340 research outputs found
J/Psi suppression in colliding nuclei: statistical model analysis
We consider the suppression at a high energy heavy ion collision. An
ideal gas of massive hadrons in thermal and chemical equilibrium is formed in
the central region. The finite-size gas expands longitudinally in accordance
with Bjorken law. The transverse expansion in a form of the rarefaction wave is
taken into account. We show that suppression in such an environment,
when combined with the disintegration in nuclear matter, gives correct
evaluation of NA38 and NA50 data in a broad range of initial energy densities.Comment: 14 pages, 13 figures. Accepted for publication in Phys. Rev.
Leakage Effect on J/psi Pt Distributions in Different Centrality Bins for Pb-Pb Collisions at E/A=160 GeV
A transport approach including a leakage effect for J/psi's in the transverse
phase space is used to calculate the ratios between the J/psi transverse
momentum distributions in several centrality bins for Pb-Pb collisions at E/A =
160 GeV. From the comparison with the CERN-SPS data, where the centrality is
characterized by the transverse energy Et, the leakage effect is extremely
important in the region of high transverse momentum and high transverse energy,
and both the threshold and the comover models can describe the ratio well for
all centrality bins except the most central one (Et < 100 GeV), for which the
comover model calculation is considerably better than the threshold one.Comment: 4 pages, 2 figures, REVTEX3.1, accepted for publication in Phys. Rev.
suppression in Pb+Pb collisions and broadening
We have analysed the NA50 data, on the centrality dependence of
broadening of 's, in Pb+Pb collisions, at the CERN-SPS. The data were
analysed in a QCD based model, where 's are suppressed in 'nuclear'
medium. Without any free parameter, the model could explain the NA50
broadening data. The data were also analysed in a QGP based threshold model,
where suppression is 100% above a critical density. The QGP based
model could not explain the NA50 broadening data. We have also predicted
the centrality dependence of suppression and broadening at RHIC
energy. Both the models, the QGP based threshold model and the QCD based
nuclear absorption model, predict broadening very close to each other.Comment: The paper was completely revised. The conclusion is also changed. 5
pages, 4 figure
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
A systematic study of J/psi suppression in cold nuclear matter
Based on a Glauber model, a statistical analysis of all mid-rapidity J/psi
hadroproduction and leptoproduction data on nuclear targets is carried out.
This allows us to determine the J/psi-nucleon inelastic cross section, whose
knowledge is crucial to interpret the J/psi suppression observed in heavy-ion
collisions, at SPS and at RHIC. The values of sigma are extracted from each
experiment. A clear tension between the different data sets is reported. The
global fit of all data gives sigma=3.4+/-0.2 mb, which is significantly smaller
than previous estimates. A similar value, sigma=3.5+/-0.2 mb, is obtained when
the nDS nuclear parton densities are included in the analysis, although we
emphasize that the present uncertainties on gluon (anti)shadowing do not allow
for a precise determination of sigma. Finally, no significant energy dependence
of the J/psi-N interaction is observed, unless strong nuclear modifications of
the parton densities are assumed.Comment: 25 pages, 5 figure
Coherence Time Effects on J/psi Production and Suppression in Relativistic Heavy Ion Collisions
Using a coherence time extracted from high precision proton-nucleus Drell-Yan
measurements and a nuclear absorption cross section extracted from pA
charmonium production experiments, we study J/psi production and absorption in
nucleus-nucleus collisions. We find that coherence time effects are large
enough to affect the measured J/psi-to-Drell-Yan ratio. The S+U data at 200A
GeV/c measured by NA38 are reproduced quantitatively without the introduction
of any new parameters. However, when compared with recent NA50 measurements for
Pb+Pb at 158A GeV/c, the data is not reproduced in trend or in magnitude.Comment: 8 pages, 2 figure
Shadowing Effects on Vector Boson Production
We explore how nuclear modifications to the nucleon structure functions,
shadowing, affect massive gauge boson production in heavy ion collisions at
different impact parameters. We calculate the dependence of , and
production on rapidity and impact parameter to next-to-leading order in
Pb+Pb collisions at 5.5 TeV/nucleon to study quark shadowing at high . We
also compare our Pb+Pb results to the rapidity distributions at 14 TeV.Comment: 25 pages ReVTeX, 12 .eps figures, NLO included, version accepted for
publication in Physical Review
Open charm and charmonium production at relativistic energies
We calculate open charm and charmonium production in reactions at
= 200 GeV within the hadron-string dynamics (HSD) transport approach
employing open charm cross sections from and reactions that are
fitted to results from PYTHIA and scaled in magnitude to the available
experimental data. Charmonium dissociation with nucleons and formed mesons to
open charm ( pairs) is included dynamically. The 'comover'
dissociation cross sections are described by a simple phase-space model
including a single free parameter, i.e. an interaction strength , that
is fitted to the suppression data for collisions at SPS
energies. As a novel feature we implement the backward channels for charmonium
reproduction by channels employing detailed balance. From our
dynamical calculations we find that the charmonium recreation is comparable to
the dissociation by 'comoving' mesons. This leads to the final result that the
total suppression at = 200 GeV as a function of centrality
is slightly less than the suppression seen at SPS energies by the NA50
Collaboration, where the 'comover' dissociation is substantial and the backward
channels play no role. Furthermore, even in case that all directly produced
mesons dissociate immediately (or are not formed as a mesonic state),
a sizeable amount of charmonia is found asymptotically due to the + meson channels in central collisions of at =
200 GeV which, however, is lower than the yield expected from binary
scaling of collisions.Comment: 42 pages, including 14 eps figures, discussions extended and
references added, to be published in Phys. Rev.
Quark exchange model for charmonium dissociation in hot hadronic matter
A diagrammatic approach to quark exchange processes in meson-meson scattering
is applied to the case of inelastic reactions of the type
(Q\barQ)+(q\barq)\rightarrow (Q\barq) + (q\barQ), where and refer to
heavy and light quarks, respectively. This string-flip process is discussed as
a microscopic mechanism for charmonium dissociation (absorption) in hadronic
matter. The cross section for the reaction is
calculated using a potential model, which is fitted to the meson mass spectrum.
The temperature dependence of the relaxation time for the \J/Psi distribution
in a homogeneous thermal pion gas is obtained. The use of charmonium for the
diagnostics of the state of hot hadronic matter produced in ultrarelativistic
nucleus-nucleus collisions is discussed.Comment: 24 pages, 3 tables, 7 figure
Emerging Artificial Societies Through Learning
The NewTies project is implementing a simulation in which societies of agents are expected to de-velop autonomously as a result of individual, population and social learning. These societies are expected to be able to solve environmental challenges by acting collectively. The challenges are in-tended to be analogous to those faced by early, simple, small-scale human societies. This report on work in progress outlines the major features of the system as it is currently conceived within the project, including the design of the agents, the environment, the mechanism for the evolution of language and the peer-to-peer infrastructure on which the simulation runs
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