arXiv subjects

A. Capella

Publications and source records attributed to A. Capella.

At least 55 records · Page 3Linked to original sources

arXiv2001

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.

nucl-th↗

arXiv2001

J/psi suppression at SPS and RHIC in the comovers approach

The NA50 collaboration data on the $J/ψ$ suppression are compared with the results obtained in a comovers approach based on the Dual Parton Model (DPM). Predictions for the $J/ψ$ suppression versus the charged multiplicity - measured in the rapidity region of the dimuon trigger - are given for SPS and RHIC energies.

nucl-th↗

arXiv2000

Single Spin Asymmetry in Heavy Flavor Photoproduction as a Test of pQCD

We analyze in the framework of pQCD the properties of the single spin asymmetry in heavy flavor production by linearly polarized photons. At leading order, the parallel- perpendicular asymmetry in azimuthal distributions of both charm and bottom quark is predicted to be about 20% in a wide region of initial energy. Our analysis shows that the next-to-leading order corrections practically do not affect the Born predictions for the azimuthal asymmetry at energies of the fixed target experiments. Both leading and next-to-leading order predictions for the asymmetry are insensitive to within few percent to theoretical uncertainties in the QCD input parameters: $m_{Q}$, $μ_{R}$, $μ_{F}$, $Λ_{QCD}$ and in the gluon distribution function. We estimate also nonperturbative contributions to azimuthal distributions due to the gluon transverse motion in the target and the final quark fragmentation. Our calculations show that nonperturbative corrections to a $B$-meson azimuthal asymmetry are negligible. We conclude that measurements of the single spin asymmetry would provide a good test of pQCD applicability to heavy flavor production at fixed target energies.

hep-ph↗

arXiv2000

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 $σ=0.2$ mb, we can explain the observed hyperon enhancement in PbPb collisions at CERN SPS.

hep-ph↗

arXiv2000

Deep Inelastic Scattering Data and the Problem of Saturation in Small-x Physics

We investigate the role of unitarization effects in virtual photon-proton (γ^*p) interactions at small x. The q\bar{q}-fluctuation of the initial photon is separated into a small distance and a large distance component and a model for the unitarization of each component is proposed. The Born approximation for the small size component is calculated using QCD perturbation theory. Reggeon diagram technique is used in order to obtain a self-consistent scheme for both total γ^*p cross section and diffractive production. The model gives a good description of HERA data in the small-x region, with a single Pomeron of intercept 1.2.

hep-ph↗

arXiv2000

A unitary model for structure functions and diffractive production at small x

We propose a unified approach which describes both structure functions in the small-$x$ region and diffractive production in $γ^*p$-interactions. It is shown that the model, based on reggeon calculus and a quark-parton picture of the interaction, gives a good description of available experimental data in a broad region of $Q^2$ (including $Q^2 =0$) with a single Pomeron of intercept $α_P(0) = 1.2$. Predictions for very small $x$ are given and the problem of saturation of parton densities is discussed.

hep-ph↗

arXiv2000

Non-saturation of the J/psi suppression at large transverse energy in the comovers approach

We show that, contrary to recent claims, the $J/ψ$ suppression resulting from its interaction with comovers does not saturate at large transverse energy $E_T$. On the contrary, it shows a characteristic structure - change of curvature near the knee of the $E_T$ distribution - which is due to the $E_T$ (or multiplicity) fluctuation, and agrees with recent experimental results.

hep-ph↗

arXiv1999

Evolution of average multiplicities of quark and gluon jets

The energy evolution of average multiplicities of quark and gluon jets is studied in perturbative QCD. Higher order (3NLO) terms in the perturbative expansion of equations for the generating functions are found. First and second derivatives of average multiplicities are calculated. The mean multiplicity of gluon jets is larger than that of quark jets and evolves more rapidly with energy. It is shown which quantities are most sensitive to higher order perturbative and nonperturbative corrections. We define the energy regions where the corrections to different quantities are important. The latest experimental data are discussed.

hep-ph↗

arXiv1999

Standard Sources of Particle Production in Heavy Ion Collisions

We describe particle production in the framework of an independent string model: the dual parton model. We show that an improved version of this model, containing a diquark breaking component, allows to describe the bulk of particle production and, in particular, baryon stopping and most of the observed enhancement of strange baryons. Only for very rare processes, such as $Ω$ or $J/ψ$ production, the model has to be supplemented with final state interaction (comovers interaction). Recent data on event-by-event fluctuations in $p_{T}$ are also described by the model. Predictions for RHIC and LHC are presented and the effect of nuclear shadowing is discussed.

hep-ph↗

arXiv1999

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.

hep-ph↗

arXiv1999

Event-by-event fluctuations in heavy-ion collisions and the quark-gluon string model

We apply dynamical string models of heavy-ions collisions at high energies to the analysis of event-by-event fluctuations. Main attention is devoted to a new variable proposed to study "equilibration" in heavy-ions collisions. Recent results of the NA49 collaboration at CERN SPS are compared with predictions of the Quark-Gluon String Model (QGSM), which gives a good description of different aspects of multiparticle production for collisions of nucleons and nuclei. It is shown that the new observable and other results of the NA49 analysis of event-by-event fluctuations are correctly reproduced in the model. We discuss dynamical effects responsible for these fluctuations and give the predictions for p-p, p-Pb and Pb-Pb collisions at RHIC and higher energies.

hep-ph↗

arXiv1999

Gribov Theory of Nuclear Interactions and Particle Densities at Future Heavy-Ion Colliders

Gribov approach to high-energy interactions of hadrons and nuclei is reviewed and applied to calculation of particle production in heavy-ions collisions. It is pointed out that the AGK (Abramovsky, Gribov, Kancheli) cutting rules is a powerful tool to investigate particle spectra in these processes. It leads, in the Glauber approximation, to a simple formula for the density of hadrons produced in the central rapidity region in nucleus-nucleus interactions. An estimate of this density for RHIC and LHC is presented and compared with results of Monte-Carlo calculations. It is shown that the Glauber approximation substantially overestimate particle densities compared to the results of the complete Gribov theory. This is due to extra shadowing in the system, related to large mass diffraction which leads to a strong decrease of particle densities at mid rapidities. Our method of calculation of these effects has been applied to the problem of shadowing of nuclear structure functions and a good agreement with experimental data has been obtained.

hep-ph↗

arXiv1999

Baryon and Antibaryon Production in Hadronic and Nuclear Interactions

We introduce a new formulation of the diquark breaking mechanism which describes with no free parameters the huge nuclear stopping observed in central nucleus-nucleus collisions. Supplemented, in the dual parton model, with strings originating from diquark--antidiquark pairs from the nucleon sea, it gives a substantial increase of hyperon and antihyperon yields from $pPb$ to central $PbPb$ collisions. Compared to data, this increase is slightly underestimated for $Λ$'s and $Ξ$'s but is five times too small for $Ω$'s. Introducing final state interactions, a reasonable description of all baryon and antibaryon yields is achieved.

hep-ph↗

arXiv1998

Charmonium Suppression in Lead-Lead Collisions: Is There a Break in the $J/ψ$ Cross-Section?

In the framework of a model based on nuclear absorption plus comover interaction, we compute the $E_{T}$ distribution of the $J/ψ$ 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 $J/ψ$ 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 $J/ψ$ suppression at RHIC and LHC energies are also given.

hep-ph↗

arXiv1998

A quantitative reanalysis of charmonium suppression in nuclear collisions

We present a quantitative description of $J/ψ$ and $ψ^\prime$ suppression in proton-nucleus and nucleus-nucleus collisions at CERN energies. We use a conventional hadronic framework based on nuclear absorption plus final state interaction of the $J/ψ$ or $ψ^\prime$ with co-moving hadrons.

hep-ph↗

arXiv1997

Multiparticle production in small-x deep inelastic scattering and the nature of the Pomeron

Properties of multiparticle final states produced in the central rapidity region of small-x deep inelastic scattering (DIS) are discussed. It is pointed out that these properties contain important information on the nature of the Pomeron - used for the description of total cross section and diffractive production in DIS. It is shown that models based on universality of the Pomeron predict charged particle multiplicities in small-x DIS which are in good agreement with recent HERA results.

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↗

arXiv1997

Probabilistic versus Field-Theoretical Description of Heavy Flavour Production off Nuclei

The absorptive corrections resulting from the rescattering of a heavy flavour (the so-called nuclear absorption) are usually calculated with a probabilistic formula valid only in the low energy limit. We extend this formula to all energies using a quantum field theoretical approach. For charmonium and bottonium we find that the absorptive corrections in the rigorous treatment are very similar to the ones in the probabilistic approach. On the contrary, at sufficiently high energy, open charm and bottom are absorbed as much as charmonium and bottonium - in spite of the fact that their absorptive cross-sections are zero, and therefore they are not absorbed in the probabilistic model. At high enough energies there are also absorptive corrections due to the shadowing of the nucleus structure function, which are present for all systems including Drell-Yan pair production. These shadowing corrections cancel in the low energy limit.

hep-ph↗