1,692 research outputs found
Strangeness counting in high energy collisions
The estimates of overall strange quark production in high energy e+e-, pp and
ppbar collisions by using the statistical-thermal model of hadronisation are
presented and compared with previous works. The parametrization of strangeness
suppression within the model is discussed. Interesting regularities emerge in
the strange/non-strange produced quark ratio which turns out to be fairly
constant in elementary collisions while it is twice as large in SPS heavy ion
collision.Comment: talk given at Strangeness in Quark Matter 98, submitted to J. Phys.
A Monte-Carlo generator for statistical hadronization in high energy e+e- collisions
We present a Monte-Carlo implementation of the Statistical Hadronization
Model in e+e- collisions. The physical scheme is based on the statistical
hadronization of massive clusters produced by the event generator Herwig within
the microcanonical ensemble. We present a preliminary comparison of several
observables with measurements in e+e- collisions at the Z peak. Although a fine
tuning of the model parameters is not carried out, a general good agreement
between its predictions and data is found.Comment: 19 pages, 28 figures, 6 tables. v2: added sections on comparison
between the Statistical Hadronization Model and the Cluster Model and on the
interplay between Herwig cluster splitting algorithm and Statistical
Hadronization Model predictions. Fixed typos and references added. Version
accepted for publication in EPJ
Hadronization conditions in relativistic nuclear collisions and the QCD pseudo-critical line
We compare the reconstructed hadronization conditions in relativistic nuclear
collisions in the nucleon-nucleon centre-of-mass energy range 4.7-2760 GeV in
terms of temperature and baryon-chemical potential with lattice QCD
calculations, by using hadronic multiplicities. We obtain hadronization
temperatures and baryon chemical potentials with a fit to measured
multiplicities by correcting for the effect of post-hadronization rescattering.
The post-hadronization modification factors are calculated by means of a
coupled hydrodynamical-transport model simulation under the same conditions of
approximate isothermal and isochemical decoupling as assumed in the statistical
hadronization model fits to the data. The fit quality is considerably better
than without rescattering corrections, as already found in previous work. The
curvature of the obtained "true" hadronization pseudo-critical line kappa is
found to be 0.0048 +- 0.0026, in agreement with lattice QCD estimates; the
pseudo-critical temperature at vanishing mu_B is found to be 164.3+-1.8 MeV.Comment: 9 pages, 2 figures. Minor corrections, version published in PL
Chemical equilibrium study in nucleus-nucleus collisions at relativistic energies
We present a detailed study of chemical freeze-out in nucleus-nucleus
collisions at beam energies of 11.6, 30, 40, 80 and 158A GeV. By analyzing
hadronic multiplicities within the statistical hadronization approach, we have
studied the strangeness production as a function of centre of mass energy and
of the parameters of the source. We have tested and compared different versions
of the statistical model, with special emphasis on possible explanations of the
observed strangeness hadronic phase space under-saturation. We show that, in
this energy range, the use of hadron yields at midrapidity instead of in full
phase space artificially enhances strangeness production and could lead to
incorrect conclusions as far as the occurrence of full chemical equilibrium is
concerned. In addition to the basic model with an extra strange quark
non-equilibrium parameter, we have tested three more schemes: a two-component
model superimposing hadrons coming out of single nucleon-nucleon interactions
to those emerging from large fireballs at equilibrium, a model with local
strangeness neutrality and a model with strange and light quark non-equilibrium
parameters. The behaviour of the source parameters as a function of colliding
system and collision energy is studied. The description of strangeness
production entails a non-monotonic energy dependence of strangeness saturation
parameter gamma_S with a maximum around 30A GeV. We also present predictions of
the production rates of still unmeasured hadrons including the newly discovered
Theta^+(1540) pentaquark baryon.Comment: 36 pages, 14 figures. Revised version published in Phys. Rev. C:
title changed, one paragraph added in section 2, other typos correcte
What is the meaning of the statistical hadronization model?
The statistical model of hadronization succeeds in reproducing particle
abundances and transverse momentum spectra in high energy collisions of
elementary particles as well as of heavy ions. Despite its apparent success,
the interpretation of these results is controversial and the validity of the
approach very often questioned. In this paper, we would like to summarize the
whole issue by first outlining a basic formulation of the model and then
comment on the main criticisms and different kinds of interpretations, with
special emphasis on the so-called "phase space dominance". While the ultimate
answer to the question why the statistical model works should certainly be
pursued, we stress that it is a priority to confirm or disprove the fundamental
scheme of the statistical model by performing some detailed tests on the rates
of exclusive channels at lower energy.Comment: 14 pages, to be published in the Proceedings of the International
workshop "Focus on multiplicity", Bari (Italy) June 17-19 200
Global hyperon polarization at local thermodynamic equilibrium with vorticity, magnetic field and feed-down
The system created in ultrarelativistic nuclear collisions is known to behave
as an almost ideal liquid. In non-central collisions, due to the large orbital
momentum, such a system might be the fluid with the highest vorticity ever
created under laboratory conditions. Particles emerging from such a highly
vorticous fluid are expected to be globally polarized with their spins on
average pointing along the system angular momentum. Vorticity-induced
polarization is the same for particles and antiparticles, but the intense
magnetic field generated in these collisions may lead to the splitting in
polarization. In this paper we outline the thermal approach to the calculation
of the global polarization phenomenon for particles with spin and we discuss
the details of the experimental study of this phenomenon, estimating the effect
of feed-down. A general formula is derived for the polarization transfer in
two-body decays and, particularly, for strong and electromagnetic decays. We
find that accounting for such effects is crucial when extracting vorticity and
magnetic field from the experimental data.Comment: 14 pages, 1 figure. Final version published in PRC with one more
formula and slightly revised tex
Energy and system size dependence of chemical freeze-out in relativistic nuclear collisions
We present a detailed study of chemical freeze-out in p-p, C-C, Si-Si and
Pb-Pb collisions at beam momenta of 158A GeV as well as Pb-Pb collisions at
beam momenta of 20A, 30A, 40A and 80A GeV. By analyzing hadronic multiplicities
within the statistical hadronization model, we have studied the parameters of
the source as a function of the number of the participating nucleons and the
beam energy. We observe a nice smooth behaviour of temperature, baryon chemical
potential and strangeness under-saturation parameter as a function of energy
and nucleus size. Interpolating formulas are provided which allow to predict
the chemical freeze-out parameters in central collisions at centre-of-mass
energies > 4.5 GeV and for any colliding ions. Specific discrepancies between
data and model emerge in particle ratios in Pb-Pb collisions at SPS between 20A
and 40A GeV of beam energy which cannot be accounted for in the considered
model schemes.Comment: 22 pages, 10 figures. References added and updated. Table correcte
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