arXiv subjects

L. Gerland

Publications and source records attributed to L. Gerland.

33 records · Page 2Linked to original sources

arXiv1998

J/Ψproduction, $χ$ polarization and Color Fluctuations

The hard contributions to the heavy quarkonium-nucleon cross sections are calculated based on the QCD factorization theorem and the nonrelativistic quarkonium model. We evaluate the nonperturbative part of these cross sections which dominates at $\sqrt{s_{NN}}\approx 20$ GeV at the Cern Super Proton Synchrotron (SPS) and becomes a correction at $\sqrt{s_{NN}}\approx 6$ TeV at the CERN Large Hadron Collider (LHC). $\J$ production at the CERN SPS is well described by hard QCD, when the larger absorption cross sections of the $χ$ states predicted by QCD are taken into account. We predict an $A$-dependent polarization of the $χ$ states. The expansion of small wave packets is discussed.

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arXiv1998

Microscopic Models for Ultrarelativistic Heavy Ion Collisions

In this paper, the concepts of microscopic transport theory are introduced and the features and shortcomings of the most commonly used ansatzes are discussed. In particular, the Ultrarelativistic Quantum Molecular Dynamics (UrQMD) transport model is described in great detail. Based on the same principles as QMD and RQMD, it incorporates a vastly extended collision term with full baryon-antibaryon symmetry, 55 baryon and 32 meson species. Isospin is explicitly treated for all hadrons. The range of applicability stretches from $E_{lab}< 100$ MeV/nucleon up to $E_{lab}> 200$ GeV/nucleon, allowing for a consistent calculation of excitation functions from the intermediate energy domain up to ultrarelativistic energies. The main physics topics under discussion are stopping, particle production and collective flow.

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arXiv1998

Can Momentum Correlations Proof Kinetic Equilibration in Heavy Ion Collisions at 160 AGeV?

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.

hep-ph↗

arXiv1998

The Origin of Transverse Flow at the SPS

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.

hep-ph↗

arXiv1997

Are we close to the QGP? - Hadrochemical vs. microscopic analysis of particle production in ultrarelativistic heavy ion collisions

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.

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arXiv1997

A microscopic calculation of secondary Drell-Yan production in heavy ion collisions

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 contributions from meson-baryon interactions, small in hadron-nucleus interactions, are found to be substantial in nucleus-nucleus collisions at low dilepton masses. Preresonance collisions of partons may further increase the yields.

hep-ph↗

arXiv1997

Microscopic Analysis of Thermodynamic Parameters from 160 MeV/n - 160 GeV/n

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.

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arXiv1996

Signatures of dense hadronic matter in ultrarelativistic heavy ion reactions

The behavior of hadronic matter at high baryon densities is studied within Ultrarelativistic Quantum Molecular Dynamics (URQMD). Baryonic stopping is observed for Au+Au collisions from SIS up to SPS energies. The excitation function of flow shows strong sensitivities to the underlying equation of state (EOS), allowing for systematic studies of the EOS. Dilepton spectra are calculated with and without shifting the $ρ$ pole. Except for S+Au collisions our calculations reproduce the CERES data.

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arXiv1996

Microscopic calculations of stopping and flow from 160AMeV to 160AGeV

The behavior of hadronic matter at high baryon densities is studied within Ultrarelativistic Quantum Molecular Dynamics (URQMD). Baryonic stopping is observed for Au+Au collisions from SIS up to SPS energies. The excitation function of flow shows strong sensitivities to the underlying equation of state (EOS), allowing for systematic studies of the EOS. Effects of a density dependent pole of the $ρ$-meson propagator on dilepton spectra are studied for different systems and centralities at CERN energies.

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arXiv1996

Collective flow in heavy ion reactions and the properties of excited nuclear matter

Quantum Molecular Dynamics (QMD) 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 intermediate reaction stages show that the event shapes are more complex and that equilibrium is reached only in very special cases but not in event samples which cover a wide range of impact parameters as it is the case in experiments. The basic features of a new molecular dynamics model (UQMD) for heavy ion collisions from the Fermi energy regime up to the highest presently available energies are outlined.

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arXiv1996

Extracting the equation of state from a microscopic non-equilibrium model

We study the thermodynamic properties of infinite nuclear matter with the Ultrarelativistic Quantum Molecular Dynamics (URQMD), a semiclassical transport model, running in a box with periodic boundary conditions. It appears that the energy density rises faster than $T^4$ at high temperatures of $T\approx 200-300$~MeV. This indicates an increase in the number of degrees of freedom. Moreover, We have calculated direct photon production in Pb+Pb collisions at 160~GeV/u within this model. The direct photon slope from the microscopic calculation equals that from a hydrodynamical calculation without a phase transition in the equation of state of the photon source.

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arXiv1996

Dynamics of Strangeness Production and Strange Matter Formation

The dynamics of a hadronizing quark matter drop is investigated with a hybrid-like model, which takes into account equilibrium as well as non-equilibrium features of the process. We study the the particle rates from the hadronizing plasma, which have to be put in relation to the whole time evolution of the system. The impact of finite-size effects on the distillation process is considered. $H^0$-Dibaryon yields are estimated. Microscopic calculations of hypercluster formation at AGS and SPS are presented.

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arXiv1996

Baryon Stopping and Strangeness Production in Ultra-Relativistic Heavy Ion Collisions

The stopping behaviour of baryons in massive heavy ion collisions (at SPS, RHIC and LHC) is investigated within different microscopic models. At SPS-energies the predictions range from full stopping to virtually total transparency. Experimental data are indicating strong stopping. The initial baryo-chemical potentials and temperatures at collider energies and their impact on the formation probability of strange baryon clusters and strangelets are discussed.

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arXiv1996

Nucleus-Nucleus Collisions at Highest Energies

The microscopic phasespace approach URQMD is used to investigate the stopping power and particle production in heavy systems at SPS and RHIC energies. This framework bridges with one model consistently the entire available range of energies from below SIS to CERN, even for the heaviest system Pb+Pb. URQMD is a hadronic transport model based on the propagation of hadrons on classical trajectories in the framework of Lorentz-invariant dynamics. It includes secondary interactions e.g. annihilation of produced mesons leading to the formation of resonances and the excitation of strings. The collision term contains 50 different baryon species (including nucleon, delta and hyperon resonances with masses up to 2 GeV) and 25 different meson species (including strange meson resonances), which are supplemented by their corresponding antiparticle and all isospin-projected states. Within this approach we find no gap in the baryon rapidity distribution even at RHIC. For CERN (SPS) energies URQMD shows a pile up of baryons and a supression of multi-nucleon clusters (e.g. He) at midrapidity.

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arXiv1996

Hypermatter - Properties and Formation in Relativistic Nuclear Collisions

The extension of the Periodic System into hitherto unexplored domains - antimatter 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 studied 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 absolutely stable hypermatter is unlikely to be produced in heavy ion collisions. New attention should be focused on short lived metastable hyperclusters ($τ\propto 10^{-10}$s) and on intensity interferometry of multi-strange-baryon correlations.

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