137 research outputs found
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 (GeV/c). 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 C and 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 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
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