9 research outputs found
Twist-three observables in deeply virtual Compton scattering on the nucleon
We study twist-three effects in deeply virtual Compton scattering on an
unpolarized spin-1/2 target. A careful definition of observables as Fourier
moments w.r.t. the azimuthal angle allows for a clear separation of twist-two
and -three effects. Although the latter are power suppressed, they give leading
contributions to the twist-three asymmetries and do not affect the twist-two
observables.Comment: 10 pages, LaTe
Twist-three analysis of photon electroproduction with pion
We study twist-three effects in spin, charge, and azimuthal asymmetries in
deeply virtual Compton scattering on a spin-zero target. Contributions which
are power suppressed in 1/Q generate a new azimuthal angle dependence of the
cross section which is not present in the leading twist results. On the other
hand the leading twist terms are not modified by the twist three contributions.
They may get corrected at twist four level. In the Wandzura-Wilczek
approximation these new terms in the Fourier expansion with respect to the
azimuthal angle are entirely determined by the twist-two skewed parton
distributions. We also discuss more general issues like the general form of the
angular dependence of the differential cross section, validity of factorization
at twist-three level, and a relation of skewed parton distributions to spectral
functions.Comment: 21 pages, LaTeX, 2 figures, text clarifications, an equation, a note
and references adde
Theory of deeply virtual Compton scattering on the nucleon
We compute the cross section for leptoproduction of the real photon off the
nucleon, which is sensitive to the deeply virtual Compton scattering amplitude
with power accuracy. Our considerations go beyond the leading twist and involve
the complete analysis in the twist-three approximation. We discuss consequences
of the target and lepton beam polarizations for accessing the generalized
parton distributions from experimental measurements of the azimuthal angular
dependence of the final state photon or nucleon. We introduce several sets of
asymmetries, defined as Fourier moments with respect to the azimuthal angle,
which allow for a clear separation of the twist-two and -three sectors. Relying
on a simple ansatz for the generalized parton distributions, we give
quantitative estimates for azimuthal and spin asymmetries, discuss the
uncertainties of these predictions brought in by radiative corrections, and
compare them with experimental data as well as other theoretical expectations.
Furthermore, we derive a general parametrization of the DVCS amplitudes in the
region of small Bjorken variable.Comment: 76 pages, LaTeX, 16 figures, 3 tables, minor correction
Unraveling hadron structure with generalized parton distributions
The generalized parton distributions, introduced nearly a decade ago, have
emerged as a universal tool to describe hadrons in terms of quark and gluonic
degrees of freedom. They combine the features of form factors, parton densities
and distribution amplitudes--the functions used for a long time in studies of
hadronic structure. Generalized parton distributions are analogous to the
phase-space Wigner quasi-probability function of non-relativistic quantum
mechanics which encodes full information on a quantum-mechanical system. We
give an extensive review of main achievements in the development of this
formalism. We discuss physical interpretation and basic properties of
generalized parton distributions, their modeling and QCD evolution in the
leading and next-to-leading orders. We describe how these functions enter a
wide class of exclusive reactions, such as electro- and photo-production of
photons, lepton pairs, or mesons. The theory of these processes requires and
implies full control over diverse corrections and thus we outline the progress
in handling higher-order and higher-twist effects. We catalogue corresponding
results and present diverse techniques for their derivations. Subsequently, we
address observables that are sensitive to different characteristics of the
nucleon structure in terms of generalized parton distributions. The ultimate
goal of the GPD approach is to provide a three-dimensional spatial picture of
the nucleon, direct measurement of the quark orbital angular momentum, and
various inter- and multi-parton correlations.Comment: 370 pages, 62 figures; Dedicated to Anatoly V. Efremov on occasion of
his 70th anniversar