Hydrodynamic simulation of elliptic flow

We use a hydrodynamic model to study the space-time evolution transverse to the beam direction in ultrarelativistic heavy-ion collisions with nonzero impact parameters. We focus on the influence of early pressure on the development of radial and elliptic flow. We show that at high energies elliptic flow is generated only during the initial stages of the expansion while radial flow continues to grow until freeze-out. Quantitative comparisons with SPS data from semiperipheral Pb+Pb collisions suggest the applicability of hydrodynamical concepts already $\approx$ 1 fm/c after impact.Comment: 4 pages, 5 figures, proceedings for Quark Matter 9

Rapidity Dependence of Strange Particle Ratios in Nuclear Collisions

It was recently found that in sulphur-induced nuclear collisions at 200 A GeV the observed strange hadron abundances can be explained within a thermodynamic model where baryons and mesons separately are in a state of relative chemical equilibrium, with overall strangeness being slightly undersaturated, but distributed among the strange hadron channels according to relative chemical equilibrium with a vanishing strange quark chemical potential. We develop a consistent thermodynamic formulation of the concept of relative chemical equilibrium and show how to introduce into the partition function deviations from absolute chemical equilibrium, e.~g.~an undersaturation of overall strangeness or the breaking of chemical equilibrium between mesons and baryons. We then proceed to test on the available data the hypothesis that the strange quark chemical potential vanishes everywhere, and that the rapidity distributions of all the observed hadrons can be explained in terms of one common, rapidity-dependent function $\mu_{\rm q}(\eta)$ for the baryon chemical potential only. The aim of this study is to shed light on the observed strong rapidity dependence of the strange baryon ratios in the NA36 experiment.Comment: uses REVTeX, 14 pages, 17 ps-figures (uuencoded) added with figures comman

Strange Messages: Chemical and Thermal Freeze-out in Nuclear Collisions

Thermal models are commonly used to interpret heavy-ion data on particle yields and spectra and to extract the conditions of chemical and thermal freeze-out in heavy-ion collisions. I discuss the usefulness and limitations of such thermal model analyses and review the experimental and theoretical evidence for thermalization in nuclear collisions. The crucial role of correlating strangeness production data with single particle spectra and two-particle correlation measurements is pointed out. A consistent dynamical picture for the heavy-ion data from the CERN SPS involves an initial prehadronic stage with deconfined color and with an appreciable isotropic pressure component. This requires an early onset of thermalization.Comment: 15 pages, 2 figures, talk given at Strange Quark Matter '98, Padova, Italy, 20-24 July 1998, to be published in J. Phys. G 25; final version with updated reference

Entropy production by resonance decays

We investigate entropy production for an expanding system of particles and resonances with isospin symmetry -- in our case pions and $\rho$ mesons -- within the framework of relativistic kinetic theory. A cascade code to simulate the kinetic equations is developed and results for entropy production and particle spectra are presented.Comment: 17 pages, 10 ps-figures included, only change: preprint number adde

Aspects of thermal and chemical equilibration of hadronic matter

We study thermal and chemical equilibration in 'infinite' hadron matter as well as in finite size relativistic nucleus-nucleus collisions using a BUU cascade transport model that contains resonance and string degrees-of-freedom. The 'infinite' hadron matter is simulated within a cubic box with periodic boundary conditions. The various equilibration times depend on baryon density and energy density and are much shorter for particles consisting of light quarks then for particles including strangeness. For kaons and antikaons the chemical equilibration time is found to be larger than $\simeq$ 40 fm/c for all baryon and energy densities considered. The inclusion of continuum excitations, i.e. hadron 'strings', leads to a limiting temperature of $T_s\simeq$ 150 MeV. We, furthermore, study the expansion of a hadronic fireball after equilibration. The slope parameters of the particles after expansion increase with their mass; the pions leave the fireball much faster then nucleons and accelerate subsequently heavier hadrons by rescattering ('pion wind'). If the system before expansion is close to the limiting temperature $T_s$, the slope parameters for all particles after expansion practically do not depend on (initial) energy and baryon density. Finally, the equilibration in relativistic nucleus-nucleus collision is considered. Since the reaction time here is much shorter than the equilibration time for strangeness, a chemical equilibrium of strange particles in heavy-ion collisions is not supported by our transport calculations. However, the various particle spectra can approximately be described within the blast model.Comment: 39 pages, LaTeX, including 18 postscript figures, Nucl. Phys. A, in pres

Bose-condensation through resonance decay

We show that a system described by an equation of state which contains a high number of degrees of freedom (resonances) can create a considerable amount of superfluid (condensed) pions through the decay of short-lived resonances, if baryon number and entropy are large and the dense matter decouples from chemical equilibrium earlier than from thermal equilibrium. The system cools down faster in the presence of a condensate, an effect that may partially compensate the enhancement of the lifetime expected in the case of quark-gluon-plasma formation.Comment: 12 pages GSI-93-27 PREPRIN

Chemical equilibration of strangeness

Thermal models are very useful in the understanding of particle production in general and especially in the case of strangeness. We summarize the assumptions which go into a thermal model calculation and which differ in the application of various groups. We compare the different results to each other. Using our own calculation we discuss the validity of the thermal model and the amount of strangeness equilibration at CERN-SPS energies. Finally the implications of the thermal analysis on the reaction dynamics are discussed.Comment: 23 pages, LaTeX (figures included); Talk given at the Int. Symposium on Strangeness in Quark Matter 1997, Santorini (Greece), April 199

Flow effects on the freeze-out phase-space density in heavy ion collisions

The strong longitudinal expansion of the reaction zone formed in relativistic heavy-ion collisions is found to significantly reduce the spatially averaged pion phase-space density, compared to naive estimates based on thermal distributions. This has important implications for data interpretation and leads to larger values for the extracted pion chemical potential at kinetic freeze-out.Comment: 5 pages, 3 figures included via epsfig, added discussion of different transverse density profiles, 1 new figur

Thermal phenomenology of hadrons from 200 AGeV S+S collisions

We develop a complete and consistent description for the hadron spectra from heavy ion collisions in terms of a few collective variables, in particular temperature, longitudinal and transverse flow. To achieve a meaningful comparison with presently available data, we also include the resonance decays into our picture. To disentangle the influences of transverse flow and resonance decays in the $m_T$-spectra, we analyse in detail the shape of the $m_T$-spectra.Comment: 31 pages, 13 figs in seperate uuencoded file, for LaTeX, epsf.sty and dvips, TPR-93-16 and BNL-(no number yet

Thermal analysis of hadron multiplicities from relativistic quantum molecular dynamics

Some questions arising in the application of the thermal model to hadron production in heavy ion collisions are studied. We do so by applying the thermal model of hadron production to particle yields calculated by the microscopic transport model RQMD(v2.3). We study the bias of incomplete information about the final hadronic state on the extraction of thermal parameters.It is found that the subset of particles measured typically in the experiments looks more thermal than the complete set of stable particles. The hadrons which show the largest deviations from thermal behaviour in RQMD(v2.3) are the multistrange baryons and antibaryons. We also looked at the influence of rapidity cuts on the extraction of thermal parameters and found that they lead to different thermal parameters and larger disagreement between the RQMD yields and the thermal model.Comment: 12 pages, 2 figures, uses REVTEX, only misprint and stylistic corrections, to appear in Physical Review