Centrality Dependence of Hadron Multiplicities in Nuclear Collisions in the Dual Parton Model

We show that, even in purely soft processes, the hadronic multiplicity in nucleus-nucleus interactions contains a term that scales with the number of binary collisions. In the absence of shadowing corrections, this term dominates at mid rapidities and high energies. Shadowing corrections are calculated as a function of impact parameter and the centrality dependence of mid-rapidity multiplicities is determined. The multiplicity per participant increases with centrality with a rate that increases between SPS and RHIC energies, in agreement with experiment.Comment: 15 pages, 4 postscript figure

Hyperon Enhancement in the Dual Parton Model

We review the two sources of hyperon enhancement in the dual parton model: strings originating from diquark-antidiquark pairs in the nucleon sea and net baryons containing two or three sea quarks with a yield controlled by the observed stopping. We show that adding final state interactions (including strangeness exchange reactions as well as the inverse reactions required by detailed balance) with a single averaged cross-section $\sigma=0.2$ mb, we can explain the observed hyperon enhancement in PbPb collisions at CERN SPS.Comment: 6 pages, 2 eps-figure

Nucleus-nucleus collisions: what we have learned from the heavy ion program at CERN

In the first part, I give a brief description of the quark-gluon plasma search at CERN and of some experimental results. In the second part, I review a dynamical model of nucleus-nucleus interactions and propose a physical interpretation of those results.Comment: LaTeX, 11 pages, uses fleqn.sty and espcrc2.sty. Figures available from the author. Invited talk at the XVth European Cosmic Ray Symposium, Perpignan (France), August 1996; to appear in the proceeding

J/ψ\psi suppression at s=\sqrt{{\bf s}} = 200 GeV in the comovers interaction model

The yield of $J/\psi$ per binary nucleon-nucleon collision in $AuAu$ and $CuCu$ collisions at $\sqrt{s} = 200$ GeV is computed in the framework of the dual parton model, supplemented with final state interaction (comovers interaction). For the latter we use the same value of the cross-section, $\sigma_{co} = 0.65$ mb, which describes the anomalous $J/\psi$ suppression observed at CERN-SPS energies. Several possibilities for the value of the absorptive cross-section are considered. Shadowing is introduced in both the comovers and the $J/\psi$ yields. A comparison with the results at CERN-SPS, including a prediction for $InIn$ collisions, is also presented.Comment: 15 pages, 3 figure

Mechanisms of multiparticle production in heavy ion collisions at high energy

In the framework of a microscopic string model inclusive charged particle distribution and baryon and antibaryon production are described. The emphasis is put on high energies (RHIC) where shadowing corrections play a crucial role. Some recent developments on $J/\psi$ suppression at CERN-SPS are also discussed. Possible consequences for the crucial issue of thermal equilibration of the produced system are considered.Comment: 35 pages, Latex style, 12 Figures, 2 Tables. To be published in a special issue of ``Acta Physica Polonica'' in homage to Jan Kwiecinsk

Coherence Effects in Charmonium Production off Nuclei : Consequences for J/\psi Suppression

The probabilistic Glauber formula for nuclear absorption used in the literature is only valid at low energies and $x_+ \simeq 0$. Due to energy conservation, $\sigma_{abs}$ is replaced by an effective cross-section $\sigma_{abs} + x_+^{\gamma}$ ($\sigma_{c\bar{c}-N} - \sigma_{abs})$ which increases with $x_+$ and tends to the total $c\bar{c}-N$ cross-section $\sigma_{c\bar{c}-N}$. Experimental data can be described with $\sigma_{abs} \sim 4 \div 5$mb and $\sigma_{c\bar{c}-N} \sim 15 \div 20$mb. At high energies, due to the increase of the coherence length, this formula changes. The main change is the replacement of $\sigma_{abs}$ by $\sigma_{c\bar{c}-N}$ -- for all values of $x_+$, as $s \to \infty$. Thus, if $\sigma_{c\bar{c}-N} > \sigma_{abs}$ the $J/\psi$ suppression due to nuclear interaction will increase with energy.Comment: Talk presented at the International Workshop on Charm Production, ECT*, Trento, June 200

Q2Q^2-dependence of backward pion multiplicity in neutrino-nucleus interactions

The production of pions emitted backward in inelastic neutrino-nucleus interactions is analyzed within the impulse approximation in the framework of the dual parton model. We focus on the $Q^2$-dependence of the multiplicity of negative pions, normalized to the total cross section of the reaction $\nu + A \to \mu + X$. The inclusion of planar (one-Reggeon exchange) and cylindrical (one-Pomeron exchange) graphs leads to a multiplicity that decreases as $Q^2$ increases, in agreement with recent measurements carried out at CERN by the NOMAD collaboration. A realistic treatment of the high momentum tail of the nucleon distribution in a nucleus also allows for a satisfactory description of the semi-inclusive spectrum of backward pions.Comment: 15 pages, 4 figure

Microscopic Models of Heavy Ion Interactions

An introduction to dynamical microscopic models of hadronic and nuclear interactions is presented. Special emphasis is put in the relation between multiparticle production and total cross-section contributions. In heavy ion collisions, some observables, considered as signals of the production of a Quark Gluon Plasma (QGP), are studied. It is shown that they can only be described if final state interactions are introduced. It is argued that the cross-sections required are too small to drive the system to thermal equilibrium within the duration time of the final state interaction.Comment: 39 pages, 18 figures. Lectures at VIII Hispalensis International Summer School, Seville (Spain), to be published by Springer Verla

Multiplicity and Transverse Energy Distributions Associated to Rare Events in Nucleus-Nucleus Collisions

We show that in high energy nucleus-nucleus collisions the transverse energy or multiplicity distribution P_C, associated to the production of a rare, unabsorbed event C, is universally related to the standard or minimum bias distribution P by the equation $P_C(\nu)={\nu\over}P(\nu)$, with $\sum P(\nu)=1$ and $\nu\equiv E_T$ or n. Deviations from this formula are discussed, in particular having in view the formation of the plasma of quarks and gluons. This possibility can be distinguished from absortion or interaction of comovers, looking at the curvature of the $J/\Psi$ over Drell-Yan pairs as a function of E_T.Comment: 8 pages, 4 Postscript figure