7,031 research outputs found

### 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/$\psi$ suppression at $\sqrt{{\bf s}} =$ 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

### $Q^2$-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

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