285 research outputs found
Unitarity Corrections to the Proton Structure Functions through the Dipole Picture
We study the dipole picture for the description of the deep inelastic
scattering, focusing on the structure functions which are driven directly by
the gluon distribution. One performs estimates using the effective dipole cross
section given by the Glauber-Mueller approach in QCD, which encodes the
corrections due to the unitarity effects associated with the saturation
phenomenon. We also address issues about frame invariance of the calculations
when analysing the observables.Comment: 16 pages, 8 figures. Version to be published in Phys. Rev.
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 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 in the DLA limit.Comment: 3 pages. To be published in the proceedings of 7th International
Workshop on Deep Inelastic Scattering and QCD (DIS 99), Zeuthen, Germany, 19
- 23 Apr 199
Seeking the Shadowing in eA Processes
We consider the behavior of the slope of the nuclear structure function
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.Comment: 10 pages, 1 figur
The Description of at Very High Parton Densities
The behavior of the structure function at high energies (high densities) is
directly associated to the gluon distribution. In this paper we analyze the
asymptotic behavior of the structure function at very high densities
considering a leading and a higher twist relation between these two quantities.
We verify that (a) if the leading twist relation between the structure function
and the gluon distribution is used, we recover the black disc limit, and (b) a
softer behavior is predicted if a higher twist relation between and
is considered. While the first behavior is well-established by the current high
density approaches, the higher twist behavior is a new result. In both cases,
the structure function unitarizes and the Froissart boundary is not
violated in the asymptotic regime of high density QCD.Comment: 14 pages. Accepted for publication in Physics Letters
Non-perturbative gluons in diffractive photo-production of J/Psi
The modifications induced in the calculation of the cross section of the
diffractive process gamma gamma -> J/Psi J/Psi when the gluon propagator is
changed are analyzed. Instead of the usual perturbative gluon propagator,
alternative forms obtained using non-perturbative methods like Dyson-Schwinger
equations are used to consider in a more consistent way the contributions of
the infrared region. The result shows a reduction in the differential
cross-section for low momentum transfer once compared with the perturbative
result, to be confirmed with future experimental results from TESLA.Comment: 17 pages, 5 figures, RevTex, Accepted for publication in Phys. Lett.
Using dileptons to probe the Color Glass Condensate
The rapidity and transverse momentum dependence of the nuclear modification
ratio for dilepton production at RHIC and LHC is presented, calculated in the
Color Glass Condensate (CGC) framework. The transverse momentum ratio is
compared for two distinct dilepton mass values and a suppression of the Cronin
peak is verified even for large mass. The nuclear modification ratio
suppression in the dilepton rapidity spectra, as obtained experimentally for
hadrons at RHIC, is verified for LHC energies at large transverse momentum,
although not present at RHIC energies. The ratio between LHC and RHIC nuclear
modification ratios is evaluated in the CGC, showing the large saturation
effects at LHC compared with the RHIC results. These results consolidate the
dilepton as a most suitable observable to investigate the QCD high density
approaches.Comment: 5 pages, version to appear in Phys. Lett.
Diffractive photon dissociation in the saturation regime from the Good and Walker picture
Combining the QCD dipole model with the Good and Walker picture, we formulate
diffractive dissociation of a photon of virtuality Q^2 off a hadronic target,
in the kinematical regime in which Q is close to the saturation scale and much
smaller than the invariant mass of the diffracted system. We show how the
obtained formula compares to the HERA data and discuss what can be learnt from
such a phenomenology. In particular, we argue that diffractive observables in
these kinematics provide useful pieces of information on the saturation regime
of QCD.Comment: 17 pages, 7 figures, revte
A dynamical gluon mass solution in a coupled system of the Schwinger-Dyson equations
We study numerically the Schwinger-Dyson equations for the coupled system of
gluon and ghost propagators in the Landau gauge and in the case of pure gauge
QCD. We show that a dynamical mass for the gluon propagator arises as a
solution while the ghost propagator develops an enhanced behavior in the
infrared regime of QCD. Simple analytical expressions are proposed for the
propagators, and the mass dependency on the scale and its
perturbative scaling are studied. We discuss the implications of our results
for the infrared behavior of the coupling constant, which, according to fits
for the propagators infrared behavior, seems to indicate that as .Comment: 17 pages, 7 figures - Revised version to be consistent with erratum
to appear in JHE
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 \kappa = \as
\frac{3 \pi \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 . We claim that
the SC in 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 in the
region of very small . 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.Comment: latex file 53 pages, 27 figures in eps file
Quarkonium+ production in coherent hadron - hadron interactions at LHC energies
In this paper we study the ( and )
production in coherent hadron - hadron interactions at LHC energies.
Considering the ultrarelativistic protons as a source of photons, we estimate
the cross section using the
non-relativistic QCD (NRQCD) factorization formalism and considering different
sets of values for the matrix elements. Our results for the total cross sections and rapidity distributions at
and 14 TeV demonstrate that the experimental analysis of the
production at LHC is feasible.Comment: 6 pages, 3 figures, 1 table. Improved version with a new figure.
Version to be published in European Physical Journal
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