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Fri, 14 Jul 2006 10:15:51 +0200Fri, 14 Jul 2006 10:15:51 +0200Structure of the vacuum in nuclear matter: a nonperturbative approach
http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2383
We compute the vacuum polarization correction to the binding energy of nuclear matter in the Walecka model using a nonperturbative approach. We first study such a contribution as arising from a ground-state structure with baryon-antibaryon condensates. This yields the same results as obtained through the relativistic Hartree approximation of summing tadpole diagrams for the baryon propagator. Such a vacuum is then generalized to include quantum effects from meson fields through scalar-meson condensates which amounts to summing over a class of multiloop diagrams. The method is applied to study properties of nuclear matter and leads to a softer equation of state giving a lower value of the incompressibility than would be reached without quantum effects. The density-dependent effective sigma mass is also calculated including such vacuum polarization effects.Amruta Mishra; P. K. Panda; Stefan Schramm; Joachim Reinhardt; Walter Greinerarticlehttp://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2383Fri, 14 Jul 2006 10:15:51 +0200Transition to delta matter from hot, dense nuclear matter within a relativistic mean field formulation of the nonlinear sigma and omega model
http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2382
An investigation of the transition to delta matter is performed based on a relativistic mean field formulation of the nonlinear sigma and omega model. We demonstrate that in addition to the Delta-meson coupling, the occurrence of the baryon resonance isomer also depends on the nucleon-meson coupling. Our results show that for the favored phenomenological value of m* and K, the Delta isomer exists at baryon density ~ 2–3 p0 if beta=1.31 is adopted. For universal coupling of the nucleon and Delta, the Delta density at baryon density ~ 2–3 p0 and temperature ~ 0.4–0.5 fm-1 is about normal nuclear matter density, which is in accord with a recent experimental finding.Zhuxia Li; Guangjun Mao; Yizhong Zhuo; Walter Greinerarticlehttp://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2382Fri, 14 Jul 2006 10:15:10 +0200Hot nuclear matter in the quark meson coupling model
http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2381
We study here hot nuclear matter in the quark meson coupling model which incorporates explicitly quark degrees of freedom, with quarks coupled to scalar and vector mesons. The equation of state of nuclear matter including the composite nature of the nucleons is calculated at finite temperatures. The calculations are done taking into account the medium-dependent bag constant. Nucleon properties at finite temperatures as calculated here are found to be appreciably different from the value at T=0.P. K. Panda; Amruta Mishra; Judah M. Eisenberg; Walter Greinerarticlehttp://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2381Fri, 14 Jul 2006 10:14:40 +0200"Pressure equilibration" in ultrarelativistic heavy ion collisions
http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/4576
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).Jörg Brachmann; Adrian Dumitru; Christian Spieles; Joachim A. Maruhn; Horst Stöcker; Walter Greinerpreprinthttp://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/4576Thu, 19 Jan 2006 13:53:26 +0100Modelling the many-body dynamics of heavy ion collisions
http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/3010
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.Christoph Hartnack; Rajeev Kumar Puri; Jörg Aichelin; Jens Konopka; Steffen A. Bass; Horst Stöcker; Walter Greinerpreprinthttp://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/3010Tue, 17 Jan 2006 10:10:35 +0100"Soft'' transverse expansion and flow in a multi-fluid model without phase transition
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 Greinerpreprinthttp://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/3009Tue, 17 Jan 2006 10:01:46 +0100Nonequilibrium fluid-dynamics in the early stage of ultrarelativistic heavy-ion collisions
http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/3008
To describe ultrarelativistic heavy-ion collisions we construct a three-fluid hydrodynamical model. In contrast to one-fluid hydrodynamics, it accounts for the finite stopping power of nuclear matter, i.e. for nonequilibrium e ects in the early stage of the reaction. Within this model, we study baryon dynamics in the BNL-AGS energy range. For the system Au+Au we find that kinetic equilibrium between projectile and target nucleons is established only after a time teq CM H 5 fm/c C 2RAu/³CM. Observables which are sensitive to the early stage of the collision (like e.g. nucleon flow) therefore di er considerably from those calculated in the one-fluid model.Jörg Brachmann; Adrian Dumitru; Joachim A. Maruhn; Horst Stöcker; Walter Greiner; Dirk-Hermann Rischkepreprinthttp://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/3008Tue, 17 Jan 2006 09:57:57 +0100Probes for the early reaction dynamics of heavy ion collisions at AGS and SPS
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 Greinerpreprinthttp://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/3022Mon, 16 Jan 2006 10:45:00 +0100A Microscopic calculation of secondary Drell-Yan production in heavy ion collisions
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 Vogtpreprinthttp://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/3040Thu, 05 Jan 2006 09:42:26 +0100Chiral Lagrangian for strange hadronic matter
http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/3039
A generalized Lagrangian for the description of hadronic matter based on the linear SU(3)L × SU(3)R -model is proposed. Besides the baryon octet, the spin-0 and spin-1 nonets, a gluon condensate associated with broken scale invariance is incorporated. The observed values for the vacuum masses of the baryons and mesons are reproduced. In mean-field approximation, vector and scalar interactions yield a saturating nuclear equation of state. We discuss the di culties and possibilities to construct a chiral invariant baryon-meson interaction that leads to a realistic equation of state. It is found that a coupling of the strange condensate to nucleons is needed to describe the hyperon potentials correctly. The effective baryon masses and the appearance of an abnormal phase of nearly massless nucleons at high densities are examined. A nonlinear realization of chiral symmetry is considered, to retain a Yukawa-type baryon-meson interaction and to establish a connection to the Walecka-model.Panajotis Papazoglou; Stefan Schramm; Jürgen Schaffner-Bielich; Horst Stöcker; Walter Greinerpreprinthttp://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/3039Thu, 05 Jan 2006 09:34:39 +0100Hypermatter in chiral field theory
http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/3038
Abstract. A generalized Lagrangian for the description of hadronic matter based on the linear SU(3)L × SU(3)R -model is proposed. Besides the baryon octet, the spin-0 and spin-1 nonets, a gluon condensate associated with broken scale invariance is incorporated. The observed values for the vacuum masses of the baryons and mesons are reproduced. In mean-field approximation, vector and scalar interactions yield a saturating nuclear equation of state. Finite nuclei can be reasonably described, too. The condensates and the e ective baryon masses at finite baryon density and temperature are discussed.Panajotis Papazoglou; Detlef Zschiesche; Stefan Schramm; Horst Stöcker; Walter Greinerpreprinthttp://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/3038Thu, 05 Jan 2006 09:33:08 +0100Relativistic transport theory of N, Delta and N* (1440) interacting through sigma, omega and pi mesons.
http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/3048
A self-consistent relativistic integral-di erential equation of the Boltzmann- Uehling-Uhlenbeck-type for the N*(1440) resonance is developed based on an effective Lagrangian of baryons interacting through mesons. The closed time-path Green s function technique and semi-classical, quasi-particle and Born approxima- tions are employed in the derivation. The non-equilibrium RBUU-type equation for the N*(1440) is consistent with that of nucleon s and delta s which we derived before. Thus, we obtain a set of coupled equations for the N,Delta and N*(1440) distribution functions. All the N (1440)-relevant in-medium two-body scattering cross sections within the N,Delta and N*(1440) system are derived from the same effective Lagrangian in addition to the mean field and presented analytically, which can be directly used in the study of relativistic heavy-ion collisions. The theoreticalprediction of the free pp - pp* (1440) cross section is in good agreement with the experimental data. We calculate the in-medium N+N - N+N* , N* +N - N+N and N*+N - N* +N cross sections in cold nuclear matter up to twice the nuclear matter density. The influence of different choices of the N* N* coupling strengths, which can not be obtained through fitting certain experimental data, are discussed. The results show that the density dependence of predicted in-medium cross sections are sensitive to the N* N* coupling strengths used. An evident density dependence will appear when a large scalar coupling strength of g^(sigma) N*N* is assumed. PACS number(s): 24.10.Cn; 25.70.-z; 21.65.+fGuangjun Mao; Ludwig Neise; Horst Stöcker; Walter Greiner; Zhuxia Lipreprinthttp://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/3048Wed, 21 Dec 2005 11:00:59 +0100Chemical freezeout in relativistic A+A collisions: is it close to the QGP?
http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/3066
Preliminary experimental data for particle number ratios in the collisions of Au+Au at the BNL AGS (11A GeV/c) and Pb+Pb at the CERN SPS (160A GeV/c) are analyzed in a thermodynamically consistent hadron gas model with excluded volume. Large values of temperature, T = 140 185 MeV, and baryonic chemical potential, µb = 590 270 MeV, close to the boundary of the quark-gluon plasma phase are found from fitting the data. This seems to indicate that the energy density at the chemical freezeout is tremendous which would be indeed the case for the point-like hadrons. However, a self-consistent treatment of the van der Waals excluded volume reveals much smaller energy densities which are very far below a lowest limit estimate of the quark-gluon plasma energy density. PACS number(s): 25.75.-q, 24.10.PaMark I. Gorenstein; Horst Stöcker; Granddon D. Yen; Shin Nan Yang; Walter Greinerpreprinthttp://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/3066Mon, 19 Dec 2005 13:15:10 +0100Dilepton production by bremsstrahlung of meson fields in nuclear collisions
http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/3065
We study the bremsstrahlung of virtual omega mesons due to the collective deceleration of nuclei at the initial stage of an ultrarelativistic heavy ion collision. It is shown that electromagnetic decays of these mesons may give an important contribution to the observed yields of dileptons. Mass spectra of e+e and µ+µ pairs produced in central Au+Au collisions are calculated under some simplifying assumptions on the space time variation of the baryonic current in a nuclear collision process. Comparison with the CERES data for 160 AGev Pb+Au collisions shows that the proposed mechanism gives a noticeable fraction of the observed e+e pairs in the intermediate region of invariant masses. Sensi tivity of the dilepton yield to the in medium modification of masses and widths of vector mesons is demonstrated.Igor N. Mishustin; Leonid M. Satarov; Horst Stöcker; Walter Greinerpreprinthttp://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/3065Mon, 19 Dec 2005 13:10:58 +0100Collective mechanism of dilepton production in high-energy nuclear collisions.
http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/3073
Collective bremsstrahlung of vector meson fields in relativistic nuclear collisions is studied within the time dependent Walecka model. Mutual deceleration of the colliding nuclei is described by introducing the e ective stopping time and average rapidity loss of baryons. It is shown that electromagnetic decays of virtual ω mesons produced by bremsstrahlung mechanism can provide a substantial contribution to the soft dilepton yield at the SPS bombarding energies. In particular, it may be responsible for the dilepton enhancement observed in 160 AGev central Pb+Au collisions. Suggestions for future experiments to estimate the relative contribution of the collective mechanism are given.Igor N. Mishustin; Leonid M. Satarov; Horst Stöcker; Walter Greinerpreprinthttp://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/3073Fri, 16 Dec 2005 11:10:43 +0100Direct photons in Pb+Pb at CERN-SPS from microscopic transport theory
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 Greinerpreprinthttp://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/3075Thu, 15 Dec 2005 11:52:03 +0100Microscopic analysis of thermodynamic parameters from 160 MeV/n - 160 GeV/n
http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/4808
Microscopic calculations of central collisions between heavy nuclei are used to study fragment production and the creation of collective flow. It is shown that the final phase space distributions are compatible with the expectations from a thermally equilibrated source, which in addition exhibits a collective transverse expansion. However, the microscopic analyses of the transient states in the reaction stages of highest density and during the expansion show that the system does not reach global equilibrium. Even if a considerable amount of equilibration is assumed, the connection of the measurable final state to the macroscopic parameters, e.g. the temperature, of the transient ''equilibrium'' state remains ambiguous.Marcus Bleicher; Steffen A. Bass; Mohamed Belkacem; Jörg Brachmann; Mathias Brandstetter; Christoph Ernst; Lars Gerland; Jens Konopka; Sven Soff; Christian Spieles; Henning Weber; Horst Stöcker; Walter Greinerpreprinthttp://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/4808Thu, 14 Apr 2005 09:25:30 +0200Are we close to the QGP? - Hadrochemical vs. microscopic analysis of particle production in ultrarelativistic heavy ion collisions
http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/4811
Ratios of hadronic abundances are analyzed for pp and nucleus-nucleus collisions at sqrt(s)=20 GeV using the microscopic transport model UrQMD. Secondary interactions significantly change the primordial hadronic cocktail of the system. A comparison to data shows a strong dependence on rapidity. Without assuming thermal and chemical equilibrium, predicted hadron yields and ratios agree with many of the data, the few observed discrepancies are discussed.Steffen A. Bass; Mohamed Belkacem; Mathias Brandstetter; Marcus Bleicher; Lars Gerland; Jens Konopka; Ludwig Neise; Christian Spieles; Sven Soff; Henning Weber; Horst Stöcker; Walter Greinerpreprinthttp://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/4811Wed, 13 Apr 2005 17:38:36 +0200