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 $p\bar{p}$ 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

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 $x$. 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 $\sigma (\gamma^* A) < A\sigma (\gamma^* N)$ known as shadowing. A comparison of the shadowing of quarks to E665 data for $^{40}Ca$ and $^{207}Pb$ shows good agreement.Comment: 9 pages, 3 eps figure

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, $\sqrt{s}$ = 200 GeV, for both Au+Au and Cu+Cu collisions. Predictions for LHC, $\sqrt{s}$ = 5.5 TeV, are presented and the assumptions and extrapolations involved are discussed.Comment: 16 pages, 5 figure

Charmonium Suppression in Lead-Lead Collisions: Is There a Break in the J/ψJ/\psi Cross-Section?

In the framework of a model based on nuclear absorption plus comover interaction, we compute the $E_{T}$ distribution of the $J/\psi$ in $PbPb$ 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 $E_{T}$ dependence of the $J/\psi$ transverse momentum has been computed. At large $E_T$ it turns out to be much flatter in the comover approach than in a phase transition framework. Estimates of the $J/\psi$ 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.