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A. Capella

Publications and source records attributed to A. Capella.

At least 37 records · Page 2Linked to original sources
arXiv2004

Anomalous suppression of $π^0$ production at large transverse momentum in Au + Au and d + Au collisions at $\sqrt{{\bf s}_{\bf NN}} =$ 200 GeV

We propose a model of suppression of large $p_T$-pions in heavy ion collisions based on the interaction of the large $p_T$ pion with the dense medium created in the collision. The model is practically the same as the one previously introduced to describe $J/ψ$ suppression. Both the $p_T$ and the centrality dependence of the data are reproduced. In deuteron-gold collisions, the effect of the final state interaction with the dense medium turns out to be negligibly small. Here the main features of the data are also reproduced both at mid and at forward rapidities.

hep-ph↗
arXiv2004

Nuclear Shadowing and Diffraction

The relation between diffraction in lepton-proton collisions and shadowing of nuclear structure functions which arises from Gribov inelastic shadowing, is described. A model realizing such relation, which produces a parameter-free description of experimental data on nuclear structure functions at small $x$, is presented. The application to the description of multiplicities in nuclear collisions is discussed and related to other approaches.

hep-ph↗
arXiv2004

Large transverse momentum suppression of $π^0$'s in Au+Au and d+Au collisions at $\sqrt{s}=200$ GeV

We propose a model of suppression of $π^0$'s based on two different effects: at low $p_T$ we take into account the shadowing corrections, while at large $p_T$ the suppression is produced due to the interaction of the large $p_T$ pion with the dense medium created in the collision. The main features of the data on $AuAu$ and $dAu$ collisions are reproduced both at mid and at forward rapidities.

nucl-th↗
arXiv2003

Nuclear Structure Functions at Small x from Inelastic Shadowing and Diffraction

A model for shadowing in nuclear structure functions at small $x$ and small or moderate $Q^2$ is presented using the relation with diffraction that arises from Gribov's reggeon calculus. A reasonable description of the experimental data is obtained with no fitted parameters. A comparison with predictions from other models is performed.

hep-ph↗
arXiv2003

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.

hep-ph↗
arXiv2003

Nuclear Structure Functions at Small x from Inelastic Shadowing and Diffraction

Nuclear structure functions at small x and small or moderate $Q^2$ are studied using the relation with diffraction on nucleons which arises from Gribov's Reggeon Calculus. A reasonable description of experimental data is obtained with no fitted parameters. A comparison with other models and predictions for future lepton-ion colliders are provided. Consequences for the reduction of multiplicities in nucleus-nucleus collisions at energies of RHIC and LHC are examined.

hep-ph↗
arXiv2003

New J/$ψ$ suppression data and the comovers interaction model

New data on the $J/ψ$ suppression both in proton-nucleus and in lead-lead interactions have been presented recently by the NA50 collaboration. We show that these data, together with the final ones on sulfur-uranium interactions, can be described in the framework of the comovers interaction model with a unique set of three parameters : the nuclear absorption cross-section, the comovers interaction cross-section and a single (rescaled) absolute normalization. Expectations for $J/ψ$ suppression at RHIC are also discussed.

nucl-th↗
arXiv2003

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 $J/ψ$ suppression at CERN-SPS are also discussed. Possible consequences for the crucial issue of thermal equilibration of the produced system are considered.

nucl-th↗
arXiv2002

Coherence Effects in Charmonium Production off Nuclei : Consequences for J/ψSuppression

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, $σ_{abs}$ is replaced by an effective cross-section $σ_{abs} + x_+^γ$ ($σ_{c\bar{c}-N} - σ_{abs})$ which increases with $x_+$ and tends to the total $c\bar{c}-N$ cross-section $σ_{c\bar{c}-N}$. Experimental data can be described with $σ_{abs} \sim 4 ÷5$mb and $σ_{c\bar{c}-N} \sim 15 ÷20$mb. At high energies, due to the increase of the coherence length, this formula changes. The main change is the replacement of $σ_{abs}$ by $σ_{c\bar{c}-N}$ -- for all values of $x_+$, as $s \to \infty$. Thus, if $σ_{c\bar{c}-N} > σ_{abs}$ the $J/ψ$ suppression due to nuclear interaction will increase with energy.

nucl-th↗
arXiv2002

J/$ψ$ suppression as a function of the energy of the zero degree calorimeter : can it discriminate between deconfining and comovers interaction models ?

The ratio of $J/ψ$ over Drell-Yan cross-sections as a function of the energy of the zero degree calorimeter in $Pb$ $Pb$ collisions has been computed in a comovers interaction model and compared with the results of a deconfining model. The predictions of the two models are different both for peripheral events and for very central ones. These differences are analyzed and the results of the models confronted with available data -- not only for $Pb$ $Pb$ but also for $pA$ and $SU$ collisions.

nucl-th↗
arXiv2002

Absence of anomalous stopping in heavy ion collisions

We show that the baryon stopping observed in heavy ion collisions both at CERN-SPS and at RHIC can be derived from the one observed in proton-proton collisions. No increase in the size of the baryon junction component is required between small size $(pp)$ and large size (AA) systems.

nucl-th↗
arXiv2002

Strange particle production at RHIC in the Dual Parton Model

We compute the mid-rapidity densities of pions, kaons, baryons and antibaryons in $Au$-$Au$ collisions at $\sqrt{s} =$ 130 GeV in the Dual Parton Model supplemented with final state interaction (comovers interaction). The ratios $B/n_{part}$ ($\bar{B}/n_{part}$) increase between peripheral ($n_{part} = 18$) and central ($n_{part} = 350)$ collisions by a factor 2.4 (2.0) for $Λ$'s, 4.8 (4.1) for $Ξ$'s and 16.5 (13.5) for $Ω$'s. The ratio $K^-/π^-$ increases by a factor 1.3 in the same centrality range. A comparison with available data is presented.

nucl-th↗
arXiv2001

Why is the J/$ψ$ suppression enhanced at large transverse energy ?

We study the ratio of $J/ψ$ over minimum bias in $Pb$ $Pb$ collisions at SPS energy. The NA50 data exhibit a sharp turn-over at $E_T \sim 100$ GeV (close to the knee of the $E_T$ distribution) followed by a steady, steep decrease at larger $E_T$. We show that this behaviour can be explained by the combined effects of a small decrease of the hadronic $E_T$ in the $J/ψ$ event sample (due to the $E_T$ taken by the $J/ψ$ trigger), together with the sharp decrease of the $E_T$ distributions in this $E_T$ region (tail). This phenomenon does not affect the (true) ratio $J/ψ$ over $DY$ (obtained by the NA50 standard analysis), but does affect the one obtained by the so-called minimum bias analysis. A good agreement is obtained with the data coming from both analysis -- as well as with the ratios of $J/ψ$ and $DY$ over minimum bias -- in the whole $E_{T}$ region.

nucl-th↗
arXiv2001

Charged multiplicities and $J/ψ$ suppression at SPS and RHIC energies

Charged multiplicities in nucleus--nucleus collisions are calculated in the Dual Parton Model taking into account shadowing corrections. Its dependence on the number of collisions and participants is analyzed and found in agreement with experiment at SPS and RHIC energies. Using these results, we compute the $J/ψ$ suppression at SPS as a function of the transverse energy and of the energy of the zero degree calorimeter. Predictions for RHIC are presented.

nucl-th↗
arXiv2001

Baryon Number Transfer in Hadronic Interaction

The process of baryon number transfer due to string junction propagation in rapidity is considered. It has a significant effect in the net baryon production in $pp$ collisions at mid-rapidities and an even larger effect in the forward hemisphere in the cases of $πp$ and $γp$ interactions. The results of numerical calculations in the framework of the Quark-Gluon String model are in reasonable agreement with the data.

hep-ph↗
arXiv2001

Charged Multiplicities at SPS and RHIC and consequences for $J/ψ$ suppression

Hadron multiplicities in nucleus--nucleus interactions are calculated in the Dual Parton Model and its dependence on the number of collisions and the number of participants is analyzed. Shadowing corrections are calculated as a function of impact parameter and the multiplicity per participant as a funtion of centrality is found to be in agreement with experiment at SPS and RHIC energies. The obtained results are used to compute the $J/ψ$ suppression in a comover approach.

nucl-th↗
arXiv2001

DIS data and the problem of saturation in small-x physics

New experimental data from HERA explore the region of very small x and small and moderate $Q^2$. We study the role of unitarity corrections in the description of these data. For that, we propose a model separating the small and large components of the $q\bar q$ fluctuation. This model gives a unified description of total and diffractive production in $γ^*p$ interactions. In this model unitarity corrections are controlled by diffraction data.

hep-ph↗