159 research outputs found

    On local and global equilibrium in heavy ion collisions

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    The thermal model is commonly used in two different ways for the description of hadron production in ultra-relativistic heavy ion collision. One is the application of the thermal model to 4pi integrated data and the other is the thermal description of central dN/dy ratios. While the first method implicitly assumes global equilibrium the other scenario assumes Bjorken scaling within the investigated rapidity range. Both assumptions are only approximations for real physical collision systems. We study the impact of both approximations for the extraction of thermal parameters on the exemplary case of S+S collisions at SPS energies. The particle distributions are modeled by a hydrodynamical description of the relevant collision system.Comment: 13 pages, 2 figures included, uses REVTE

    Mass number scaling in ultra-relativistic nuclear collisions from a hydrodynamical approach

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    We study the different nucleus-nucleus collisions, O+Au, S+S, S+Ag, S+Au and Pb+Pb, at the CERN-SPS energy in a one-fluid hydrodynamical approach using a parametrization based on baryon stopping in terms of the thickness of colliding nuclei. Good agreement with measured particle spectra is achieved. We deduce the mass number scaling behaviour of the initial energy density. We find that the equilibration time is nearly independent of the size of the colliding nuclei.Comment: 27 pages, figures included, submitted to European Physical Journa

    Unified Description of Freeze-Out Parameters in Relativistic Heavy Ion Collisions

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    It is shown that the chemical freeze-out parameters obtained at CERN/SPS, BNL/AGS and GSI/SIS energies all correspond to a unique value of 1 GeV per hadron in the local rest frame of the system, independent of the beam energy and of the target and beam particles.Comment: revtex, 1 figur

    Rapidity Dependence of Strange Particle Ratios in Nuclear Collisions

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    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 őľq(ő∑)\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

    Entropy production by resonance decays

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    We investigate entropy production for an expanding system of particles and resonances with isospin symmetry -- in our case pions and ŌĀ\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

    Chemical equilibration of strangeness

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    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
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