418 research outputs found
Dispersion relations and the spin polarizabilities of the nucleon
A forward dispersion calculation is implemented for the spin polarizabilities
of the proton and the neutron. These polarizabilities
are related to the spin structure of the nucleon at low energies and are
structure-constants of the Compton scattering amplitude at . In the absence of a direct experimental measurement of these
quantities, a dispersion calculation serves the purpose of constraining the
model building, and of comparing with recent calculations in heavy baryon
chiral perturbation theory.Comment: 14 pages Latex, 1 ps figur
The E2/M1 and C2/M1 ratios and form factors in N -> Delta transitions
A partial wave analysis of pion photoproduction has been obtained in the
framework of fixed-t dispersion relations valid from threshold up to 500 MeV.
In the resonance region we have precisely determined the electromagnetic
properties of the \Delta(1232) resonance, in particular the E2/M1 ratio
R_{EM}=(-2.5 +- 0.1) %. For pion electroproduction recent experimental data
from Mainz, Bates and JLab for Q^2 up to 4.0 (GeV/c)^2 have been analyzed with
two different models, an isobar model (MAID) and a dynamical model. The E2/M1
ratios extracted with these two models show, starting from a small and negative
value at the real photon point, a clear tendency to cross zero, and become
positive with increasing Q^2. This is a possible indication of a very slow
approach toward the pQCD region. The C2/M1 ratio near the photon point is found
as R_{SM}(0)=(-6.5 +- 0.5) %. At high Q^2 the absolute value of the ratio is
strongly increasing, a further indication that pQCD is not yet reached.Comment: 10 pages LaTeX including 3 figures. Talk given at the XVIIth European
conference on Few-Body Problems in Physics, Evora, Portugal, 11 - 16
September 2000; to be published in Nucl.Phys.
Gender Mainstreaming: Who Wins? Gender & Irrigated Urban Vegetable Production in West Africa
Gender roles in agriculture can be quite specific, not only in view of particular labor inputs during the production cycle but also in terms of who farms and who trades certain crops. Using data collected over ten years in West Africa, this study looked at market-oriented urban vegetable production in West Africa and Ghana in particular. Gender disaggregated data on key issues such as access and control of resources, division of tasks, decision-making process and challenges faced was collected from farmers and traders. With several exceptions, a clear gender distinction emerged across the sub-region: men dominate urban vegetable farming, while women manage vegetable marketing. The general differentiation is attributed to societal norms, but other factors play a role as well. Female farmers, for example, feel constrained by existing irrigation practices that are energy-intensive and conflict with household duties. Male farmers, on the other hand, feel significantly oppressed by their dependency on credit and prices dictated by market women, and feel disadvantaged when entering the vegetable retail market. Improved irrigation technology could facilitate a better gender balance on the farm, but mainstreaming gender balance in vegetable wholesale and retail is likely to disadvantage women
On-farm treatment options for wastewater, greywater and fecal sludge with special reference to West Africa
Low-energy Compton scattering on the nucleon and sum rules
The Gerasimov-Drell-Hearn and Baldin-Lapidus sum rules are evaluated in the
dressed K-matrix model for photon-induced reactions on the nucleon. For the
first time the sum of the electric and magnetic polarisabilities
and the forward spin polarisability are explicitly calculated in two
alternative ways -- from the sum rules and from the low-energy expansion of the
real Compton scattering amplitude -- within the {\em same} framework. The two
methods yield compatible values for but differ somewhat for
. Consistency between the two ways of determining the
polarisabilities is a measure of the extent to which basic symmetries of the
model are obeyed.Comment: 9 pages, 4 figures, using REVTeX. More concise version, results
unchanged. To appear in Phys. Rev.
A dispersion theoretical approach to the threshold amplitudes of pion photoproduction
We give predictions for the partial wave amplitudes of pion photoproduction
near threshold by means of dispersion relations at fixed t. The free parameters
of this approach are determined by a fit to experimental data in the energy
range 160 MeV 420 MeV. The observables near threshold are
found to be rather sensitive to the amplitudes in the resonance region, in
particular to the (1232) and (1440). We obtain a good agreement
with the existing threshold data for both charged and neutral pion production.
Our predictions also agree well with the results of chiral perturbation theory,
except for neutral pion production off the neutron.Comment: 16 pages LATEX including 4 postscript figure
Compton scattering in a unitary approach with causality constraints
Pion-loop corrections for Compton scattering are calculated in a novel
approach based on the use of dispersion relations in a formalism obeying
unitarity. The basic framework is presented, including an application to
Compton scattering. In the approach the effects of the non-pole contribution
arising from pion dressing are expressed in terms of (half-off-shell) form
factors and the nucleon self-energy. These quantities are constructed through
the application of dispersion integrals to the pole contribution of loop
diagrams, the same as those included in the calculation of the amplitudes
through a K-matrix formalism. The prescription of minimal substitution is used
to restore gauge invariance. The resulting relativistic-covariant model
combines constraints from unitarity, causality, and crossing symmetry.Comment: 25 pages, 9 ps-figure
The position and the residues of the delta resonance pole in pion photoproduction
We have analyzed the and multipole
amplitudes of pion photoproduction in the framework of fixed- dispersion
relations. Applying the speed plot technique to our results for these
multipoles, we have determined the position and the residues of the
(1232) resonance pole. The pole is found at total energy MeV on the second Riemann sheet, and the ratio of the electric and
magnetic residues is , resulting in an E2/M1
ratio for the "dressed" delta resonance of .Comment: 16 pages LATEX including 5 postscript figures in a self-extracting
uufile archiv
The GDH Sum Rule and Related Integrals
The spin structure of the nucleon resonance region is analyzed on the basis
of our phenomenological model MAID. Predictions are given for the
Gerasimov-Drell-Hearn sum rule as well as generalized integrals over spin
structure functions. The dependence of these integrals on momentum transfer is
studied and rigorous relationships between various definitions of generalized
Gerasimov-Drell-Hearn integrals and spin polarizabilities are derived. These
results are compared to the predictions of chiral perturbation theory and
phenomenological models.Comment: 15 pages LaTeX including 5 figure
Higher moments of nucleon spin structure functions in heavy baryon chiral perturbation theory and in a resonance model
The third moment of the twist-3 part of the nucleon spin structure
function is generalized to arbitrary momentum transfer and is
evaluated in heavy baryon chiral perturbation theory (HBChPT) up to order
and in a unitary isobar model (MAID). We show how to link
as well as higher moments of the nucleon spin structure functions
and to nucleon spin polarizabilities. We compare our results with the
most recent experimental data, and find a good description of these available
data within the unitary isobar model. We proceed to extract the twist-4 matrix
element which appears in the suppressed term in the twist
expansion of the spin structure function for proton and neutron.Comment: 30 pages, 7 figure
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