312 research outputs found
Pedagogic model for Deeply Virtual Compton Scattering with quark-hadron duality
We show how quark-hadron duality can emerge for valence spin averaged
structure functions, and for the non-forward distributions of Deeply Virtual
Compton Scattering. Novel factorisations of the non-forward amplitudes are
proposed. Some implications for large angle scattering and deviations from the
quark counting rules are illustrated.Comment: Version accepted by Phys. Rev.
Positivity bounds on generalized parton distributions in impact parameter representation
New positivity bounds are derived for generalized (off-forward) parton
distributions using the impact parameter representation. These inequalities are
stable under the evolution to higher normalization points. The full set of
inequalities is infinite. Several particular cases are considered explicitly.Comment: 8 page
Deeply Virtual Compton Scattering on nucleons and nuclei in generalized vector meson dominance model
We consider Deeply Virtual Compton Scattering (DVCS) on nucleons and nuclei
in the framework of generalized vector meson dominance (GVMD) model. We
demonstrate that the GVMD model provides a good description of the HERA data on
the dependence of the proton DVCS cross section on Q^2, W (at Q^2=4 GeV^2) and
t. At Q^2 = 8 GeV^2, the soft W-behavior of the GVMD model somewhat
underestimates the W-dependence of the DVCS cross section due to the hard
contribution not present in the GVMD model. We estimate 1/Q^2 power-suppressed
corrections to the DVCS amplitude and the DVCS cross section and find them
large. We also make predictions for the nuclear DVCS amplitude and cross
section in the kinematics of the future Electron-Ion Collider. We predict
significant nuclear shadowing, which matches well predictions of the
leading-twist nuclear shadowing in DIS on nuclei.Comment: 25 pages, 9 figures, 1 tabl
Inequalities for nucleon generalized parton distributions with helicity flip
Several positivity bounds are derived for generalized parton distributions
(GPDs) with helicity flip.Comment: 20 page
Off-forward parton distributions and Shuvaev's transformations
We review Shuvaev's transformations, that relate off-forward parton
distributions (OFPDs) to so-called effective forward parton distributions
(EFPDs). The latter evolve like conventional forward partons. We express
nonforward amplitudes, depending on OFPDs, directly in terms of EFPDs and
construct a model for the EFPDs, which allows to consistently express them in
terms of the conventional forward parton distributions and nucleon form
factors. Our model is self-consistent for arbitrary x, xi, mu, and t.Comment: 13 pages, 7 eps-figures, LaTeX2e, added references, corrected typo
Probing partonic structure in gamma* gamma -> pi pi near threshold
Hadron pair production gamma* gamma -> h hbar in the region where the c.m.
energy is much smaller than the photon virtuality can be described in a
factorized form, as the convolution of a partonic handbag diagram and
generalized distribution amplitudes which are new non-perturbative functions
describing the exclusive fragmentation of a quark-antiquark pair into two
hadrons. Scaling behavior and a selection rule on photon helicity are
signatures of this mechanism. The case where h is a pion is emphasized.Comment: 8 pages, 1 figure, LaTeX2
Integral representations for nonperturbative GPDs in terms of perturbative diagrams
An integral representation is suggested for generalized parton distributions
which automatically satisfies the polynomiality and positivity constraints.
This representation has the form of an integral of perturbative triangle
diagrams over the masses of three propagators with an appropriate weight
depending on these masses. An arbitrary D term can be added.Comment: 15 page
DVCS amplitude at tree level: Transversality, twist-3, and factorization
We study the virtual Compton amplitude in the generalized Bjorken region (q^2
-> Infinity, t small) in QCD by means of a light-cone expansion of the product
of e.m. currents in string operators in coordinate space. Electromagnetic gauge
invariance (transversality) is maintained by including in addition to the
twist-2 operators 'kinematical' twist-3 operators which appear as total
derivatives of twist-2 operators. The non-forward matrix elements of the
elementary twist-2 operators are parametrized in terms of two-variable spectral
functions (double distributions), from which twist-2 and 3 skewed distributions
are obtained through reduction formulas. Our approach is equivalent to a
Wandzura-Wilczek type approximation for the twist-3 skewed distributions. The
resulting Compton amplitude is manifestly transverse up to terms of order
t/q^2. We find that in this approximation the tensor amplitude for longitudinal
polarization of the virtual photon is finite, while the one for transverse
polarization contains a divergence already at tree level. However, this
divergence has zero projection on the polarization vector of the final photon,
so that the physical helicity amplitudes are finite.Comment: 34 pages, revtex, 1 eps figure included using epsf. Misprints
corrected, one reference adde
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
Off-diagonal parton distributions and their evolution
We construct off-diagonal parton distributions defined on the interval 0 < X
< 1 starting from the off-forward distributions defined by Ji. We emphasize the
particular role played by the symmetry relations in the "ERBL-like" region. We
find the evolution equations for the off-diagonal distributions which conserve
these symmetries. We present numerical results of the evolution, and verify
that the analytic asymptotic forms of the parton distributions are reproduced.
We also compare the constructed off-diagonal distributions with the non-forward
distributions defined by Radyushkin and comment on the singularity structure of
the basic amplitude written in terms of the off-diagonal distributions.Comment: 22 pages, Latex, 6 ps figures. Improved presentation after
discussions with X.Ji and A.Radyushki
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