185 research outputs found
On local and global equilibrium in heavy ion collisions
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
Dependence of lepton pair emission on EoS and initial state
We present results from a hydrodynamic calculation for thermal emission of
lepton pairs in central lead-lead collisions at the CERN SPS energy. Dependence
of the emission on the initial conditions and Equation of State (EoS) is
considered and the spectra are compared with CERES data and calculated
distribution of Drell--Yan pairs.Comment: 4 pages, includes 4 ps-figures, talk at Quark Matter'97, Tsukuba,
Japa
On chemical equilibrium in nuclear collisions
The data on average hadron multiplicities in central A+A collisions measured at CERN SPS are analysed with the ideal hadron gas model. It is shown that the full chemical equilibrium version of the model fails to describe the experimental results. The agreement of the data with the off-equilibrium version allowing for partial strangeness saturation is significantly better. The freeze-out temperature of about 180 MeV seems to be independent of the system size (from S+S to Pb+Pb) and in agreement with that extracted in e+e-, pp and p{\bar p} collisions. The strangeness suppression is discussed at both hadron and valence quark level. It is found that the hadronic strangeness saturation factor gamma_S increases from about 0.45 for pp interactions to about 0.7 for central A+A collisions with no significant change from S+S to Pb+Pb collisions. The quark strangeness suppression factor lambda_S is found to be about 0.2 for elementary collisions and about 0.4 for heavy ion collisions independently of collision energy and type of colliding syste
Mass number scaling in ultra-relativistic nuclear collisions from a hydrodynamical approach
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
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
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 1 fm/c after impact.Comment: 4 pages, 5 figures, proceedings for Quark Matter 9
Unified Description of Freeze-Out Parameters in Relativistic Heavy Ion Collisions
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
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 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
We investigate entropy production for an expanding system of particles and
resonances with isospin symmetry -- in our case pions and 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
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