193 research outputs found
Relativistic Heavy--Ion Collisions in the Dynamical String--Parton Model
We develop and extend the dynamical string parton model. This model, which is
based on the salient features of QCD, uses classical Nambu-Got\=o strings with
the endpoints identified as partons, an invariant string breaking model of the
hadronization process, and interactions described as quark-quark interactions.
In this work, the original model is extended to include a phenomenological
quantization of the mass of the strings, an analytical technique for treating
the incident nucleons as a distribution of string configurations determined by
the experimentally measured structure function, the inclusion of the gluonic
content of the nucleon through the introduction of purely gluonic strings, and
the use of a hard parton-parton interaction taken from perturbative QCD
combined with a phenomenological soft interaction. The limited number of
parameters in the model are adjusted to and -- data. Utilizing
these parameters, the first calculations of the model for -- and
-- collisions are presented and found to be in reasonable agreement with
a broad set of data.Comment: 26 pages of text with 23 Postscript figures placed in tex
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
Energy dependence of kaon production in central Pb+Pb collisions
Recent results from the NA49 experiment on the energy dependence of charged
kaon production in central Pb+Pb collisions are presented. First results from
the new data at 80 AGeV beam energy are compared with those from lower and
higher energies. A difference in the energy dependence of the /
and / ratios is observed. The / ratio shows a
non-monotonic behaviour with a maximum near 40 AGeV.Comment: 8 pages, 7 figures, proceedings of talk at SQM2001, Frankfurt,
Germany, to appear in J. Phys.
Modelling J/psi production and absorption in a microscopic nonequilibrium approach
Charmonium production and absorption in heavy ion collisions is studied with
the Ultrarelativisitic Quantum Molecular Dynamics model. We compare the
scenario of universal and time independent color-octet dissociation cross
sections with one of distinct color-singlet J/psi, psi' and chi_c states,
evolving from small, color transparent configurations to their asymptotic
sizes. The measured J/psi production cross sections in pA and AB collisions at
SPS energies are consistent with both - purely hadronic - scenarios. The
predicted rapidity dependence of J/psi suppression can be used to discriminate
between the two experimentally. The importance of interactions with secondary
hadrons and the applicability of thermal reaction kinetics to J/psi absorption
are investigated. We discuss the effect of nuclear stopping and the role of
leading hadrons. The dependence of the psi' to J/psi ratio on the model
assumptions and the possible influence of refeeding processes is also studied.Comment: 35 pages, 16 figure
Baryon stopping and strange baryon/antibaryon production at SPS energies
The amount of proton stopping in central Pb+Pb collisions from 20-160 AGeV as
well as hyperon and antihyperon rapidity distributions are calculated within
the UrQMD model in comparison to experimental data at 40, 80 and 160 AGeV taken
recently from the NA49 collaboration. Furthermore, the amount of baryon
stopping at 160 AGeV for Pb+Pb collisions is studied as a function of
centrality in comparison to the NA49 data. We find that the strange baryon
yield is reasonably described for central collisions, however, the rapidity
distributions are somewhat more narrow than the data. Moreover, the
experimental antihyperon rapidity distributions at 40, 80 and 160 AGeV are
underestimated by up to factors of 3 - depending on the annihilation cross
section employed - which might be addressed to missing multi-meson fusion
channels in the UrQMD model.Comment: 18 pages, including 7 eps figures, to be published in Phys. Rev.
Violation of energy-per-hadron scaling in a resonance matter
Yields of hadrons, their average masses and energies per hadron at the stage
of chemical freeze-out in (ultra)relativistic heavy-ion collisions are analyzed
within the statistical model. The violation of the scaling / = 1 GeV
observed in Au+Au collisions at = 130 AGeV is linked to the
formation of resonance-rich matter with a considerable fraction of baryons and
antibaryons. The rise of the energy-per-hadron ratio in baryon-dominated matter
is discussed. A violation of the scaling condition is predicted for a very
central zone of heavy-ion collisions at energies around 40 AGeV.Comment: 5 pages incl. 3 figures and 2 tables, to be published in Phys. Rev.
Strangeness Conservation in Hot Nuclear Fireballs
A constraint between thermal fireball parameters arises from the requirement
that the balance of strangeness in a fireball is (nearly) zero. We study the
impact of this constraint on (multi-)strange (anti-)baryon multiplicities and
compare the hadron gas and quark-gluon plasma predictions. We explore the
relation between the entropy content and particle multiplicities and show that
the data are compatible with the quark-gluon plasma hypothesis, but appear to
be inconsistent with the picture of an equilibrated hadron gas fireball. We
consider the implications of the results on the dynamics of evolution and decay
of the particle source.Comment: 35 pages, 11 postscript figures, report PAR/LPTHE/92--2
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
Criticality, Fractality and Intermittency in Strong Interactions
Assuming a second-order phase transition for the hadronization process, we
attempt to associate intermittency patterns in high-energy hadronic collisions
to fractal structures in configuration space and corresponding intermittency
indices to the isothermal critical exponent at the transition temperature. In
this approach, the most general multidimensional intermittency pattern,
associated to a second-order phase transition of the strongly interacting
system, is determined, and its relevance to present and future experiments is
discussed.Comment: 15 pages + 2 figures (available on request), CERN-TH.6990/93,
UA/NPPS-5-9
Thermal hadron production in high energy collisions
It is shown that hadron abundances in high energy e+e-, pp and p{\bar p}
collisions, calculated by assuming that particles originate in hadron gas
fireballs at thermal and partial chemical equilibrium, are in very good
agreement with the data. The freeze-out temperature of the hadron gas fireballs
turns out to be nearly constant over a large center of mass energy range and
not dependent on the initial colliding system. The only deviation from chemical
equilibrium resides in the incomplete strangeness phase space saturation.
Preliminary results of an analysis of hadron abundances in S+S and S+Ag heavy
ion collisions are presented.Comment: 10 pages, 1 .eps figure, talk given at the Strangeness and Quark
Matter 97 conferenc
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