92 research outputs found
Helicity amplitudes and electromagnetic decays of strange baryon resonances
We present results for the helicity amplitudes of the lowest-lying hyperon
resonances Y*, 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 helicity amplitudes are genuine predictions.
Some hyperon resonances are seen to couple more strongly to a virtual photon
with finite Q^2 than to a real photon. Other Y*'s, such as the S_{01}(1670)
Lambda resonance or the S_{11}(1620) Sigma resonance, have large
electromagnetic decay widths and couple very strongly to real photons. The
negatively-charged and neutral members of a Sigma* triplet may couple only
moderately to the Sigma(1193), while the positively-charged member of the same
Sigma* triplet displays a relatively large coupling to the Sigma^+(1193) state.
This illustrates the necessity of investigating all isospin channels in order
to obtain a complete picture of the hyperon spectrum.Comment: 4 pages, 1 figure, 1 table, Proceedings of the Conference
"International Workshop on the Physics of Excited Baryons NSTAR 05",
Tallahassee, Florida (USA), Oct. 2005, contributed tal
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
Helicity amplitudes and electromagnetic decays of hyperon resonances
We present results for the helicity amplitudes of the lowest-lying hyperon
resonances Y*, computed within the framework of the Bonn constituent-quark
model, which is based on the Bethe-Salpeter approach. The seven parameters
entering the model were fitted to the best known baryon masses. Accordingly,
the results for the helicity amplitudes are genuine predictions. Some hyperon
resonances are seen to couple more strongly to a virtual photon with finite Q^2
than to a real photon. Other Y*'s, such as the S_{01}(1670) Lambda resonance or
the S_{11}(1620) Sigma resonance, couple very strongly to real photons. We
present a qualitative argument for predicting the behaviour of the helicity
asymmetries of baryon resonances at high Q^2.Comment: 20 pages, 26 figures, uses svjour.cls style, submitted to the
European Physical Journal
Fermion family recurrences in the Dyson-Schwinger formalism
We study the multiple solutions of the truncated propagator Dyson-Schwinger
equation for a simple fermion theory with Yukawa coupling to a scalar field.
Upon increasing the coupling constant , other parameters being fixed, more
than one non-perturbative solution breaking chiral symmetry becomes possible
and we find these numerically. These ``recurrences'' appear as a mechanism to
generate different fermion generations as quanta of the same fundamental field
in an interacting field theory, without assuming any composite structure. The
number of recurrences or flavors is reduced to a question about the value of
the Yukawa coupling, and has no special profound significance in the Standard
Model. The resulting mass function can have one or more nodes and the
measurement that potentially detects them can be thought of as a collider-based
test of the virtual dispersion relation for the charged
lepton member of each family. This requires three independent measurements of
the charged lepton's energy, three-momentum and off-shellness. We illustrate
how this can be achieved for the (more difficult) case of the tau lepton
Electric and magnetic form factors of strange baryons
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.
Kaon photoproduction: background contributions, form factors and missing resonances
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.
Times of Minima of 116 Eclipsing Binary Systems (2010-2015)
We present 201 times of CCD photometric minima collected from observing sites in Belgium and Greece during the years 2010 till 2015 for 116 eclipsing binary systems
The nature of the Lambda(1405)
We present here some results supporting the nature of the
resonance as dynamically generated from the meson baryon interaction in coupled
channels and resulting from the superposition of two close-by poles. We find
support for this picture in the reaction,
which shows a different shape than the one obtained from the reaction. We also call the attention to the with in the region, which shows a narrow
peak in the calculations around 1420 MeV. We also report on recent calculations
of the radiative decay of the two states and on reactions to
obtain information on these decay modes. Finally, we present results for the
reaction recently measured at ANKE/COSY and compare
them with theoretical results.Comment: Talk given at the NSTAR2007 Workshop, Bonn September 200
Constraints on background contributions from K+ Lambda electroproduction
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 figur
The field high-amplitude SX Phe variable BL Cam: results from a multisite photometric campaign. II. Evidence of a binary - possibly triple - system
Short-period high-amplitude pulsating stars of Population I ( Sct
stars) and II (SX Phe variables) exist in the lower part of the classical
(Cepheid) instability strip. Most of them have very simple pulsational
behaviours, only one or two radial modes being excited. Nevertheless, BL Cam is
a unique object among them, being an extreme metal-deficient field
high-amplitude SX Phe variable with a large number of frequencies. Based on a
frequency analysis, a pulsational interpretation was previously given. aims
heading (mandatory) We attempt to interpret the long-term behaviour of the
residuals that were not taken into account in the previous Observed-Calculated
(O-C) short-term analyses. methods heading (mandatory) An investigation of the
O-C times has been carried out, using a data set based on the previous
published times of light maxima, largely enriched by those obtained during an
intensive multisite photometric campaign of BL Cam lasting several months.
results heading (mandatory) In addition to a positive (161 3) x 10
yr secular relative increase in the main pulsation period of BL Cam, we
detected in the O-C data short- (144.2 d) and long-term ( 3400 d)
variations, both incompatible with a scenario of stellar evolution. conclusions
heading (mandatory) Interpreted as a light travel-time effect, the short-term
O-C variation is indicative of a massive stellar component (0.46 to 1
M_{\sun}) with a short period orbit (144.2 d), within a distance of 0.7 AU
from the primary. More observations are needed to confirm the long-term O-C
variations: if they were also to be caused by a light travel-time effect, they
could be interpreted in terms of a third component, in this case probably a
brown dwarf star ( 0.03 \ M_{\sun}), orbiting in 3400 d at a
distance of 4.5 AU from the primary.Comment: 7 pages, 5 figures, accepted for publication in A&
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