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The relativistically covariant constituent quark model developed by the Bonn group is used to compute the EM form factors of strange baryons. We present form-factor results for the ground-state and some excited hyperons. The computed magnetic moments agree well with the experimental values and the magnetic form factors follow a dipole $Q^2$ dependence., Comment: 4 pages, 1 figure, Proceedings for NSTAR '04 conference in Grenoble, France, March 24-27, 2004 (World Scientific)

The problem of extracting information on new and known $N^{*}$ resonances by fitting isobar models to photonuclear data is addressed. A new fitting strategy, incorporating a genetic algorithm, is outlined. As an example, the method is applied to a typical tree-level analysis of published $p(\gamma,K^{+})\Lambda$ data. It is shown that, within the limitations of this tree-level analysis, a resonance in addition to the known set is required to obtain a reasonable fit. An additional $P_{11}$ resonance, with a mass of about 1.9 GeV, gives the best agreement with the published data, but additional $S_{11}$ or $D_{13}$ resonances cannot be ruled out. Our genetic algorithm method predicts that photon beam asymmetry and double polarization $p(\gamma,K^{+})\Lambda$ measurements should provide the most sensitive information with respect to missing resonances., Comment: 34 pages, 11 figures; Revised version accepted for publication by Nuclear Physics A

The reaction $\gamma p \to \pi^0 \pi^0 p$ has been measured using the TAPS BaF$_2$ calorimeter at the tagged photon facility of the Mainz Microtron accelerator. Chiral perturbation theory (ChPT) predicts that close to threshold this channel is significantly enhanced compared to double pion final states with charged pions. In contrast to other reaction channels, the lower order tree terms are strongly suppressed in 2$\pi^0$ photoproduction. The consequence is the dominance of pion loops in the 2$\pi^0$ channel close to threshold - a result that opens new prospects for the test of ChPT and in particular its inherent loop terms. The present measurement is the first which is sensitive enough for a conclusive comparison with the ChPT calculation and is in agreement with its prediction. The data also show good agreement with a calculation in the unitary chiral approach., Comment: Submitted to PLB

Predictions for the electromagnetic form factors of the Lambda$, Sigma and Xi hyperons are presented. The numerical calculations are performed within the framework of the fully relativistic constituent-quark model developed by the Bonn group. The computed magnetic moments compare favorably with the experimentally known values. Most magnetic form factors G_M(Q^2) can be parametrized in terms of a dipole with cutoff masses ranging from 0.79 to 1.14 GeV., Comment: 15 pages, 8 figures, 3 tables, submitted to Eur. Phys. J. A

At sufficiently large proton energies, Glauber multiple-scattering theory offers good opportunities for describing the final state interactions in electro-induced proton emission off nuclear targets. A fully unfactorized relativistic formulation of Glauber multiple-scattering theory is presented. The effect of truncating the Glauber multiple-scattering series is discussed. Relativistic effects in the description of the final-state interactions are found not to exceed the few percent level. Also the frequently adopted approximation of replacing the wave functions for the individual scattering nucleons by some average density, is observed to have a minor impact on the results. Predictions for the separated $^4$He$(e,e'p)$ response functions are given in quasi-elastic kinematics and a domain corresponding with $1 \leq Q^2 \leq 2$ (GeV)$^2$., Comment: 30 pages, 12 figures

Nucleon resonance information is often obtained from fitting hadrodynamical model calculations to data, where model parameters such as resonance coupling constants are the free parameters in the fitting procedure. For reactions with a limited data set, such as p(gamma, K+)Lambda, complications in the extraction of reliable N* information occur not only through theoretical uncertainties, but also due to technical difficulties in the fitting procedure. In this article we outline a fitting strategy based on a genetic algorithm and illustrate the kind of ambiguities which can arise., Comment: 8 pages, 6 figures, submitted to Phys. Lett. B

Results for response functions for kaon electroproduction on the proton are presented. A tree-level hadrodynamical model is adopted and it is shown that some of the electroproduction response functions are particularly powerful with the eye on gaining control over the parameterization of the background diagrams. The existing data set for the p(e,e'K+)Lambda reaction appears to rule out the use of a g_{K+ Lambda p} coupling constant beyond the boundaries of softly broken SU(3) flavor symmetry. Also the use of soft hadronic form factors, which has been proposed as a valid alternative for a hadrodynamical description of the p(gamma,K+)Lambda data in the resonance region, seems to be disfavored by the magnitude of the measured p(e,e'K+)Lambda cross sections., Comment: Accepted for publication in PRC. Includes new data, additional paragraph and figure

A study of p(gamma,K)Sigma processes in an isobar model at tree level is reported. By comparing model calculations to the published SAPHIR data, we explore the possible role of different isospin I=1/2 (N*) and I=3/2 (Delta*) resonances in the reaction dynamics. In our analysis, the inclusion of the ``missing'' D_{13}(1895) resonance does only slightly improve the global description of the Sigma photoproduction data. More convincing signals for the presence of such a ``missing'' resonance emerged in the analysis of the isospin related p(gamma,K+)Lambda reaction. Various implementations of the nonresonant part of the Sigma photoproduction amplitude are presented. The sensitivity of the computed observables and extracted resonance parameters to the uncertainties inherent to the treatment of the nonresonant (background) diagrams are discussed., Comment: subsection and table added, to appear in Phys. Rev. C

We outline a relativistic and unfactorized framework to treat the final-state interactions in quasi-elastic A(e,e'p) reactions for four-momentum transfers Q$^{2} \gtrsim 0.3$ (GeV/c)$^{2}$. The model, which relies on the eikonal approximation, can be used in combination with optical potentials, as well as with the Glauber multiple-scattering method. We argue that such a model can bridge the gap between a typical ``low'' and ``high-energy'' description of final-state interactions, in a reasonably smooth fashion. This argument is made on the basis of calculated structure functions, polarization observables and nuclear transparencies for the target nuclei $^{12}$C and $^{16}$O., Comment: revised version

The photoproduction p(gamma, K+)Lambda process is studied within a field-theoretic approach. It is shown that the background contributions constitute an important part of the reaction dynamics. We compare predictions obtained with three plausible techniques for dealing with these background contributions. It appears that the extracted resonance parameters drastically depend on the applied technique. We investigate the implications of the corrections to the functional form of the hadronic form factor in the contact term, recently suggested by Davidson and Workman (Phys. Rev. C 63, 025210). The role of background contributions and hadronic form factors for the identification of the quantum numbers of ``missing'' resonances is discussed., Comment: 11 pages, 7 eps figures, submitted to Phys. Rev. C

We discuss the role of hyperon resonances in the u-channel when modeling p (gamma,K+)Lambda processes in an effective Lagrangian approach. Without the introduction of hyperon resonances, one is forced to use soft hadronic form factors with a cutoff mass which is at best two times the kaon mass. After inclusion of the hyperon resonances in the u-channel, we obtain a fair description of the data with a cutoff mass of the order of 1.8 GeV., Comment: 7 pages, 7 eps figures, submitted to Eur. Phys. J. A

A fully relativistic model for the description of exclusive (e,e'p) reactions off nuclear targets at high energies and momentum transfers is outlined. It is based on the eikonal approximation for the ejectile scattering wave function and a relativistic mean-field approximation to the Walecka model. Results for ^{12}C(e,e'p) and ^{16}O(e,e'p) differential cross sections and separated structure functions are presented for four-momenta in the range 0.8 \leq Q^{2} \leq 20 (GeV/c)^{2}. The regions of applicability of the eikonal approximation are studied and observed to be confined to proton knockout in a relatively small cone about the momentum transfer. A simple criterium defining the boundaries of this cone is determined. The Q^2 dependence of the effect of off-shell ambiguities on the different (e,e'p) structure functions is addressed. At sufficiently high values of Q^2 their impact on the cross sections is illustrated to become practically negligible. It is pointed out that for the whole range of Q^2 values studied here, the bulk of the relativistic effects arising from the coupling between the lower components in the wave functions, is manifesting itself in the longitudinal-transverse interference term., Comment: 13 pages,11 figures

The effect of tensor nucleon-nucleon correlations upon exclusive and semi-exclusive electronuclear reactions is studied. Differential cross sections for the semi-exclusive ^{16}O(e,e'p) and exclusive ^{16}O(e,e'pn) processes are computed by explicitly evaluating the dynamical electromagnetic coupling to a tensor correlated nucleon pair. In both reaction channels the tensor correlations contribute in a very substantial way. Tensor correlations are found to generate more electronuclear strength than central Jastrow correlations do., Comment: 5 pages, 2 figures, Accepted for publication in Phys. Rev. C