499 research outputs found
Multipole analysis for pion photoproduction with MAID and a dynamical models
Results of new analysis of pion photoproduction data obtained with Dynamical
and MAID models are presentedComment: 5 pages, 1 figure, talk given at the NSTAR2001 Workshop, Mainz,
Germany, March 7-10, 200
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.
Singularity structure of the pi N scattering amplitude in a meson-exchange model up to energies W < 2.0 GeV
Within the previously developed Dubna-Mainz-Taipei meson-exchange model, the
singularity structure of the pi N scattering amplitudes has been investigated.
For all partial waves up to F waves and c.m. energies up to W = 2 GeV, the
T-matrix poles have been calculated by three different techniques: analytic
continuation into the complex energy plane, speed-plot and the regularization
method. For all 4-star resonances, we find a perfect agreement between the
analytic continuation and the regularization method. We also find resonance
poles for resonances that are not so well established, but in these cases the
pole positions and residues obtained by analytic continuation can substantially
differ from the results predicted by the speed-plot and regularization methods.Comment: 21 pages, 4 figures, 4 table
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
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
Real and Virtual Compton Scattering: the nucleon polarisabilities
We give an overview of low-energy Compton scattering (gamma^(*) p --> gamma
p) with a real or virtual incoming photon. These processes allow the
investigation of one of the fundamental properties of the nucleon, i.e. how its
internal structure deforms under an applied static electromagnetic field. Our
knowledge of nucleon polarisabilities and their generalization to non-zero
four-momentum transfer will be reviewed, including the presently ongoing
experiments and future perspectives.Comment: 20 pages, 12 figures. Minireview/Proceedings of "Many-Body Structure
of Strongly Interacting Systems", Mainz, Germany, Feb. 23-25 2011 . V2: typos
corrected. version to appear in EPJ Special Topic
Higher Order Polarizabilities of the Proton
Compton scattering results are used to probe proton structure via measurement
of higher order polarizabilities. Values for
determined via
dispersion relations are compared to predictions based upon chiral symmetry and
from the constituent quark model. Extensions to spin-polarizabilities are also
discussed.Comment: 18 pages, revised version with one reference adde
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