arXiv subjects

M. B. Gay Ducati

Publications and source records attributed to M. B. Gay Ducati.

At least 91 records · Page 5Linked to original sources

arXiv2000

The Asymptotic Regime of High Density QCD

We discuss the distinct approaches for high density QCD (hdQCD) in the asymptotic regime of large values of parton density. We derive the AGL equation for running coupling constant and obtain the asymptotic solution, demonstrating that the property of partial saturation of the solution of the AGL equation is not modified by the running of the coupling constant. We show that in this kinematical regime, the solution of the AGL equation coincides with the solution of an evolution equation, obtained recently using the McLerran-Venugopalan approach. Using the asymptotic behavior of the gluon distribution we calculate the $F_2$ structure function assuming first that the leading twist relation between these two quantities is valid and second that this relation is modified by the higher twist terms associated to the unitarity corrections. In the first case we obtain that the corresponding $F_2$ structure function is linearly proportional to $ln s$, which agrees with the results obtained recently by Kovchegov using a distinct approach. In the second case a softer behavior is obtained. In both cases, the $F_2$ structure function unitarizes and the Froissart boundary is not violated in the asymptotic regime of high density QCD. We conclude that the partial saturation of the gluon distribution and the unitarization of the structure function are general results, well-established by three distinct approaches for high dense systems.

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arXiv1999

The High Density Parton Dynamics from Eikonal and Dipole Pictures

In this contribution we discuss distinct approaches for the high density QCD (hdQCD) and the demonstration of the equivalence between the AGL approach and the K approach is presented. Our conclusion is that the AGL equation is a good candidate for an unitarized evolution equation at high densities in the DLA limit.

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arXiv1999

The AGL Equation from the Dipole Picture

The AGL equation includes all multiple pomeron exchanges in the double logarithmic approximation (DLA) limit, leading to an unitarized gluon distribution in the small x regime. This equation was originally obtained using the Glauber-Mueller approach. We demonstrate in this paper that the AGL equation and, consequently, the GLR equation, can also be obtained from the dipole picture in the double logarithmic limit, using an evolution equation, recently proposed, which includes all multiple pomeron exchanges in the leading logarithmic approximation. Our conclusion is that the AGL equation is a good candidate for an unitarized evolution equation at small x in the DLA limit.

hep-ph↗

arXiv1999

The Color Evaporation Model in Diffractive $J/ψ$ Photoproduction

Our goal is the study of color effects on the $J/ψ$ and $D$ mesons production, as well as the study of their elastic production. We use the Color Evaporation Model (CEM), where a single factor takes into account the probability that the $c\bar{c}$ produced in a color octet state turns into a physical (colorless) quarkonium or continues as a color octet, producing open charm. The main result of this work, besides investigating the range of applicability of the CEM, is to show that this model can be used to describe the elastic processes of $J/ψ$ and open charm photoproduction without the use of a Pomeron.

hep-ph↗

arXiv1999

Seeking the Shadowing in eA Processes

We consider the behavior of the slope of the nuclear structure function $F_2^A$ in the kinematic region which will be explored in the eA HERA collider. We demonstrate that, similarly to the nucleon case, a `turn over' is predicted in this observable. Moreover, we predict that the presence of the shadowing corrections implies that the maximum value of the slope is dependent of the number of nucleons A, differently from the DGLAP predictions. Our conclusion is that the measurement of this observable will allow to evidentiate the presence of the shadowing corrections.

hep-ph↗

arXiv1999

The AGL Equation from a Dipole Picture

The AGL equation includes all multiple pomeron exchanges in the double logarithmic approximation (DLA) limit, leading to an unitarized gluon distribution in the small $x$ regime. This equation was originally obtained using the Glauber-Mueller approach. We demonstrate in this contribution that the AGL equation can also be obtained from the dipole picture. Our conclusion is that the AGL equation is a good candidate for an unitarized evolution equation at small $x$ in the DLA limit.

hep-ph↗

arXiv1999

The small $x$ nuclear shadowing at DIS

We estimate the perturbative nuclear shadowing in the nuclear structure function $F_2^A(x,Q^2)$, mainly at the kinematic region of HERA using the Glauber-Mueller approach. The contributions of the quark and gluon sectors to the nuclear shadowing are estimated. We predict that the nuclear shadowing corrections are important and that saturation of the ratio $R_1 = F_2^{A}(x,Q^2)/A F^p_2(x,Q^2)$ occurs once the shadowing in the gluon sector is considered.

hep-ph↗

arXiv1998

High Density Parton Shadowing Corrections in DIS Scaling Violations

The description of the dynamics at high density parton regime is one of the main open questions of the strong interactions theory. In this paper we address the shadowing corrections (SC) in the scaling violations of the $F_2$ structure function using the eikonal approach. We propose a procedure to estimate the distinct contributions to the SC for $F_2$ and its slope and show that the recent ZEUS data can be described if the SC in the quark and gluon sectors are considered. The radius dependence of the SC is estimated. Moreover, we calculate the superior limit above which the unitarity corrections cannot be disregarded at low $Q^2$ and show that the recent HERA data overcomes this bound.

hep-ph↗

arXiv1998

The DLLA limit of BFKL in the Dipole Picture

In this work we obtain the DLLA limit of BFKL in the dipole picture and compare it with HERA data. We demonstrate that in leading-logarithmic- approximation, where $α_s$ is fixed, a transition between the BFKL dynamics and the DLLA limit can be obtained in the region of $Q^2 \approx 150 GeV^2$. We compare this result with the DLLA predictions obtained with $α_s$ running. In this case a transition is obtained at low $Q^2$ $(\le 5 GeV^2)$. This demonstrates the importance of the next-to-leading order corrections to the BFKL dynamics. Our conclusion is that the $F_2$ structure function is not the best observable for the determination of the dynamics, since there is great freedom in the choice of the parameters used in both BFKL and DLLA predictions.

hep-ph↗

arXiv1998

On the Determination of Small x Shadowing Corrections at HERA

The recent $F_2$ data can be described using the DGLAP evolution equations with an appropriate choice of input distributions and the choice of the starting scale for the $Q^2$ evolution. We demonstrate in this paper that we cannot conclude that there are no significant shadowing corrections at HERA kinematic region using $F_2$ data only. In this paper we calculate the shadowing corrections to the longitudinal structure function $F_L(x,Q^2)$ and to the charm component of the proton structure function $F_2^c(x,Q^2)$ at HERA kinematic region using an eikonal approach. We demonstrate that the shadowing corrections to these observables are very large and that the charm production is strongly modified at small-$x$. Our results agree with the recent few H1 data.

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arXiv1997

Scaling violation and shadowing corrections at HERA

We study the value of shadowing corrections (SC) in HERA kinematic region in Glauber - Mueller approach. Since the Glauber - Mueller approach was proven in perturbative QCD in the double logarithmic approximation (DLA), we develop the DLA approach for deep inelastic structure function which takes into account the SC. Our estimates show small SC for $F_2$ in HERA kinematic region while they turn out to be sizable for the gluon structure function. We compare our estimates with those for gluon distribution in leading order (LO) and next to leading order (NLO) in the DGLAP evolution equations.

hep-ph↗

arXiv1997

Parton Densities in a Nucleon

In this paper we re-analyse the situation with the shadowing corrections (SC) in QCD for the proton deep inelastic structure functions. We reconsider the Glauber - Mueller approach for the SC in deep inelastic scattering (DIS) and suggest a new nonlinear evolution equation. We argue that this equation solves the problem of the SC in the wide kinematic region where $\as κ= \as \frac{3 π\as}{2 Q^2R^2} x G(x,Q^2) \leq 1$. Using the new equation we estimate the value of the SC which turn out to be essential in the gluon deep inelastic structure function but rather small in $F_2(x,Q^2)$. We claim that the SC in $xG(x,Q^2)$ is so large that the BFKL Pomeron is hidden under the SC and cannot be seen even in such "hard" processes that have been proposed to test it. We found that the gluon density is proportional to $\ln(1/x)$ in the region of very small $x$. This result means that the gluon density does not reach saturation in the region of applicability of the new evolution equation. It should be confronted with the solution of the GLR equation which leads to saturation.

hep-ph↗

arXiv1996

Analysis of Low-$x$ Gluon Density from the $F_2$ Scaling Violations

We present an analysis of the current methods of approximate determination of the gluon density $G(x,Q^2)$ at low-$x$ from the scaling violations of the proton structure function $F_2(x,Q^2)$. The gluon density is obtained from DGLAP equation by expansion at distinct points of expansion. We show that the different results given by the proposed methods can be obtained from one new expression considering the adequate points of expansion. It is shown in which case the approximate determination of $G(x,Q^2)$ is reasonable.

hep-ph↗

arXiv1996

Gluon density in nuclei

In this talk we present our detail study ( theory and numbers) [1] on the shadowing corrections to the gluon structure functions for nuclei. Starting from rather contraversial information on the nucleon structure function which is originated by the recent HERA data, we develop the Glauber approach for the gluon density in a nucleus based on Mueller formula [2] and estimate the value of the shadowing corrections in this case. Than we calculate the first corrections to the Glauber approach and show that these corrections are big. Based on this practical observation we suggest the new evolution equation which takes into account the shadowing corrections and solve it. We hope to convince you that the new evolution equation gives a good theoretical tool to treat the shadowing corrections for the gluons density in a nucleus and, therefore, it is able to provide the theoretically reliable initial conditions for the time evolution of the nucleus - nucleus cascade.

hep-ph↗

arXiv1996

Froissart boundary for deep inelastic structure functions

In this letter we derive the Froissart boundary in QCD for the deep inelastic structure function in low $x$ kinematic region. We show that the comparison of the Froissart boundary with the new HERA experimental data gives rise to a challenge for QCD to explain the matching between the deep inelastic scattering and real photoproduction process.

hep-ph↗

arXiv1996

QCD evolution of the gluon density in a nucleus

The Glauber approach to the gluon density in a nucleus, suggested by A. Mueller, is developed and studied in detail. Using the GRV parameterization for the gluon density in a nucleon, the value as well as energy and $Q^2$ dependence of the gluon density in a nucleus is calculated. It is shown that the shadowing corrections are under theoretical control and are essential in the region of small $x$. They change crucially the value of the gluon density as well as the value of the anomalous dimension of the nuclear structure function, unlike of the nucleon one. The systematic theoretical way to treat the correction to the Glauber approach is developed and a new evolution equation is derived and solved. It is shown that the solution of the new evolution equation can provide a selfconsistent matching of ``soft" high energy phenomenology with ``hard" QCD physics.

hep-ph↗

arXiv1995

Gluon structure function for deeply inelastic scattering with nucleus in QCD

In this talk we present the first calculation of the gluon structure function for nucleus in QCD. We discuss the Glauber formula for the gluon structure function and the violation of this simple approach that we anticipate in QCD. (Talk given by E. Levin at QCD and nuclear target session at the Workshop on Deep Inelastic Scattering and QCD, Paris, April 1995).

hep-ph↗

arXiv1993

The Pomeron as Massive Gluons

A QCD-Pomeron composed by two non-perturbative gluons with a dynamically generated mass, is constructed in a gauge invariant way. The gluon propagator is infrared-finite. The model properly describes data on elastic scattering, exclusive $ρ$ production in deep inelastic scattering (DIS) and the $J/Ψ$-nucleon total cross-section in terms of a single gluon mass $m_g\simeq0.37$~GeV. The total cross sections of hadrons with small radii, such as $J/Ψ$, are very sensitive on the effective gluon mass.

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