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A. Capella

Publications and source records attributed to A. Capella.

66 records · Page 4Linked to original sources
arXiv1997

Interpretations of $J/ψ$ suppression

We review the two main interpretations of $J/ψ$ suppression proposed in the literature. The phase transition (or deconfining) scenario assumes that below some critical value of the local energy density (or of some other geometrical quantity which depends both on the colliding systems and on the centrality of the collision), there is only nuclear absorption. Above this critical value the absorptive cross-section is taken to be infinite, i.e. no $J/ψ$ can survive in this hot region. In the hadronic scenario the $J/ψ$ dissociates due both to nuclear absorption and to its interactions with co-moving hadrons produced in the collision. No discontinuity exists in physical observables. We show that an equally good description of the present data is possible in either scenario.

hep-ph↗
arXiv1997

Structure Functions of Nuclei at Small x and Diffraction at HERA

Gribov theory is applied to investigate the shadowing effects in the structure functions of nuclei. In this approach these effects are related to the process of diffractive dissociation of a virtual photon. A model for this diffractive process, which describes well the HERA data, is used to calculate the shadowing in nuclear structure functions. A reasonable description of the x, Q^2 and A-dependence of nuclear shadowing is achieved.

hep-ph↗
arXiv1996

$J/Ψ$ and $Ψ'$ Suppression in Heavy Ion Collisions

We study the combined effect of nuclear absorption and final state interaction with co-moving hadrons on the $J/Ψ$ and $Ψ'$ suppression in proton-nucleus and nucleus-nucleus collisions. We show that reasonable description of the experimental data can be achieved with theoretically meaningful values of the cross-sections involved and without introducing any dicontinuity in the $J/Ψ$ or $Ψ'$ survival probabilities.

hep-ph↗
arXiv1996

Strangeness Enhancement Scenarios: Fireball or Independent Strings?

Due to the long-standing discrepancy between NA35 and NA36 data on $Λ$ production, two drastically different scenarios of strangeness enhancement are still possible. Independent string models, such as the dual parton model, lead to results close to the NA36 data. On the contrary, the NA35 results can only be described by introducing full final state rescattering of the produced particles. The corresponding predictions for central $Pb$-$Pb$ collisions at CERN energies differ by a factor 3 to 4. Preliminary data on the net proton ($p$-$\bar{p}$) rapidity distribution in $Pb$-$Pb$ collisions favor the independent string scenario.

hep-ph↗
arXiv1996

Moment Analysis of Multiplicity Distributions

Moment analysis of global multiplicity distribution previously done for $e^{+}e^{-}$ and $hh$ processes is applied to $hA$ and $AA$ collisions. The oscillations of cumulants as functions of their rank are found in all the cases. Some phenomenological approaches and quark-gluon string models are confronted to experimental data. It has been shown that the analysis is a powerful tool for revealing the tiny features of the distributions, and its qualitative results in various processes are rather stable for different multiplicity cut-offs determined by experimental (or Monte-Carlo) statistics.

hep-ph↗
arXiv1996

Novel Mechanism of Nucleon Stopping in Heavy Ion Collisions

When a diquark does not fragment directly but breaks in such a way that only one of its quarks gets into the produced baryon, the latter is produced closer to mid rapidities. The relative size of this diquark breaking component increases quite fast with increasing energy. We show that at a given energy it also increases with the atomic mass number and with the centrality of the collision and that it allows to explain the rapidity distribution of the net baryon number ($p$-$\bar{p}$) in $SS$ central collisions. Predictions for $Pb$-$Pb$ collisions are presented.

hep-ph↗
arXiv1995

Strange Baryon Production in Heavy Ion Collisions

The rapidity distribution of $Λ$ and $\barΛ$ produced in nucleus-nucleus collisions at CERN energies is studied in the framework of an independent string model - with quark-antiquark as well as diquark-antidiquark pairs in the nucleon sea. It is shown that, besides the $Λ$-$\barΛ$ pair production resulting from the fragmentation of sea diquarks, final state interactions of co-moving secondaries $π+ N \to K + Λ$ and $π+ \bar{N} \to K + \barΛ$ are needed in order to reproduce the data. Predictions for $Pb$-$Pb$ collisions are presented.

hep-ph↗
arXiv1995

Hard Diffraction at HERA and the Gluonic Content of the Pomeron

We show that the previously introduced CKMT model, based on conventional Regge theory, gives a good description of the HERA data on the structure function F_2^D for large rapidity gap (diffractive) events. These data allow, not only to determine the valence and sea quark content of the Pomeron, but also, through their Q^2 dependence, give information on its gluonic content. Using DGLAP evolution, we find that the gluon distribution in the Pomeron is very hard and the gluons carry more momentum than the quarks. This indicates that the Pomeron, unlike ordinary hadrons, is a mostly gluonic object. With our definition of the Pomeron flux factor the total momentum carried by quarks and gluons turns out to be 0.3-0.4 - strongly violating the momentum sum rule.

hep-ph↗
arXiv1995

Strangeness enhancement in heavy ion collisions

The enhancement of strange particle production observed in nucleus-nucleus collisions at CERN is explained by the combined effect of the increase in the relative number of strings containing strange constitutents at the ends, and the final state interaction of co-moving secondaries~: $π+ N \to K + Λ/Σ$.

hep-ph↗
arXiv1994

Diffractive Dissociation in Deep Inelastic Scattering at HERA

The diffraction dissociation of virtual photons is considered in the framework of conventional Regge theory. It is shown that the recent HERA data on large rapidity gap events can be successfully described in terms of the Pomeron structure function. Using Regge factorization, the latter can be related to the deuteron structure function. The parameters which relate these two structure functions are determined from soft hadronic diffraction data. The size of the shadowing corrections at low x and large Q**2 is also obtained.

hep-ph↗
arXiv1994

Structure functions and low x physics

In the framework of conventional Regge theory we present a common description of total photon-proton cross-section and proton structure functions in the region $0 \leq Q^2 \leq 5 \ {\rm GeV}^2$. Using it as an initial condition in the perturbative QCD evolution equation, we describe structure functions at any $Q^2$ and $x$ - including the small $x$ region explored at HERA.

hep-ph↗