132 research outputs found
Generalized Parton Distributions and Nucleon Resonances
We discuss the relations between Generalized Parton Distributions (GPDs) and
nucleon resonances. We first briefly introduce the concept of "transition"
GPDs. Then we discuss a straightforward application to the modelization of the
N- magnetic transition form factor. Finally, we discuss the
experimental aspects of the subject and present first preliminary experimental
investigations in this field.Comment: 6 pages, 4 figures, Nstar02 conference proceedin
Eta photoproduction off the neutron at GRAAL: Evidence for a resonant structure at W=1.67 GeV
New (preliminary) data on eta photoproduction off the neutron are presented.
These data reveal a resonant structure at W=1.67 GeV.Comment: 8 pages, 4 figures. Published in Proceedings of Workshop on the
Physics of Excited Nucleons NSTAR2004, Grenoble, France, March 24 - 27,
pg.19
Eta photoproduction off the neutron at GRAAL
The gamma n -> eta n quasi-free cross section reveals a resonant structure at
W ~ 1.675 GeV. This structure may be a manifestation of a baryon resonance. A
priori its properties, the possibly narrow width and the strong photocoupling
to the neutron, look surprising. This structure may also signal the existence
of a narrow state.Comment: To appear in Proceedings of Workshop on the Physics of Excited
Nucleons NSTAR2005, 12 - 15 October 2005, Tallahassee, Florida, US
Eta photoproduction on the neutron at GRAAL: Measurement of the differential cross section
In this contribution, we will present our first preliminary measurement of
the differential cross section for the reaction gamma+n->eta+n. Comparison of
the reactions gamma+p->eta+p for free and bound proton (D2 target) will also be
discussed.Comment: 6 pages, 4 figures, Proceedings of the 10th International Symposium
on Meson-Nucleon Physics and the Structure of the Nucleon, August
29-September 4 2004, Beijing, Chin
Lowering the Light Speed Isotropy Limit: European Synchrotron Radiation Facility Measurements
The measurement of the Compton edge of the scattered electrons in GRAAL
facility in European Synchrotron Radiation Facility (ESRF) in Grenoble with
respect to the Cosmic Microwave Background dipole reveals up to 10 sigma
variations larger than the statistical errors. We now show that the variations
are not due to the frequency variations of the accelerator. The nature of
Compton edge variations remains unclear, thus outlining the imperative of
dedicated studies of light speed anisotropy
Precise Measurement of Sigma Beam Asymmetry for Positive Pion Photoproduction on the Proton from 800 to 1500 Mev
The Sigma beam asymmetry for positive pion photoproduction on the proton has
been measured over an angular range of 40-170 deg at photon energies from 0.8
to 1.5 GeV. The resulting data set includes 237 accurate points, 136 of these
belonging to an almost unexplored domain above 1.05 GeV. Data of such high
precision provide severe constraints for partial wave analyses. The influence
of this experiment on the GW multipole analysis is demonstrated. Significant
changes are found in multipoles connected to the S31(1620) and P13(1720)
resonances. Comparisons using the MAID analysis are also presented.Comment: 11 pages, 4 eps figures. to be published in Physics Letters
Differential cross section measurement of eta photoproduction on the proton from threshold to 1100 MeV
The differential cross section for the reaction p(gamma, eta p) has been
measured from threshold to 1100 MeV photon laboratory energy. For the first
time, the region of the S11(1535) resonance is fully covered in a
photoproduction experiment and allows a precise extraction of its parameters at
the photon point. Above 1000 MeV, S-wave dominance vanishes while a P-wave
contribution is observed whose nature will have to be clarified. These high
precision data together with the already measured beam asymmetry data will
provide stringent constraints on the extraction of new couplings of baryon
resonances to the eta meson.Comment: 10 pages, 5 figures, submitted to Phys. Letters B. Typos corrected.
Some more information on the S11(1535) parameter
Measurement of eta photoproduction on the proton from threshold to 1500 MeV
Beam asymmetry and differential cross section for the reaction gamma+p->eta+p
were measured from production threshold to 1500 MeV photon laboratory energy.
The two dominant neutral decay modes of the eta meson, eta->2g and eta->3pi0,
were analyzed. The full set of measurements is in good agreement with
previously published results. Our data were compared with three models. They
all fit satisfactorily the results but their respective resonance contributions
are quite different. The possible photoexcitation of a narrow state N(1670) was
investigated and no evidence was found.Comment: 18 pages, 14 figures, 4 tables Submitted to EPJ
Exclusive electroproduction on the proton at CLAS
The reaction has been measured, using the 5.754
GeV electron beam of Jefferson Lab and the CLAS detector. This represents the
largest ever set of data for this reaction in the valence region. Integrated
and differential cross sections are presented. The , and
dependences of the cross section are compared to theoretical calculations based
on -channel meson-exchange Regge theory on the one hand and on quark handbag
diagrams related to Generalized Parton Distributions (GPDs) on the other hand.
The Regge approach can describe at the 30% level most of the features
of the present data while the two GPD calculations that are presented in this
article which succesfully reproduce the high energy data strongly underestimate
the present data. The question is then raised whether this discrepancy
originates from an incomplete or inexact way of modelling the GPDs or the
associated hard scattering amplitude or whether the GPD formalism is simply
inapplicable in this region due to higher-twists contributions, incalculable at
present.Comment: 29 pages, 29 figure
- …