1,194 research outputs found
Dispersion relation formalism for virtual Compton scattering and the generalized polarizabilities of the nucleon
A dispersion relation formalism for the virtual Compton scattering (VCS)
reaction on the proton is presented, which for the first time allows a
dispersive evaluation of 4 generalized polarizabilities at a four-momentum
transfer 0.5 GeV. The dispersive integrals are calculated using
a state-of-the-art pion photo- and electroproduction analysis. 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: 4 pages, 2 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
Rural-urban food, nutrient and virtual water flows in selected West African cities
Food consumption / Water quality / Nutrients / Urban agricuture / Food production
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
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
The Counting of Generalized Polarizabilities
We demonstrate a concise method to enumerate the number of generalized
polarizabilities---quantities characterizing the independent observables in
singly-virtual Compton scattering---for a target particle of arbitrary spin s.
By using crossing symmetry and J^{PC} conservation, we show that this number is
(10s+1+delta_{s,0}).Comment: 10 pages, revtex4, no figures. Version to appear in Phys. Rev. D.
Paper now divided into sections and clarifying comments added, but physics
content unchange
Electromagnetic Excitation of Nucleon Resonances
Recent progress on the extraction of electromagnetic properties of nucleon
resonance excitation through pion photo- and electroproduction is reviewed.
Cross section data measured at MAMI, ELSA, and CEBAF are analyzed and compared
to the analysis of other groups. On this basis, we derive longitudinal and
transverse transition form factors for most of the four-star nucleon
resonances. Furthermore, we discuss how the transition form factors can be used
to obtain empirical transverse charge densities. Contour plots of the thus
derived densities are shown for the Delta, Roper, S11, and D13 nucleon
resonances.Comment: 30 pages, 19 figures, 9 table
Informal irrigation in urban West Africa: An overview
Irrigated farming / Urban agriculture / Suburban agriculture / Farm size / Farming systems / Health hazards / Water pollution / Farm income
The polarizability of the pion: no conflict between dispersion theory and chiral perturbation theory
Recent attempts to determine the pion polarizability by dispersion relations
yield values that disagree with the predictions of chiral perturbation theory.
These dispersion relations are based on specific forms for the absorptive part
of the Compton amplitudes. The analytic properties of these forms are examined,
and the strong enhancement of intermediate-meson contributions is shown to be
connected with spurious singularities. If the basic requirements of dispersion
relations are taken into account, the results of dispersion theory and
effective field theory are not inconsistent.Comment: 30 pages, 8 figures, 6 table
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