996 research outputs found
Elliptic Flow and Fixed p_T Suppression in a Final State Interaction Model
It has been shown that a final state interaction model, used to describe
J/psi suppression, can also describe the fixed p_T suppression of the pi^0 (and
charged pions) yield at all values of p_T, with a final state interaction
cross-section sigma close to one milibarn. We propose an extension of the model
to the pion motion in the transverse plane - which introduces a dependence of
the suppression on the azimuthal angle theta_R. Using the same value of sigma,
we obtain values of the elliptic flow v_2 close to the experimental ones, for
all values of p_T, including the soft p_T region.Comment: 21 pages, 6 figure
Baryon stopping and hyperon enhancement in the improved dual parton model
We present an improved version of the dual parton model which contains a new
realization of the diquark breaking mechanism of baryon stopping. We reproduce
in this way the net baryon yield in nuclear collisions. The model, which also
considers strings originating from diquark-antidiquark pairs in the nucleon
sea, reproduces the observed yields of p and Lambda and their antiparticles and
underestimates cascades by less than 50 %. However, Omega's are underestimated
by a factor five. Agreement with data is restored by final state interaction,
with an averaged cross-section as small as 0.14 mb. Hyperon yields increase
significantly faster than antihyperons, in agreement with experiment.Comment: 40 pages, 18 postscript figure
A model-independent analysis of the dependence of the anomalous J/psi suppression on the number of participant nucleons
A recently published experimental dependence of the J/psi to Drell-Yan ratio
on the measured, by a zero degree calorimeter, forward energy E_ZDC in Pb+Pb
collisions at the CERN SPS is analyzed. Using a model-independent approach it
is shown that the data are at variance with an earlier published experimental
dependence of the same quantity on the transverse energy of neutral hadrons
E_T. The discrepancy is related to a moderate centrality region: 100 < N_p <
200 (N_p is the number of participant nucleons) and is peculiar only to the
data obtained within the `minimum bias' analysis (using the `theoretical
Drell-Yan'). This could result from systematic experimental errors in the
minimum bias sample. A possible source of the errors is discussed.Comment: 10 pages, LaTeX, 3 PS-figures. V2: Misprints are correcte
Hard diffraction in hadron--hadron interactions and in photoproduction
Hard single diffractive processes are studied within the framework of the
triple--Pomeron approximation. Using a Pomeron structure function motivated by
Regge--theory we obtain parton distribution functions which do not obey
momentum sum rule. Based on Regge-- factorization cross sections for hard
diffraction are calculated. Furthermore, the model is applied to hard
diffractive particle production in photoproduction and in
interactions.Comment: 13 pages, Latex, 13 uuencoded figure
Charmonium dissociation by mesons in heavy-ion collisions
The charmonium dissociation by mesons in relativistic heavy-ion reactions is
analyzed within the Regge approach. It is shown that the inclusion of the
initial and final state interactions in the dissociation of J/Psi to bar{D}* D*
close to threshold increases the cross section significantly and can not be
neglected in comparison to the total dissociation rate. This is due to resonant
bar{D}*-D* interactions in sqrt{s} close to the masses of the Psi(4.04) and
Psi(4.16) mesons. We also investigate thermal effects of the (c bar{c}) width
for such processes in the medium. All these effects change both the magnitude
and the shape of the cross section as a function of sqrt{s}. The results
obtained should be applied in the analysis of open and hidden charm production
in heavy-ion collisions.Comment: 4 pages, 2 figures; Talk given at ICTP 5th International Conference
on Perspectives in Hadronic Physics, Trieste, Italy, 22-26 May 200
Hadron yields in Au+Au/Pb+Pb at RHIC and LHC from thermalized minijets
We calculate the yields of a variety of hadrons for RHIC and LHC energies
assuming thermodynamical equilibration of the produced minijets, and using as
input results from pQCD for the energy densities at midrapidity. In the
calculation of the production of partons and of transverse energy one has to
account for nuclear shadowing. By using two parametrizations for the gluon
shadowing one derives energy densities differing strongly in magnitude. In this
publication we link those perturbatively calculated energy densities of partons
via entropy conservation in an ideal fluid to the hadron multiplicities at
chemical freeze-out.Comment: 11 pages, 2 .eps figure
The effects of nonextensive statistics on fluctuations investigated in event-by-event analysis of data
We investigate the effect of nonextensive statistics as applied to the
chemical fluctuations in high-energy nuclear collisions discussed recently
using the event-by-event analysis of data. It turns out that very minuite
nonextensitivity changes drastically the expected experimental output for the
fluctuation measure. This results is in agreement with similar studies of
nonextensity performed recently for the transverse momentum fluctuations in the
same reactions.Comment: Revised version, to be published in J. Phys. G (2000
Charmonium dissociation and recombination at RHIC and LHC
Charmonium production at heavy-ion colliders is considered within the
comovers interaction model. The formalism is extended by including possible
secondary J/psi production through recombination and an estimate of
recombination effects is made with no free parameters involved. The comovers
interaction model also includes a comprehensive treatment of initial-state
nuclear effects, which are discussed in the context of such high energies. With
these tools, the model properly describes the centrality and the rapidity
dependence of experimental data at RHIC energy, = 200 GeV, for both
Au+Au and Cu+Cu collisions. Predictions for LHC, = 5.5 TeV, are
presented and the assumptions and extrapolations involved are discussed.Comment: 16 pages, 5 figure
Gluon- vs. Sea quark-Shadowing
We calculate the shadowing of sea quarks and gluons and show that the
shadowing of gluons is {\it not} simply given by the sea quark shadowing,
especially at small . The calculations are done in the lab frame approach by
using the generalized vector meson dominance model. Here the virtual photon
turns into a hadronic fluctuation long before the nucleus. The subsequent
coherent interaction with more than one nucleon in the nucleus leads to the
depletion known as shadowing. A
comparison of the shadowing of quarks to E665 data for and
shows good agreement.Comment: 9 pages, 3 eps figure
Charmonium Suppression in Lead-Lead Collisions: Is There a Break in the Cross-Section?
In the framework of a model based on nuclear absorption plus comover
interaction, we compute the distribution of the in
collisions at SPS and compare it with available NA50 data. Our analysis
suggests that the existence of new physics (deconfinement phase transition) in
the region E_{T} \lsim 100 GeV is unlikely and that signals of new physics
should rather be searched in the region E_T \gsim 100 GeV. The
dependence of the transverse momentum has been computed. At large
it turns out to be much flatter in the comover approach than in a phase
transition framework. Estimates of the suppression at RHIC and LHC
energies are also given.Comment: 31 pages, LaTeX, 9 postscript figures included using epsfig;
references updated and comments added; final version to appear in Phys. Rev.
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