Showing papers by "Steffen A. Bass published in 1999"

Relativistic Hadron-Hadron Collisions in the Ultra-Relativistic Quantum Molecular Dynamics Model (UrQMD)

Abstract: Hadron-hadron collisions at high energies are investigated in the Ultra-relativistic-Quantum-Molecular-Dynamics approach (UrQMD). This microscopic transport model is designed to study pp, pA and A+A collisions. It describes the phenomenology of hadronic interactions at low and intermediate energies ($\sqrt s 5$ GeV, the excitation of color strings and their subsequent fragmentation into hadrons dominates the multiple production of particles in the UrQMD model. The model shows a fair overall agreement with a large body of experimental h-h data over a wide range of h-h center-of-mass energies. Hadronic reaction data with higher precision would be useful to support the use of the UrQMD model for relativistic heavy ion collisions.

Relativistic hadron-hadron collisions in the ultra-relativistic quantum molecular dynamics model

Abstract: Hadron-hadron (h-h) collisions at high energies are investigated in the ultra-relativistic quantum molecular dynamics (UrQMD) approach. This microscopic transport model describes the phenomenology of hadronic interactions at low and intermediate energies ( 5 GeV, the excitation of colour strings and their subsequent fragmentation into hadrons dominates the multiple production of particles in the UrQMD model. The model shows a fair overall agreement with a large body of experimental h-h data over a wide range of h-h centre-of-mass energies. Hadronic reaction data with higher precision would be useful to support the use of the UrQMD model for relativistic heavy-ion collisions.

Signatures of quark-gluon plasma formation in high energy heavy-ion collisions: a critical review

Abstract: A critical review on signatures of quark-gluon plasma (QGP) formation is given and the current (1998) experimental status is discussed. After giving an introduction to the properties of QCD matter in both, equilibrium and non-equilibrium theories, we focus on observables which may yield experimental evidence for QGP formation. For each individual observable the discussion is divided into three sections: first the connection between the respective observable and QGP formation in terms of the underlying theoretical concepts is given, then the relevant experimental results are reviewed and finally the current status concerning the interpretation of both, theory and experiment, is discussed. A comprehensive summary including an outlook towards RHIC is given in the final section.

Local equilibrium in heavy ion collisions: Microscopic model versus statistical model analysis

Abstract: The assumption of local equilibrium in relativistic heavy ion collisions at energies from 10.7 AGeV (AGS) up to 160 AGeV (SPS) is checked in the microscopic transport model. Dynamical calculations performed for a central cell in the reaction are compared to the predictions of the thermal statistical model. We find that kinetic, thermal and chemical equilibration of the expanding hadronic matter are nearly approached late in central collisions at AGS energy for $t \geq 10$ fm/$c$ in a central cell. At these times the equation of state may be approximated by a simple dependence $P \cong (0.12-0.15) \epsilon$. Increasing deviations of the yields and the energy spectra of hadrons from statistical model values are observed for increasing energy, 40 AGeV and 160 AGeV. These violations of local equilibrium indicate that a fully equilibrated state is not reached, not even in the central cell of heavy ion collisions at energies above 10 AGeV. The origin of these findings is traced to the multiparticle decays of strings and many-body decays of resonances.

Strangeness Enhancement in Heavy Ion Collisions - Evidence for Quark-Gluon-Matter ?

Abstract: The centrality dependence of (multi-)strange hadron abundances is studied for Pb(158 AGeV)Pb reactions and compared to p(158 GeV)Pb collisions. The microscopic transport model UrQMD is used for this analysis. The predicted Lambda/pi-, Xi-/pi- and Omega-/pi- ratios are enhanced due to rescattering in central Pb-Pb collisions as compared to peripheral Pb-Pb or p-Pb collisions. A reduction of the constituent quark masses to the current quark masses m_s \sim 230 MeV, m_q \sim 10 MeV, as motivated by chiral symmetry restoration, enhances the hyperon yields to the experimentally observed high values. Similar results are obtained by an ad hoc overall increase of the color electric field strength (effective string tension of kappa=3 GeV/fm). The enhancement depends strongly on the kinematical cuts. The maximum enhancement is predicted around midrapidity. For Lambda's, strangeness suppression is predicted at projectile/target rapidity. For Omega's, the predicted enhancement can be as large as one order of magnitude. Comparisons of Pb-Pb data to proton induced asymmetric (p-A) collisions are hampered due to the predicted strong asymmetry in the various rapidity distributions of the different (strange) particle species. In p-Pb collisions, strangeness is locally (in rapidity) not conserved. The present comparison to the data of the WA97 and NA49 collaborations clearly supports the suggestion that conventional (free) hadronic scenarios are unable to describe the observed high (anti-)hyperon yields in central collisions. The doubling of the strangeness to nonstrange suppression factor, gamma_s \approx 0.65, might be interpreted as a signal of a phase of nearly massless particles.

Hadronic freeze-out following a first order hadronization phase transition in ultrarelativistic heavy-ion collisions

Abstract: We analyze the hadronic freeze-out in ultrarelativistic heavy-ion collisions at Relativistic Heavy Ion Collider (RHIC) in a transport approach that combines hydrodynamics for the early, dense, deconfined stage of the reaction with a microscopic nonequilibrium 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 differ drastically for the individual hadrons species. Freeze-out radii distributions are similar in width for most hadron species, even though the ${\ensuremath{\Omega}}^{\ensuremath{-}}$ 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 CERN Super Proton Synchrotron to RHIC energies.

Last Call for RHIC Predictions

Abstract: This paper contains the individual contributions of all speakers of the session on 'Last Call for RHIC Predictions' at Quark Matter 99, and a summary by the convenor.

Modeling j / psi production and absorption in a microscopic nonequilibrium approach

Abstract: 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 $J/\ensuremath{\psi},$ ${\ensuremath{\psi}}^{\ensuremath{'}}$, and ${\ensuremath{\chi}}_{c}$ states, evolving from small, color transparent configurations to their asymptotic sizes. Both purely hadronic scenarios are compared with the $\mathrm{pA}$ and $\mathrm{AB}$ data at SPS energies. The predicted rapidity dependence of $J/\ensuremath{\psi}$ suppression can be used to discriminate between the two scenarios experimentally. The importance of interactions with secondary hadrons and the applicability of thermal reaction kinetics to $J/\ensuremath{\psi}$ absorption are investigated. We discuss the effect of nuclear stopping and the role of leading hadrons. The dependence of the ${\ensuremath{\psi}}^{\ensuremath{'}}/J/\ensuremath{\psi}$ ratio on the model assumptions and the possible influence of refeeding processes are also studied.

Local Thermal and Chemical Equilibration and the Equation of State in Relativistic Heavy Ion Collisions

Abstract: Thermodynamical variables and their time evolution are studied for central relativistic heavy ion collisions from 10.7 to 160 A GeV in the microscopic ultrarelativistic quantum molecular dynamics model (UrQMD). The UrQMD model exhibits drastic deviations from equilibrium during the early high density phase of the collision. Local thermal and chemical equilibration of the hadronic matter seems to be established only at later stages of the quasi-isentropic expansion in the central reaction cell with volume 125 . Baryon energy spectra in this cell are reproduced by Boltzmann distributions at all collision energies for fm/c with a unique rapidly dropping temperature. At these times the equation of state has a simple form: . At SPS energies the strong deviation from chemical equilibrium is found for mesons, especially pions, even at the late stage of the reaction. The final enhancement of pions is supported by experimental data.

Deuterons and space momentum correlations in high-energy nuclear collisions

Abstract: Using a microscopic transport model together with a coalescence afterburner, we study the formation of deuterons in Au+Au central collisions at [radical] (s) =200A GeV. It is found that the deuteron transverse momentum distributions are strongly affected 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. [copyright] [ital 1999] [ital The American Physical Society]

Lattice gauge description of colliding nuclei

Abstract: We propose a novel formalism for simultaneously describing both the hard and soft parton dynamics in ultrarelativistic collisions of nuclei. The emission of gluons from the initially coherent parton configurations of the colliding nuclei and low-pt colour coherence effects are treated in the framework of a Yang-Mills transport equation on a coupled lattice-particle system. A collision term is added to the transport equation to account for the remaining intermediate and high-pt interactions in an infrared finite manner.

Dissociation of J/psi by mesons: thermal versus nonequilibrium scenario

Abstract: We study J/ reaction kinetics 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 framework of the microscopic transport model, the ultrarelativistic quantum molecular dynamics model, taking into account the production of charmonium states through hard parton-parton interactions and subsequent rescattering with secondary hadrons assuming constant dissociation cross sections. The thermal gas and microscopic transport scenarios are shown to be very dissimilar. Estimates of J/ survival probabilities based on thermal models of co-mover interactions in heavy-ion collisions are therefore questionable.

Analysis of reaction dynamics at ultrarelativistic energies in a combined parton-hadron transport approach

Abstract: 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 indicate 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.

Equilibrium and non-equilibrium effects in relativistic heavy ion collisions

Abstract: The hypothesis of local equilibrium (LE) in relativistic heavy ion collisions at energies from AGS to RHIC is checked in the microscopic transport model. We find that kinetic, thermal, and chemical equilibration of the expanding hadronic matter is nearly reached in central collisions at AGS energy for $t \geq 10$ fm/$c$ in a central cell. At these times the equation of state may be approximated by a simple dependence $P \cong (0.12-0.15) \epsilon$. Increasing deviations of the yields and the energy spectra of hadrons from statistical model values are observed for increasing bombarding energies. The origin of these deviations is traced to the irreversible multiparticle decays of strings and many-body $(N \geq 3)$ decays of resonances. The violations of LE indicate that the matter in the cell reaches a steady state instead of idealized equilibrium. The entropy density in the cell is only about 6% smaller than that of the equilibrium state.

Directed and elliptic flow

Abstract: We compare microscopic transport model calculations to recent data on the directed and elliptic flow of various hadrons in 2 - 10 A GeV Au+Au and Pb (158 A GeV) Pb collisions. For the Au+Au excitation function a transition from the squeeze-out to an in-plane enhanced emission is consistently described with mean field potentials corresponding to one incompressibility. For the Pb (158 A GeV) Pb system the elliptic flow prefers in-plane emission both for protons and pions, the directed flow of protons is opposite to that of the pions, which exhibit anti-flow. Strong directed transverse flow is present for protons and Lambdas in Au (6 A GeV) Au collisions as well. Both for the SPS and the AGS energies the agreement between data and calculations is remarkable.

Distinguishing hadronic cascades from hydrodynamic models in Pb(160A GeV)+Pb reactions by impact parameter variation

Abstract: We propose to study the impact parameter dependence of the {bar {Lambda}}/{bar p} ratio in Pb(160A GeV)+Pb reactions. The {bar {Lambda}}/{bar p} ratio is a sensible tool to distinguish between hadronic cascade models and hydrodynamical models, which assume chemical equilibrium and incorporate a possible phase transition to a quark-gluon plasma. {copyright} {ital 1999} {ital The American Physical Society}

RHIC predictions from the UrQMD model

Abstract: The Ultra-relativistic Quantum-Molecular-Dynamics model (UrQMD) is used to study central Pb+Pb reactions at RHIC ($\sqrt{s}=200$ AGeV). The computed collision spectrum reveals that the major part (approx. 80%) of the dynamics is governed by interactions below $\sqrt{s}=100$ GeV. Rapidity spectra of net-baryons (Protons, Hyperons) and produced particles (Pions, Kaons) are predicted. The transverse expansion is studied.

Physics opportunities at RHIC and LHC

Abstract: Nonequilibrium models (three-fluid hydrodynamics, UrQMD, and quark molecular dynamics) are used to discuss the uniqueness of often proposed experimental signatures for quark matter formation in relativistic heavy ion collisions from the SPS via RHIC to LHC. It is demonstrated that these models—although they do treat the most interesting early phase of the collisions quite differently (thermalizing QGP vs. coherent color fields with virtual particles)—all yield a reasonable agreement with a large variety of the available heavy ion data. Hadron/hyperon yields, including J/Ψ meson production/suppression, strange matter formation, dileptons, and directed flow (bounce-off and squeeze-out) are investigated. Observations of interesting phenomena in dense matter are reported. However, we emphasize the need for systematic future measurements to search for simultaneous irregularities in the excitation functions of several observables in order to come close to pinning the properties of hot, dense QCD matter from dat...