282 research outputs found
Dispersion Theory in Electromagnetic Interactions
We review various applications of dispersion relations (DRs) to the
electromagnetic structure of hadrons. We discuss the way DRs allow one to
extract information on hadron structure constants by connecting information
from complementary scattering processes. We consider the real and virtual
Compton scattering processes off the proton, and summarize recent advances in
the DR analysis of experimental data to extract the proton polarizabilities, in
comparison with alternative studies based on chiral effective field theories.
We discuss a multipole analysis of real Compton scattering data, along with a
DR fit of the energy-dependent dynamical polarizabilities. Furthermore, we
review new sum rules for the double-virtual Compton scattering process off the
proton, which allow for model independent relations between polarizabilities in
real and virtual Compton scattering, and moments of nucleon structure
functions. The information on the double-virtual Compton scattering is used to
predict and constrain the polarizability corrections to muonic hydrogen
spectroscopy.Comment: preprint version of a review to appear in Ann. Rev. Nucl. Part. Sci.
68 (2018
Light-Front Densities for Transversely Polarized Hadrons
We discuss the recent interpretation of quark distribution functions in the
plane transverse to the light-cone direction. Such a mapping is model
independent and allows one to build multidimensional pictures of the hadron and
to develop a semi-classical intuition of the quark dynamics. We comment briefly
the results obtained from the Form Factors of the nucleon. A generalization to
a target with arbitrary spin led to a set of preferred values for the
electromagnetic coupling characterizing structureless particles. Generalized
polarizabilities can also be interpreted in that frame as the distortion of the
charge densities due to an external electromagnetic field. Finally, we present
preliminary results for the Generalized Transverse-Momentum dependent
Distributions which encode in principle the most complete information about
quark distributions.Comment: 8 pages, 2 figures, 1 table; contribution to the proceedings of "The
4th Workshop on Exclusive Reactions at High Momentum Transfer", Jefferson
Lab., May 18-21, 201
Two-photon exchange contribution to elastic -proton scattering: Full dispersive treatment of states and comparison with data
We evaluate the two-photon exchange correction to the elastic electron-proton
scattering cross section within a dispersive framework. Besides the elastic
contribution, we account for all intermediate state contributions using
the phenomenological MAID fit as an input. We develop a novel method for the
analytical continuation of the two-photon exchange amplitudes into the
unphysical region and generalize our previous work to the momentum transfer
region . We
compare our results with recent OLYMPUS, CLAS and VEPP-3 data as well as with
empirical fits and estimates in the forward angular region.Comment: 43 pages, 21 figures, version published in Phys. Rev. D 96, 096001
(2017
Dispersive evaluation of the D-term form factor in deeply virtual Compton scattering
We present a dispersive representation of the D-term form factor for hard
exclusive reactions, using unsubtracted -channel dispersion relations. The
-channel unitarity relation is saturated with the contribution of two-pion
intermediate states, using the two-pion distributions amplitude for the
subprocess and reconstructing the
subprocess from available information on
pion-nucleon partial-wave helicity amplitudes. Results for the D-term form
factor as function of as well as at are discussed in comparison with
available model predictions and phenomenological parametrizations.Comment: 8 pages, 3 figures; improved presentation of the results; version to
appear in Phys. Lett.
Light-front interpretation of Proton Generalized Polarizabilities
We extend the recently developed formalism to extract light-front quark
charge densities from nucleon form factor data to the deformations of these
quark charge densities when applying an external electric field. We show that
the resulting induced polarizations can be extracted from proton generalized
polarizabilities. The available data for the generalized electric
polarizabilitiy of the proton yield a pronounced structure in its induced
polarization at large transverse distances, which will be pinned down by
forthcoming high precision virtual Compton scattering experiments.Comment: 4 pages, 4 figures, reference added, minor typos correcte
Dispersion analysis for generalized spin polarizabilities
We report on a dispersion relation formalism for the virtual Compton
scattering (VCS) reaction on the proton, which for the first time allows a
dispersive evaluation of 4 generalized polarizabilities. The dispersion
formalism provides a new tool to analyze VCS experiments above pion threshold,
thus increasing the sensitivity to the generalized polarizabilities of the
nucleon.Comment: 5pages, 2 figures, to appear in the Proceedings of the Symposium on
the Gerasimov-Drell-Hearn Sum Rule and the Spin Structure in the Nucleon
Resonance Region (GDH2000), June 14-17 2000, Mainz, German
Dispersion relation formalism for virtual Compton scattering off the proton
We present in detail a dispersion relation formalism for virtual Compton
scattering (VCS) off the proton from threshold into the
-resonance region. Such a formalism can be used as a tool to
extract the generalized polarizabilities of the proton from both unpolarized
and polarized VCS observables over a larger energy range. We present
calculations for existing and forthcoming VCS experiments and demonstrate that
the VCS observables in the energy region between pion production threshold and
the -resonance show an enhanced sensitivity to the generalized
polarizabilities.Comment: 51 pages, 15 figure
Sum rules for light-by-light scattering
We derive a set of sum rules for the light-by-light scattering and fusion:
, and verify them in lowest order QED calculations. A
prominent implication of these sum rules is the superconvergence of the
helicity-difference total cross-section for photon fusion, which in the hadron
sector reveals an intricate cancellation between the pseudoscalar and tensor
mesons. An experimental verification of superconvergence of the polarized
photon fusion into hadrons is called for, but will only be possible at and colliders with both beams polarized. We also show how
the sum rules can be used to measure various contributions to the low-energy
light-by-light scattering.Comment: 4 pages, 3 figures; minor corrections, published versio
Resonance estimates for single spin asymmetries in elastic electron-nucleon scattering
We discuss the target and beam normal spin asymmetries in elastic
electron-nucleon scattering which depend on the imaginary part of two-photon
exchange processes between electron and nucleon. We express this imaginary part
as a phase space integral over the doubly virtual Compton scattering tensor on
the nucleon. We use unitarity to model the doubly virtual Compton scattering
tensor in the resonance region in terms of
electroabsorption amplitudes. Taking those amplitudes from a phenomenological
analysis of pion electroproduction observables, we present results for beam and
target normal single spin asymmetries for elastic electron-nucleon scattering
for beam energies below 1 GeV and in the 1-3 GeV region, where several
experiments are performed or are in progress.Comment: 36 pages, 16 figure
Partonic calculation of the two-photon exchange contribution to elastic electron-proton scattering at large momentum transfer
We estimate the two-photon exchange contribution to elastic electron-proton
scattering at large momentum transfer through the scattering off a parton in
the proton. We relate the process on the nucleon to the generalized parton
distributions which also enter in other wide angle scattering processes. We
find that when taking the polarization transfer determinations of the form
factors as input, adding in the 2 photon correction, does reproduce the
Rosenbluth data.Comment: 4 pages, 4 figure
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