1,836 research outputs found
Exact Baryon, Strangeness and Charge Conservation in Hadronic Gas Models
Relativistic heavy ion collisions are studied assuming that particles can be
described by a hadron gas in thermal and chemical equilibrium. The exact
conservation of baryon number, strangeness and charge are explicitly taken into
account. For heavy ions the effect arising from the neutron surplus becomes
important and leads to a substantial increase in e.g. the ratio.
A method is developed which is very well suited for the study of small systems.Comment: 5 pages, 5 Postscript figure
Fluctuations, strangeness and quasi-quarks in heavy-ion collisions from lattice QCD
We report measurements of diagonal susceptibilities for the baryon number,
chi_B, electrical charge, chi_Q, third component of isospin, chi_I,
strangeness, chi_S, and hypercharge, chi_Y, as well as the off-diagonal chi_BQ,
chi_BY, chi_BS, etc. We show that the ratios of susceptibilities in the high
temperature phase are robust variables, independent of lattice spacing, and
therefore give predictions for experiments. We also investigate strangeness
production and flavour symmetry breaking matrix elements at finite temperature.
Finally, we present evidence that in the high temperature phase of QCD the
different flavour quantum numbers are excited in linkages which are exactly the
same as one expects from quarks. We present some investigations of these
quark-like quasi particles
Comparison of Chemical Freeze-Out Criteria in Heavy-Ion Collisions
One of the most remarkable results to emerge from heavy-ion collisions over
the past two decades is the striking regularity shown by particle yields at all
energies. This has led to several very successful proposals describing particle
yields over a very wide range of beam energies, reaching from 1 A GeV up to 200
A GeV, using only one or two parameters. A systematic comparison of these
proposals is presented here. The conditions of fixed energy per particle,
baryon+anti-baryon density, normalized entropy density as well as percolation
model are investigated. The results are compared with the most recent chemical
freeze-out parameters obtained in the thermal-statistical analysis of particle
yields. The sensitivity and dependence of the results on parameters is analyzed
and discussed. It is shown that in the energy range above the top AGS energy,
within present accuracies, all chemical freeze-out criteria give a fairly good
description of the particle yields. However, the low energy heavy-ion data
favor the constant energy per particle as a unified condition of chemical
particle freeze-out. This condition also shows the weakest sensitivity on model
assumptions and parameters.Comment: 15 pages 7 figures uses revte
Neutral Pions and Eta Mesons as Probes of the Hadronic Fireball in Nucleus-Nucleus Collisions around 1A GeV
Chemical and thermal freeze-out of the hadronic fireball formed in symmetric
collisions of light, intermediate-mass, and heavy nuclei at beam energies
between 0.8A GeV and 2.0A GeV are discussed in terms of an equilibrated,
isospin-symmetric ideal hadron gas with grand-canonical baryon-number
conservation. For each collision system the baryochemical potential mu_B and
the chemical freeze-out temperature T_c are deduced from the inclusive neutral
pion and eta yields which are augmented by interpolated data on deuteron
production. With increasing beam energy mu_B drops from 800 MeV to 650 MeV,
while T_c rises from 55 MeV to 90 MeV. For given beam energy mu_B grows with
system size, whereas T_c remains constant. The centrality dependence of the
freeze-out parameters is weak as exemplified by the system Au+Au at 0.8A GeV.
For the highest beam energies the fraction of nucleons excited to resonance
states reaches freeze-out values of nearly 15 %, suggesting resonance densities
close to normal nuclear density at maximum compression. In contrast to the
particle yields, which convey the status at chemical freeze-out, the shapes of
the related transverse-mass spectra do reflect thermal freeze-out. The observed
thermal freeze-out temperatures T_th are equal to or slightly lower than T_c,
indicative of nearly simultaneous chemical and thermal freeze-out.Comment: 42 pages, 12 figure
Equation of state of resonance-rich matter in the central cell in heavy-ion collisions at =200 AGeV
The equilibration of hot and dense nuclear matter produced in the central
cell of central Au+Au collisions at RHIC ( AGeV) energies is
studied within a microscopic transport model. The pressure in the cell becomes
isotropic at fm/ after beginning of the collision. Within the
next 15 fm/ the expansion of matter in the cell proceeds almost
isentropically with the entropy per baryon ratio , and the
equation of state in the plane has a very simple form,
. Comparison with the statistical model of an ideal hadron gas
indicates that the time fm/c may be too short to reach the fully
equilibrated state. Particularly, the creation of long-lived resonance-rich
matter in the cell decelerates the relaxation to chemical equilibrium. This
resonance-abundant state can be detected experimentally after the thermal
freeze-out of particles.Comment: LATEX, 21 pages incl. 7 figure
Multiplicity Distributions in Canonical and Microcanonical Statistical Ensembles
The aim of this paper is to introduce a new technique for calculation of
observables, in particular multiplicity distributions, in various statistical
ensembles at finite volume. The method is based on Fourier analysis of the
grand canonical partition function. Taylor expansion of the generating function
is used to separate contributions to the partition function in their power in
volume. We employ Laplace's asymptotic expansion to show that any equilibrium
distribution of multiplicity, charge, energy, etc. tends to a multivariate
normal distribution in the thermodynamic limit. Gram-Charlier expansion allows
additionally for calculation of finite volume corrections. Analytical formulas
are presented for inclusion of resonance decay and finite acceptance effects
directly into the system partition function. This paper consolidates and
extends previously published results of current investigation into properties
of statistical ensembles.Comment: 53 pages, 7 figure
Influence of Impact Parameter on Thermal Description of Relativistic Heavy Ion Collisions at GSI/SIS
Attention is drawn to the role played by the size of the system in the
thermodynamic analysis of particle yields in relativistic heavy ion collisions
at SIS energies. This manifests itself in the non-linear dependence of K+ and
K- yields in collisions at 1 -- 2 A.GeV on the number of participants. It
is shown that this dependence can be quantitatively well described in terms of
a thermal model with a canonical strangeness conservation. The measured
particle multiplicity ratios (pi+/p, pi-/pi+, d/p, K+/pi+ and K+/K- but not
eta/pi0) in central Au-Au and Ni-Ni collisions at 0.8 -- 2.0 A.GeV are also
explained in the context of a thermal model with a common freeze-out
temperature and chemical potential. Including the concept of collective flow a
consistent picture of particle energy distributions is derived with the flow
velocity being strongly impact-parameter dependent.Comment: revtex, 20 figure
Comparison of Strangeness Production between A+A and p+p Reactions from 2 to 160 AGeV
The measured K ratios from heavy-ion reactions are compared with
the K ratios from p+p reactions over the energy range 2-160 AGeV. The
K/ enhancement in heavy-ion reactions is largest at the lower energies,
consistent with strangeness production in secondary scattering becoming
relatively more important than initial collisions near the kaon production
threshold. The enhancement decreases steadily from 4 to 160 AGeV, suggesting
that the same enhancement mechanism of hadronic rescattering and decay of
strings may be applicable over this full energy range. Based on existing data,
the mid-rapidity K ratio is predicted to be for the
forthcoming Pb+Pb reactions at 40 AGeV/c.Comment: 14 pages, 4 figures, submitted to Phys. Rev.
Examine the species and beam-energy dependence of particle spectra using Tsallis Statistics
Tsallis Statistics was used to investigate the non-Boltzmann distribution of
particle spectra and their dependence on particle species and beam energy in
the relativistic heavy-ion collisions at SPS and RHIC. Produced particles are
assumed to acquire radial flow and be of non-extensive statistics at
freeze-out. J/psi and the particles containing strangeness were examined
separately to study their radial flow and freeze-out. We found that the strange
hadrons approach equilibrium quickly from peripheral to central A+A collisions
and they tend to decouple earlier from the system than the light hadrons but
with the same final radial flow. These results provide an alternative picture
of freeze-outs: a thermalized system is produced at partonic phase; the
hadronic scattering at later stage is not enough to maintain the system in
equilibrium and does not increase the radial flow of the copiously produced
light hadrons. The J/psi in Pb+Pb collisions at SPS is consistent with early
decoupling and obtains little radial flow. The J/psi spectra at RHIC are also
inconsistent with the bulk flow profile.Comment: 12 pages, 4 figures, added several references and some clarifications
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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.Comment: 12 pages, 4 figure
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