72 research outputs found
Beam normal spin asymmetry in the quasi-RCS approximation
The two-photon exchange contribution to the single spin asymmetries with the
spin orientation normal to the reaction plane is discussed for elastic
electron-proton scattering in the equivalent photon approximation. In this
case, hadronic part of the two-photon exchange amplitude describes real Compton
scattering (RCS). We show that in the case of the beam normal spin asymmetry,
this approximation selects only the photon helicity flip amplitudes of RCS. At
low energies, we make use of unitarity and estimate the contribution of the
multipoles to the photon helicity flip amplitudes. In the Regge regime,
QRCS approximation allows for a contribution from two pion exchange, and we
provide an estimate of such contributions. We furthermore discuss the
possibility of the quark and gluon GPD's contributions in the QRCS kinematics.Comment: 10 pages, 5 figures, revtex, submitted to Phys. Rev. C; new version:
references adde
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 corrections to parity violating electron scattering
We consider the dispersion correction to elastic parity violating
electron-proton scattering due to \gammaZ exchange. In a recent publication,
this correction was reported to be substantially larger than the previous
estimates. In this paper, we study the dispersion correction in greater detail.
We confirm the size of the disperion correction to be 6% for the QWEAK
experiment designed to measure the proton weak charge. We enumerate parameters
that have to be constrained to better than relative 30% in order to keep the
theoretical uncertainty for QWEAK under control.Comment: 6 pages, 3 figures, 2 tables; To be published in the proceedings of
the VIII Latin American Symposium on Nuclear Physics and Applications,
December 15-19, 2009, Santiago, Chiil
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
Analyzing power in elastic scattering of the electrons off a spin-0 target
We consider the analyzing power on a spin-0 nuclear target. This observable
is related to the imaginary part of the two-photon-exchange (box) diagram. We
consider the contributions of elastic and inelastic intermediate states. The
former requires knowledge of the elastic nuclear form factor, while the latter
uses the optical theorem as input. The elastic contribution scales as the
nuclear charge , while the inelastic contribution as the ratio of the atomic
number and nuclear charge, . We provide estimates for He and
Pb, in the kinematics of existing or upcoming experiments. In both
cases, we predict negative values of a few parts per million, and the dominant
contribution is due to inelastic contributions. The analyzing power can
contribute a substantial systematic error in parity-violating experiments.Comment: 6 pages, 8 figure, one table; submitted to Phys.Rev.C; one figure
corrrecte
Dispersion -box correction to the weak charge of the proton
We consider elastic scattering of electrons off a proton target. The parity
violating (PV) asymmetry arises at leading order in due to
interference of and exchange. The radiative corrections to this
leading mechanism were calculated in the literature and included in
experimental analyses, except for box and cross-box contributions.
We present here a dispersion calculation of these corrections in forward
kinematics. We demonstrate that at the GeV energies of current PV experiments,
such corrections are not suppressed by the small vector weak charge of the
electron, as occurs in the atomic PV. Our results suggest that the current
theoretical uncertainty in the analysis of the QWEAK experiment might be
substantially underestimated, and more accurate account of the dispersion
corrections are needed in order to interprete the PV data.Comment: 4 pages, 3 figures, revtex
Fiber bundle model with stick-slip dynamics
We propose a generic model to describe the mechanical response and failure of
systems which undergo a series of stick-slip events when subjected to an
external load. We model the system as a bundle of fibers, where single fibers
can gradually increase their relaxed length with a stick-slip mechanism
activated by the increasing load. We determine the constitutive equation of the
system and show by analytical calculations that on the macro-scale a plastic
response emerges followed by a hardening or softening regime. Releasing the
load, an irreversible permanent deformation occurs which depends on the
properties of sliding events. For quenched and annealed disorder of the failure
thresholds the same qualitative behavior is found, however, in the annealed
case the plastic regime is more pronounced.Comment: 4 pages, 4 figure
Generalized sum rules of the nucleon in the constituent quark model
We study the generalized sum rules and polarizabilities of the nucleon in the
framework of the hypercentral constituent quark model. We include in the
calculation all the well known and resonances and consider all the
generalized sum rules for which there are data available. To test the model
dependence of the calculation, we compare our results to the results obtained
in the harmonic oscillator CQM. We furthermore confront our results to the
model-independent sum rules values and to the predictions of the
phenomenological MAID model. The CQM calculations provide a good description of
most of the presented generalized sum rules in the intermediate region
(above GeV) while they encounter difficulties in describing these
observables at low , where the effects of the pion cloud, not included in
the present calculation, are expected to be important.Comment: 26 pages, 10 figure
Quark-nucleon dynamics and Deep Virtual Compton Scattering
We consider deeply virtual Compton scattering and deep inelastic scattering
in presence of Regge exchanges that are part of the non-perturbative
quark-nucleon amplitude. In particular we discuss contribution from the Pomeron
exchange and demonstrate how it leads to Regge scaling of the Compton
amplitude. Comparison with HERA data is given.Comment: 12 pages, 5 figure
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