450 research outputs found
Electroproduction of strangeness above the resonance region
A simple and elegant model, based on Reggeized t-channel exchanges is
succesful in reproducing the main features of all existing data of the
reactions ep->eKLambda and ep->eKSigma. In particular, the original way gauge
invariance is taken into account is found to be essential to describe the ratio
between the Coulomb and the Transverse cross-sections at large Q2 that has been
measured recently at JLab.Comment: 5 pages, 5 figures, submitted to Phys.Rev.Let
Electromagnetic properties of baryons
We discuss the chiral behavior of nucleon and Delta(1232) electromagnetic
properties within the framework of a SU(2) covariant baryon chiral perturbation
theory. Our one-loop calculation is complete to the order p^3 and p^4/Delta
with Delta as the Delta(1232)-nucleon energy gap. We show that the magnetic
moment of a resonance can be defined through the linear energy shift only when
an additional relation between the involved masses and the applied magnetic
field strength is fulfilled. Singularities and cusps in the pion mass
dependence of the Delta(1232) electromagnetic moments reflect a
non-fulfillment. We show results for the pion mass dependence of the nucleon
iso-vector electromagnetic quantities and present results for finite volume
effects on the iso-vector anomalous magnetic moment.Comment: 4 pages, 3 figures, prepared for Proceedings of the International
Conference on the Structure of Baryons (BARYONS'10), Osaka, Japan, Dec. 7-11,
201
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
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
Generalized Parton Distributions and Nucleon Resonances
We discuss the relations between Generalized Parton Distributions (GPDs) and
nucleon resonances. We first briefly introduce the concept of "transition"
GPDs. Then we discuss a straightforward application to the modelization of the
N- magnetic transition form factor. Finally, we discuss the
experimental aspects of the subject and present first preliminary experimental
investigations in this field.Comment: 6 pages, 4 figures, Nstar02 conference proceedin
Light-by-light scattering sum rules constraining meson transition form factors
Relating the forward light-by-light scattering to energy weighted integrals
of the \gamma* \gamma -fusion cross sections, with one real photon (\gamma) and
one virtual photon (\gamma*), we find two new exact super-convergence
relations. They complement the known super-convergence relation based on the
extension of the GDH sum rule to the light-light system. We also find a set of
sum rules for the low-energy photon-photon interaction. All of the new
relations are verified here exactly at leading order in scalar and spinor QED.
The super-convergence relations, applied to the \gamma* \gamma -production of
mesons, lead to intricate relations between the \gamma \gamma -decay widths or
the \gamma* \gamma -transition form factors for (pseudo-) scalar, axial-vector
and tensor mesons. We discuss the phenomenological implications of these
results for mesons in both the light-quark sector and the charm-quark sector.Comment: 32 pages, 7 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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