1,137 research outputs found
The structure of the nucleon
These lectures give an introduction to the structure of the nucleon as seen
with the electromagnetic probe. Particular emphasis is put on the form factors,
the strangeness content, Compton scattering and polarizabilities, pion photo-
and electroproduction, the spin structure and sum rules. The existing data are
compared to predictions obtained from chiral perturbation theory, dispersion
theory and effective Lagrangians.Comment: 57 pages LaTex including 15 postscript figures, Proceedings of HUGS
at CEBAF 199
The Gerasimov-Drell-Hearn sum rule and the single-pion photoproduction multipole E0+ close to threshold
The long-standing discrepancy between the Gerasimov-Drell-Hearn sum rule and
the analysis of pion photoproduction multipoles is greatly diminished by use of
s-wave multipoles that are in accord with the predictions of chiral
perturbation theory and describe the experimental data in the threshold region.
The remaining difference may be due to contributions of channels with more
pions and/or heavier mesons whose contributions to the sum rule remain to be
investigated by a direct measurement of the photoabsorption cross sections.Comment: 9 pages, latex, 1 figure, to appear in Phys. Rev.
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 Polarizabilities in a Constituent Quark Model
We discuss low-energy virtual Compton scattering off the proton within the
framework of a nonrelativistic constituent quark model. A simple interpretation
of the spin-averaged generalized polarizabilities is given in terms of the
induced electric polarization (and magnetization). Explicit predictions for the
generalized polarizabilities obtained from a multipole expansion are presented
for the Isgur-Karl model and are compared with results of various models.Comment: 10 pages, Latex, 2 figures, uses ws-p8-50x6-00.cls, talk given at
NSTAR2001, Workshop on "The Physics of Excited Nucleons," Mainz, Germany,
March 7-10, 200
Pion polarizabilities: No conflict between dispersion theory and ChPT
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 to non-analytic structuresComment: 9 pages, 4 figures; Proceedings of 6th International Workshop on
Chiral Dynamics, 6-10 July 2009, Bern, Switzerlan
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
Analysis of eta production using a generalized Lee model
We have investigated the processes N(, )N and N(, )N
close to eta threshold using a simple, nonrelativistic Lee model which has the
advantage of being analytically solvable. It is then possible to study the
Riemann sheets of the S-matrix and the behavior of its resonance poles
especially close to threshold. A theoretical simulation of the experimental
cusp effect at eta threshold leads to a characteristic distribution of poles on
the Riemann sheets. We find a pole located in the Riemann sheet that
up to now has not been discussed. It belongs to the cusp peak at eta threshold.
In addition we obtain the surprising result using the Lee model that the
resonance does not play a large role. The main features of the
experimental data can be reproduced without explicitly introducing this
resonance. Furthermore, we have also studied the reactions N(, )N
and N(, )N and find reasonable agreement between the data and
both models with and without the resonance.Comment: 17 pages LATEX including 13 Figurs, submitted to Z. Phys.
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