30 research outputs found
On high behavior of the pion form factor for transitions and within the nonlocal quark-pion model
The behavior of the transition pion form factor for processes \gamma^*\gamma
-> \pi^0 and \gamma^* \gamma^* -> \pi^0 at large values of space-like photon
momenta is estimated within the nonlocal covariant quark-pion model. It is
shown that, in general, the coefficient of the leading asymptotic term depends
dynamically on the ratio of the constituent quark mass and the average
virtuality of quarks in the vacuum and kinematically on the ratio of photon
virtualities. The kinematic dependence of the transition form factor allows us
to obtain the relation between the pion light-cone distribution amplitude and
the quark-pion vertex function. The dynamic dependence indicates that the
transition form factor \gamma^* \gamma -> \pi^0 at high momentum transfers is
very sensitive to the nonlocality size of nonperturbative fluctuations in the
QCD vacuum.Comment: LaTex file with 3 ps-figure
On the NLO Power Correction to Photon-Pion Transition Form Factor
We propose a perturbative evaluation for the next-to-leading-order (NLO)
power correction to the photon-pion transition form factor. The
effects of the NLO power correction are analyzed.Comment: 4 pages, 3 figures, Revtex, revised versio
The target asymmetry in hard vector-meson electroproduction and parton angular momenta
The target asymmetry for electroproduction of vector mesons is investigated
within the handbag approach. While the generalized parton distribution (GPD) H
is taken from a previous analysis of the elctroproduction cross section, we
here construct the GPD E from double distributions and constrain it by the
Pauli form factors of the nucleon, positivity bounds and sum rules. Predictions
for the target asymmetry are given for various vector mesons and discussed how
experimental data on the asymmetry will further constrain E and what we may
learn about the angular momenta the partons carry.Comment: 24 pages, 11 figures, late
Deeply Virtual Neutrino Scattering (DVNS)
We introduce the study of neutrino scattering off protons in the deeply
virtual kinematics, which describes under a unified formalism elastic and deep
inelastic neutrino scattering. A real final state photon and a recoiling
nucleon are detected in the few GeV ( GeV) region of momentum
transfer. This is performed via an extension of the notion of deeply virtual
Compton scattering, or DVCS, to the case of a neutral current exchange. The
relevance of this process and of other similar exclusive processes for the
study of neutrino interactions in neutrino factories for GeV neutrinos is
pointed out.Comment: 28 pages, 12 figures, revised final version, to appear in JHE
The dual parametrization for gluon GPDs
We consider the application of the dual parametrization for the case of gluon
GPDs in the nucleon. This provides opportunities for the more flexible modeling
unpolarized gluon GPDs in a nucleon which in particular contain the invaluable
information on the fraction of nucleon spin carried by gluons. We perform the
generalization of Abel transform tomography approach for the case of gluons. We
also discuss the skewness effect in the framework of the dual parametrization.
We strongly suggest to employ the fitting strategies based on the dual
parametrization to extract the information on GPDs from the experimental data.Comment: 37 pages, 2 figure
factorization of exclusive processes
We prove factorization theorem in perturbative QCD (PQCD) for exclusive
processes by considering and . The relevant form factors are expressed as the convolution of hard
amplitudes with two-parton meson wave functions in the impact parameter
space, being conjugate to the parton transverse momenta . The point is
that on-shell valence partons carry longitudinal momenta initially, and acquire
through collinear gluon exchanges. The -dependent two-parton wave
functions with an appropriate path for the Wilson links are gauge-invariant.
The hard amplitudes, defined as the difference between the parton-level
diagrams of on-shell external particles and their collinear approximation, are
also gauge-invariant. We compare the predictions for two-body nonleptonic
meson decays derived from factorization (the PQCD approach) and from
collinear factorization (the QCD factorization approach).Comment: 11 pages, REVTEX, 5 figure
Deeply virtual electroproduction of photons and mesons on the nucleon : leading order amplitudes and power corrections
We estimate the leading order amplitudes for exclusive photon and meson
electroproduction reactions at large Q^2 in the valence region in terms of
skewed quark distributions. As experimental investigations can currently only
be envisaged at moderate values of Q^2, we estimate power corrections due to
the intrinsic transverse momentum of the partons in the meson wavefunction and
in the nucleon. To this aim the skewed parton distribution formalism is
generalized so as to include the parton intrinsic transverse momentum
dependence. Furthermore, for the meson electroproduction reactions, we
calculate the soft overlap type contributions and compare with the leading
order amplitudes. We give first estimates for these different power corrections
in kinematics which are relevant for experiments in the near future.Comment: 59 pages, 21 figure
Analysis of the vector form factors and with light-cone QCD sum rules
In this article, we calculate the vector form factors and
within the framework of the light-cone QCD sum rules
approach. The numerical values of the are compatible with the
existing theoretical calculations, the central value of the ,
, is in excellent agreement with the values from the chiral
perturbation theory and lattice QCD. The values of the are
very large comparing with the theoretical calculations and experimental data,
and can not give any reliable predictions. At large momentum transfers with
, the form factors and can
either take up the asymptotic behavior of or decrease more
quickly than , more experimental data are needed to select the
ideal sum rules.Comment: 22 pages, 16 figures, revised version, to appear in Eur. Phys. J.
An attempt to understand exclusive pi+ electroproduction
Hard exclusive pi+ electroproduction is investigated within the handbag
approach. The prominent role of the pion-pole contribution is demonstrated. It
is also shown that the experimental data require a twist-3 effect which ensues
from the helicity-flip generalized parton distribution H_T and the twist-3 pion
wave function. The results calculated from this handbag approach are compared
in detail with the experimental data on cross sections and spin asymmetries
measured with a polarized target. It is also commented on consequences of this
approach for exclusive \pi^0 and vector-meson electroproduction.Comment: 35 pages, 12 figures, using Latex, a number of additional comments
have been included in the text, e.g. in paragraph above (3) or at end of
sect.
Modelling generalized parton distributions to describe deeply virtual Compton scattering data
We present a new model for generalized parton distributions (GPDs), based on
the aligned jet model, which successfully describes the deeply virtual Compton
scattering (DVCS) data from H1, ZEUS, HERMES and CLAS. We also present an
easily implementable and flexible algorithm for their construction. This new
model is necessary since the most widely used models for GPDs, which are based
on factorized double distributions, cannot, in their current form, describe the
DVCS data when employed in a full QCD analysis. We demonstrate explicitly the
reason for the shortcoming in the data description. We also highlight several
non-perturbative input parameters which could be used to tune the GPDs, and the
-dependence, to the DVCS data using a fitting procedure.Comment: 12 pages, 12 figures, revtex4, shortened version accepted for
publication in PRD, figures improved and references adde