frontdoor/index/index/docId/3123 Wed, 07 Dec 2005 09:42:32 +0100