Latest documents http://publikationen.stub.uni-frankfurt.de/index/index/ Tue, 28 Mar 2006 15:17:34 +0200 Tue, 28 Mar 2006 15:17:34 +0200 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2866 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. Marcus Bleicher; Christian Spieles; Christoph Ernst; Lars Gerland; Sven Soff; Horst Stöcker; Walter Greiner; Steffen A. Bass preprint http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2866 Tue, 28 Mar 2006 15:17:34 +0200 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2971 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 ¯p momentum spectra. Pacs-numbers: 14.20 Dh, 25.70.-z Christian Spieles; Marcus Bleicher; André Jahns; Raffaele Mattiello; Heinz Sorge; Horst Stöcker; Walter Greiner preprint http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2971 Wed, 08 Feb 2006 11:26:43 +0100 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2970 In the framework of the relativistic quantum molecular dynamics approach (RQMD) we investigate antideuteron (d) observables in Au+Au collisions at 10.7 AGeV. The impact parameter dependence of the formation ratios d/p2 and d/p2 is calculated. In central collisions, the antideuteron formation ratio is predicted to be two orders of magnitude lower than the deuteron formation ratio. The d yield in central Au+Au collisions is one order of magnitude lower than in Si+Al collisions. In semicentral collisions di erent configuration space distributions of p s and d s lead to a large squeeze out e ect for antideuterons, which is not predicted for the p s. Marcus Bleicher; Christian Spieles; André Jahns; Raffaele Mattiello; Heinz Sorge; Horst Stöcker; Walter Greiner preprint http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2970 Wed, 08 Feb 2006 11:18:56 +0100 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2994 The extension of the Periodic System into hitherto unexplored domains - anti- matter and hypermatter - is discussed. Starting from an analysis of hyperon and single hypernuclear properties we investigate the structure of multi-hyperon objects (MEMOs) using an extended relativistic meson field theory. These are contrasted with multi-strange quark states (strangelets). Their production mechanism is stud- ied for relativistic collisions of heavy ions from present day experiments at AGS and SPS to future opportunities at RHIC and LHC. It is pointed out that abso- lutely stable hypermatter is unlikely to be produced in heavy ion collisions. New attention should be focused on short lived metastable hyperclusters ( / 10 10s) and on intensity interferometry of multi-strange-baryon correlations. Lars Gerland; Christian Spieles; Marcus Bleicher; Panajotis Papazoglou; Jörg Brachmann; Adrian Dumitru; Horst Stöcker; Walter Greiner; Jürgen Schaffner; Carsten Greiner preprint http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2994 Mon, 23 Jan 2006 11:13:19 +0100 http://publikationen.stub.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.stub.uni-frankfurt.de/frontdoor/index/index/docId/2991 Mon, 23 Jan 2006 10:08:06 +0100 http://publikationen.stub.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.stub.uni-frankfurt.de/frontdoor/index/index/docId/3009 Tue, 17 Jan 2006 10:01:46 +0100 http://publikationen.stub.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.stub.uni-frankfurt.de/frontdoor/index/index/docId/3022 Mon, 16 Jan 2006 10:45:00 +0100 http://publikationen.stub.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.stub.uni-frankfurt.de/frontdoor/index/index/docId/3040 Thu, 05 Jan 2006 09:42:26 +0100 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/3070 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. 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 article http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/3070 Thu, 15 Dec 2005 13:51:45 +0100 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/3069 Recent reports on enhancements of intermediate and hight mass muon pairs producedin heavy ion collisions have attracted much attention. Christian Spieles; Lars Gerland; Nils Hammon; Marcus Bleicher; Steffen A. Bass; Horst Stöcker; Walter Greiner; C. Lourenco; Ramona Vogt article http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/3069 Thu, 15 Dec 2005 13:48:35 +0100 http://publikationen.stub.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.stub.uni-frankfurt.de/frontdoor/index/index/docId/3075 Thu, 15 Dec 2005 11:52:03 +0100 http://publikationen.stub.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.stub.uni-frankfurt.de/frontdoor/index/index/docId/3080 Wed, 14 Dec 2005 13:32:36 +0100 http://publikationen.stub.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.stub.uni-frankfurt.de/frontdoor/index/index/docId/3092 Fri, 09 Dec 2005 10:24:09 +0100 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/3114 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. Christian Spieles; Ramona Vogt; Lars Gerland; Steffen A. Bass; Marcus Bleicher; Horst Stöcker; Walter Greiner preprint http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/3114 Thu, 08 Dec 2005 11:05:37 +0100 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/3117 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. Christian Spieles; Ramona Vogt; Lars Gerland; Steffen A. Bass; Marcus Bleicher; Leonid Frankfurt; Mark Strikman; Horst Stöcker; Walter Greiner preprint http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/3117 Thu, 08 Dec 2005 10:46:32 +0100 http://publikationen.stub.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.stub.uni-frankfurt.de/frontdoor/index/index/docId/3108 Wed, 07 Dec 2005 13:22:18 +0100 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/3110 Noneequilibrium models (three-fluid hydrodynamics and UrQMD) use to discuss the uniqueness of often proposed experimental signatures for quark matter formation in relativistic heavy ion collisions. It is demonstrated that these two models - although they do treat the most interesting early phase of the collisions quite differently(thermalizing QGP vs. coherent color fields with virtual particles) - both yields a reasonable agreement with a large variety of the available heavy ion data. Stefan Scherer; Steffen A. Bass; Marcus Bleicher; Mohamed Belkacem; Larissa V. Bravina; Jörg Brachmann; Adrian Dumitru; Christoph Ernst; Lars Gerland; Markus Hofmann; Ludwig Neise; Manuel Reiter; Sven Soff; Christian Spieles; Henning Weber; Eugene E. Zabrodin; Detlef Zschiesche; Joachim A. Maruhn; Horst Stöcker; Walter Greiner article http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/3110 Wed, 07 Dec 2005 10:33:38 +0100 http://publikationen.stub.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.stub.uni-frankfurt.de/frontdoor/index/index/docId/3123 Wed, 07 Dec 2005 09:42:32 +0100 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/3119 We discuss a model for the space-time evolution of ultrarelativistic heavy-ion collisions which employs relativistic hydrodynamics within one region of the forward light-cone, and microscopic transport theory (i.e. UrQMD) in the complement. Our initial condition consists of a quark-gluon plasma which expands hydrodynamically and hadronizes. After hadronization the solution eventually changes from expansion in local equilibrium to free streaming, as determined selfconsistently by the interaction rates between the hadrons and the local expansion rate. We show that in such a scenario the inverse slopes of the mT -spectra of multiple strange baryons ( Xi,Omega) are practically una ected by the purely hadronic stage of the reaction, while the flow of p's and Lambda's increases. Moreover, we find that the rather soft transverse expansion at RHIC energies (due to a first-order phase transition) is not washed out by strong rescattering in the hadronic stage. The earlier kinetic freeze-out as compared to SPS-energies results in similar inverse slopes (of the mT -spectra of the hadrons in the final state) at RHIC and SPS energies. Adrian Dumitru; Steffen A. Bass; Marcus Bleicher; Horst Stöcker; Walter Greiner preprint http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/3119 Tue, 06 Dec 2005 11:29:47 +0100 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/3129 We introduce a transport approach which combines partonic and hadronic degrees of freedom on an equal footing and discuss the resulting reaction dynamics. The initial parton dynamics is modeled in the framework of the parton cascade model, hadronization is performed via a cluster hadronization model and configuration space coalescence, and the hadronic phase is described by a microscopic hadronic transport approach. The resulting reaction dynamics indicates a strong influence of hadronic rescattering on the space-time pattern of hadronic freeze-out and on the shape of transverse mass spectra. Freeze-out times and transverse radii increase by factors of 2 3 depending on the hadron species. Steffen A. Bass; Markus Hofmann; Marcus Bleicher; Larissa V. Bravina; Eugene E. Zabrodin; Horst Stöcker; Walter Greiner preprint http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/3129 Mon, 05 Dec 2005 13:45:48 +0100 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/3127 We analyze the hadronic freeze-out in ultra-relativistic heavy ion collisions at RHIC in a transport approach which combines hydrodynamics for the early, dense, deconfined stage of the reaction with a microscopic non-equilibrium model for the later hadronic stage at which the hydrodynamic equilibrium assumptions are not valid. With this ansatz we are able to self-consistently calculate the freeze-out of the system and determine space-time hypersurfaces for individual hadron species. The space-time domains of the freeze-out for several hadron species are found to be actually four-dimensional, and di er drastically for the individual hadrons species. Freeze-out radii distributions are similar in width for most hadron species, even though the is found to be emitted rather close to the phase boundary and shows the smallest freeze- out radii and times among all baryon species. The total lifetime of the system does not change by more than 10% when going from SPS to RHIC energies. Steffen A. Bass; Adrian Dumitru; Marcus Bleicher; Larissa V. Bravina; Eugene E. Zabrodin; Horst Stöcker; Walter Greiner preprint http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/3127 Mon, 05 Dec 2005 13:41:48 +0100 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/3126 Charmonium production and absorption in heavy ion collisions is studied with the Ultrarelativisitic Quantum Molecular Dynamics model. We compare the scenario of universal and time independent color-octet dissociation cross sections with one of distinct color-singlet J/psi, psi 2 and CHIc states, evolving from small, color transparent configurations to their asymptotic sizes. The measured J/psi production cross sections in pA and AB collisions at SPS energies are consistent with both purely hadronic scenarios. The predicted rapidity dependence of J/psi suppression can be used to discriminate between the two experimentally. The importance of interactions with secondary hadrons and the applicability of thermal reaction kinetics to J/psi absorption are in- vestigated. We discuss the e ect of nuclear stopping and the role of leading hadrons. The dependence of the 2/J/psi ratio on the model assumptions and the possible influence of refeeding processes is also studied. Christian Spieles; Ramona Vogt; Lars Gerland; Steffen A. Bass; Marcus Bleicher; Horst Stöcker; Walter Greiner preprint http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/3126 Mon, 05 Dec 2005 10:49:23 +0100 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/3133 We investigate event-by-event fluctuations for ensembles with non-fixed multiplicity. Moments of event observable distributions, like total energy distribution, total transverse momentum distribution, etc, are shown to be related to the multi-body correlations present in the system. For classical systems, these moments reduce in the absence of any correlations to the mo- ments of particle inclusive momentum distribution. As a consequence, a zero value for the recently introduced Phi-variable is shown to indicate the van- ishing of two-body correlations from one part, and of correlations between multiplicity and momentum distributions from the other part. It is often misunderstood as a measure of the degree of equilibration in the system. Mohamed Belkacem; Zoheir Aouissat; Marcus Bleicher; Horst Stöcker; Walter Greiner preprint http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/3133 Thu, 01 Dec 2005 10:28:43 +0100 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/3132 We investigate the hadronic cooling of a quark droplet within a microscopic model. The color flux tube approach is used to describe the hadronization of the quark phase. The model reproduces experimental particle ratios equally well compared to a static thermal hadronic source. Furthermore, the dynamics of the decomposition of a quark-gluon plasma is investigated and time dependent particle ratios are found. Markus Hofmann; Judah M. Eisenberg; Stefan Scherer; Marcus Bleicher; Ludwig Neise; Horst Stöcker; Walter Greiner preprint http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/3132 Thu, 01 Dec 2005 10:24:26 +0100 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/3143 Using a microscopic transport model together with a coalescence after-burner, we study the formation of deuterons in Au + Au central collisions at s = 200 AGeV . It is found that the deuteron transverse momentum distributions are strongly a ected by the nucleon space-momentum correlations, at the moment of freeze-out, which are mostly determined by the number of rescatterings. This feature is useful for studying collision dynamics at ultrarelativistic energies. B. Monreal; Steffen A. Bass; Marcus Bleicher; S. Esumi; Walter Greiner; Qingfeng Li; H. Liu; W.J. Llope; Raffaele Mattiello; Sergey Y. Panitkin; I. Sakrejda; Raimond Snellings; Heinz Sorge; Christian Spieles; Horst Stöcker; J. Thomas; S. Voloshin; F. Wang; Nu Xu preprint http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/3143 Thu, 01 Dec 2005 10:14:29 +0100