Electromagnetic properties of strange baryons in a relativistic quark model

We present some of our results for the electromagnetic properties of excited Σ hyperons, computed within the framework of the Bonn constituent-quark model, which is based on the Bethe-Salpeter approach. The seven parameters entering the model are fitted against the best-known baryon masses. Accordingly, the results for the form factors and helicity amplitudes are genuine predictions. We compare with the scarce experimental data available and discuss the processes in which Σ *'s may play an important role

Bridging Two Ways of Describing Final-State Interactions in A(e,e'p) Reactions

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 versio

Electromagnetic KY production from the proton in a Regge-plus-resonance approach

A Regge-plus-resonance (RPR) description of the p(\gamma,K)Y and p(e,e'K)Y processes (Y = \Lambda, \Sigma^{0,+}) is presented. The proposed reaction amplitude consists of Regge-trajectory exchanges in the t channel, supplemented with a limited selection of s-channel resonance diagrams. The RPR framework contains a considerably smaller number of free parameters than a typical effective-Lagrangian model. Nevertheless, it provides an acceptable overall description of the photo- and electroproduction observables over an extensive photon energy range. It is shown that the electroproduction response functions and polarization observables are particularly useful for fine-tuning both the background and resonance parameters.Comment: 4 pages, 3 figures, Proceedings for IX International Conference on Hypernuclear and Strange Particle Physics (HYP2006), October 10-14 2006, Main

Electromagnetic form factors of hyperons in a relativistic quark model

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 role of hyperon resonances in p(gamma,K+)Lambda processes

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.

Regge-plus-resonance treatment of the p(gamma,K^+)Sigma^0 and p(gamma,K^0)Sigma^+ reactions at forward kaon angles

An effective-Lagrangian framework for K Sigma photoproduction from the proton is presented. The proposed model is applicable at forward kaon angles and photon lab energies from threshold up to 16 GeV. The high-energy part of the p(gamma,K^+)Sigma^0 and p(gamma,K^0)Sigma^+ amplitudes is expressed in terms of Regge-trajectory exchange in the t channel. By supplementing this Regge background with a number of s-channel resonances, the model is extended towards the resonance region. The resulting ``Regge-plus-resonance'' (RPR) approach has the advantage that the background contributions involve only a few parameters, which can be largely constrained by the high-energy data. This work compares various implementations of the RPR model, and explores which resonance contributions are required to fit the data presently at hand. It is demonstrated that, through the inclusion of one K and two K* trajectories, the RPR framework provides an efficient and unified description of the K^+ Sigma^0 and K^0 Sigma^+ photoproduction channels over an extensive energy range.Comment: 33 pages, 15 figures; added discussion on new double-polarization data (Cx and Cz) in Section I

Forward-angle K+ Lambda photoproduction in a Regge-plus-resonance approach

We present an effective-Lagrangian description for forward-angle K+ Lambda photoproduction from the proton, valid for photon lab energies from threshold up to 16 GeV. The high-energy part of the amplitude is modeled in terms of t-channel Regge-trajectory exchange. The sensitivity of the calculated observables to the Regge-trajectory phase is investigated in detail. The model is extended towards the resonance region by adding a number of s-channel resonances to the t-channel background. The proposed hybrid ``Regge-plus-resonance'' (RPR) approach allows one to exploit the p(gamma,K+)Lambda data in their entirety, resulting in stronger constraints on both the background and resonance couplings. The high-energy data can be used to fix the background contributions, leaving the resonance couplings as the sole free parameters in the resonance region. We compare various implementations of the RPR model, and explore to what extent the description of the data can be improved by introducing the ``new'' resonances D13(1900) and P11(1900). Despite its limited number of free parameters, the proposed RPR approach provides an efficient description of the p(gamma,K+)Lambda dynamics in and beyond the resonance region.Comment: 31 pages, 9 figures, accepted for publication in PRC. Section IIIB modified to include the most recent data; discussion of results and conclusions changed accordingl

Helicity Amplitudes of the Lambda(1670) and two Lambda(1405) as dynamically generated resonances

We determine the helicity amplitudes A_1/2 and radiative decay widths in the transition Lambda(1670) to gamma Y (Y=Lambda or Sigma^0). The Lambda(1670) is treated as a dynamically generated resonance in meson-baryon chiral dynamics. We obtain the radiative decay widths of the Lambda(1670) to gamma Lambda as 3 \pm 2 keV and to gamma Sigma^0 as 120 \pm 50 keV. Also, the Q^2 dependence of the helicity amplitudes A_1/2 is calculated. We find that the K Xi component in the Lambda(1670) structure, mainly responsible for the dynamical generation of this resonance, is also responsible for the significant suppression of the decay ratio Gamma_{gamma Lambda}/Gamma_{gamma Sigma^0}. A measurement of the ratio would, thus, provide direct access to the nature of the Lambda(1670). To compare the result for the Lambda(1670), we calculate the helicity amplitudes A_1/2 for the two states of the Lambda(1405). Also, the analytic continuation of Feynman parameterized integrals of more complicated loop amplitudes to the complex plane is developed which allows for an internally consistent evaluation of A_1/2.Comment: 15 pages, 8 figure

Search for pulsation among suspected A-type binaries and the new multiperiodic Delta Scuti star HD217860

16 pages, 12 Postscript figures, 1 long table. Table 2 is only available in electronic form. Journal-ref: Astronomy and Astrophysics, in press (2007)We have explored a sample of suspected A-type binaries in a systematic way, both spectroscopically and photometrically. Due to their location in the H-R diagram, indications of pulsation and/or chemical peculiarities among these suspected binary (or multiple) systems may be found. High-resolution spectroscopy obtained with the ELODIE and MUSICOS spectrographs was used in combination with a few nights of differential CCD photometry in order to search for pulsation(s). Of the 32 investigated targets, eight are spectroscopic binaries, one of which is a close binary also showing eclipses, and three have been identified as Delta Scuti pulsators with rapid line-profile variations. Among the latter stars, HD 217860 reveals interesting multiperiodic photometric and spectroscopic variations, with up to eight frequencies common to two large photometric data sets. We suggest that at least one radial overtone mode is excited among the two most dominant frequencies. We furthermore found evidence for a strong modulation of the amplitude(s) and/or the (radial) frequency content of this intriguing Delta Scuti star