Exclusive electromagnetic production of strangeness on the nucleon : review of recent data in a Regge approach

In view of the numerous experimental results recently released, we provide in this letter an update on the performance of our simple Regge model for strangeness electroproduction on the nucleon. Without refitting any parameters, a decent description of all measured observables and channels is achieved. We also give predictions for spin transfer observables, recently measured at Jefferson Lab which have high sensitivity to discriminate between different theoretical approaches.Comment: 5 pages, 5 figure

A fitter code for Deep Virtual Compton Scattering and Generalized Parton Distributions

We have developped a fitting code based on the leading-twist handbag Deep Virtual Compton Scattering (DVCS) amplitude in order to extract the Generalized Parton Distributions (GPD) information from DVCS observables in the valence region. In a first stage, with simulations and pseudo-data, we show that the full GPD information can be recovered from experimental data if enough observables are measured. If only part of these observables are measured, valuable information can still be extracted, certain observables being particularly sensitive to certain GPDs. In a second stage, we make a practical application of this code to the recent DVCS Jefferson Lab Hall A data from which we can extract numerical constraints for the two $H$ GPD Compton Form Factors.Comment: 15 pages, 8 figure

Nucleon Form Factors from Generalized Parton Distributions

We discuss the links between Generalized Parton Distributions (GPDs) and elastic nucleon form factors. These links, in the form of sum rules, represent powerful constraints on parametrizations of GPDs. A Regge parametrization for GPDs at small momentum transfer, is extended to the large momentum transfer region and it is found to describe the basic features of proton and neutron electromagnetic form factor data. This parametrization is used to estimate the quark contribution to the nucleon spin.Comment: 18 pages, 5 figures, replaced with published versio

Double deeply virtual Compton scattering off the nucleon

We study the double deeply virtual Compton scattering (DDVCS) process off the nucleon, through the scattering of a spacelike virtual photon with large virtuality resulting in the production of a timelike virtual photon, decaying into an e^+ e^- pair. This process is expressed in the Bjorken regime in terms of generalized parton distributions (GPDs) and it is shown that by varying the invariant mass of the lepton pair, one can directly extract the GPDs from the observables. We give predictions for the DDVCS cross section and beam helicity asymmetry and discuss its experimental feasibility.Comment: 4 pages, 4 figure

Variation de la densité et la couleur du bois de Pterocarpus erinaceus (Poir) en fonction des conditions environnementales en Afrique de l’Ouest

National audiencePterocarpus erinaceus (Poir.) is currently spontaneous species of Guinea-Sudanese and Sudano-Sahelian areas overexploited and threatened in West Africa. This paper analyzes the density and color of the wood of the species in the agro-ecological zones of three countries in West Africa (Togo, Burkina Faso and Niger). The results of 452 samples collected from 95 trees from the three countries show significant differences in values of basic density and color parameters (P <0.05) according to agro-ecological zones (Sudan, Guinea and Sahel). At scale intra-tree, even if there is an increase in density with the number of rings counted from the pith, this increase is very small (R²≤ 0.18). This reflects the fact that age has little influence on the trees density. In addition, a significant correlation was observed between density and color parameters.Pterocarpus erinaceus (Poir.) est actuellement l’espèce spontanée des zones guinéo-soudaniennes et soudano-sahéliennes très exploitée et menacée en Afrique de l’Ouest. Le présent travail analyse la densité et la couleur du bois de l’espèce dans les zones agroécologiques de trois pays de l’Afrique de l’Ouest (Togo, Burkina Faso et Niger). Les résultats obtenus sur 452 éprouvettes prélevées de 95 arbres issus des 3 pays indiquent des différences significatives des valeurs de l’infradensité et des paramètres de couleur (P<0,05) en fonction des zones agro-écologiques (soudanienne, guinéenne et sahélienne). A l’échelle intra-arbre, même s’il ya une augmentation de la densité en fonction du nombre de cernes comptés, depuis la moelle, cette augmentation est très faible (R²≤ 0,18). Ce qui traduit le fait que l’âge des arbres influence peu l’infradensité. Par ailleurs une corrélation significative a été observée entre la densité et les paramètres de couleur

Dispersion Theory and the Low Energy Constants for Neutral Pion Photoproduction

The relativistic amplitudes of pion photoproduction are evaluated by dispersion relations at t=const. The imaginary parts of the amplitudes are taken from the MAID model covering the absorption spectrum up to center-of-mass energies W = 2.2 GeV. For sub-threshold kinematics the amplitudes are expanded in powers of the two independent variables \nu and t related to energy and momentum transfer. Subtracting the loop corrections from this power series allows one to determine the counter terms of covariant baryon chiral perturbation theory. The proposed continuation of the amplitudes into the unphysical region provides a unique framework to derive the low-energy constants to any given order as well as an estimate of the higher order terms by global properties of the absorption spectrum.Comment: 13 pages, 6 figures, 5 table

Quark model description of quasi-elastic pion knockout from the proton at JLAB

The interference term between s- and t-pole contributions to the p(e,e' pi+)n cross section is evaluated on the basis of the constituent quark model. It is shown that the contribution of baryon s-poles can be modeled by a nonlocal extension of the Kroll-Rudermann contact term. This contribution is in a destructive interference with the pion t-pole that is essential to improve the description of recent JLab data at the invariant mass W=1.95 GeV. Some predictions are made for a new JLab measurement at higher values W=2.1-2.3 GeV and Q2 centered at 1.6 and 2.45 GeV2/c2.Comment: 15 pages, 4 figures, to be published in Phys. Lett.

Hard exclusive electroproduction of decuplet baryons in the large N_c limit

The cross sections and transverse spin asymmetries in the hard exclusive electroproduction of decuplet baryons are calculated in the large N_c limit and found to be comparable to that of octet baryons. Large N_c selection rules for the production amplitudes are derived, leading to new sensitive tests of the spin aspects of the QCD chiral dynamics both in the nonstrange and strange sectors. Importance of such studies for the reliable extraction of the pion form factor from pion electroproduction is explained.Comment: 4 pages, 3 figure

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

Measurement of Unpolarized and Polarized Cross Sections for Deeply Virtual Compton Scattering on the Proton at Jefferson Laboratory with CLAS

This paper reports the measurement of polarized and unpolarized cross sections for the ep→e′p′γ reaction, which is composed of deeply virtual Compton scattering (DVCS) and Bethe-Heitler (BH) processes, at an electron beam energy of 5.88 GeV at the Thomas Jefferson National Accelerator Facility using the Large Acceptance Spectrometer CLAS. The unpolarized cross sections and polarized cross section differences have been measured over broad kinematics, 0.10 \u3c xB \u3c 0.58, 1.0 \u3c Q2 \u3c4.8 GeV2 and 0.09\u3c−t \u3c2.00 GeV2. The results are found to be consistent with previous CLAS data, and these new data are discussed in the framework of the generalized parton distribution approach. Calculations with two widely used phenomenological models are approximately compatible with the experimental results over a large portion of the kinematic range of the data