179 research outputs found
Octet and Decuplet Baryons in a Covariant and Confining Diquark-Quark Model
The baryon octet and decuplet masses and Bethe-Salpeter vertex and wave
functions are calculated in the ladder approximation to the quark exchange
between a scalar or axialvector diquark and a constituent quark. These
functions reflecting full Lorentz covariance are given in terms of an expansion
in Gegenbauer polynomials. In the rest frame of the baryon, a complete partial
wave decomposition of the Bethe-Salpeter wave function is performed. The
confinement of quarks and diquarks is implemented via a parametrisation of the
corresponding propagators. We also discuss some aspects of the momentum routing
in the ladder approximation to the Bethe-Salpeter equation. Numerical results
for the octet and decuplet masses with broken flavour SU(3) in the conserved
isospin limit are presented.Comment: 36 pages, 10 figures, LaTeX-Style article using epsfig. Bug in
program code detected; numerical results changed slightly, i.e. on the order
of a few percent, conclusions are not change
Baryon form factors in a diquark-quark bound state description
Nucleon form factors are calculated in a relativistic diquark--quark picture
based on the Nambu--Jona-Lasinio model. The nucleon wave function is obtained
in a static approximation to the quark exchange interaction between the valence
quark and the diquark. We evaluate the valence quark and --diquark
contribution to the nucleon electromagnetic and weak currents. We find
reasonable electric charge radii, magnetic moments as in the additive diquark
model, and . We discuss the dependence on the model parameters.Comment: 10 pages, latex, 1 postscript figure included (uuencoded
Diquark confinement in an extended NJL model
In a Nambu-Jona-Lasinio model supplemented with an infrared cutoff in
addition to the ultraviolet cutoff we study the issue whether diquarks are
confined when the model is extended beyond the rainbow-ladder approximation.
The gap equation, obtained in a truncation scheme motivated via a nontrivial
quark-gluon vertex function, is solved to determine the constituent quark mass
if chiral symmetry is spontaneously broken. In a second step, the
Bethe-Salpeter equations for mesons and diquarks beyond ladder approximation
are derived, taking care to preserve Goldstone's theorem in the pion channel.
While the obtained masses of pseudoscalar and vector mesons are only moderately
shifted compared to the values in ladder approximation, we observe that scalar
diquarks disappear from the physical spectrum and therefore are confined. For
axialvector diquarks we observe indications, that the same mechanism may also
work, but the NJL model allows no conclusive answer in this channel.Comment: 17 pages, LaTeX, 4 figures, uses epsf.st
Nucleons or diquarks? Competition between clustering and color superconductivity in quark matter
We study the instabilities of quark matter in the framework of a generalized
Nambu--Jona-Lasinio model, in order to explore possible competition between
three-quark clustering to form nucleons and diquark formation leading to color
superconductivity. Nucleon and solutions are obtained for the
relativistic Faddeev equation at finite density and their binding energies are
compared with those for the scalar and axial-vector diquarks found from the
Bethe-Salpeter equation. In a model with interactions in both scalar and axial
diquark channels, bound nucleons exist up to nuclear matter density. However,
except at densities below about a quarter of that of nuclear matter, we find
that scalar diquark formation is energetically favored. This raises the
question of whether a realistic phase diagram of baryonic matter can be
obtained from any model which does not incorporate color confinement.Comment: 23 pages (RevTeX), 5 figures (epsf
Baryons in O(4) and Vibron Model
The structure of the reported excitation spectra of the light unflavored
baryons is described in terms of multi-spin valued Lorentz group
representations of the so called Rarita-Schwinger (RS) type (K/2, K/2)* [(1/
2,0)+ (0,1/2)] with K=1,3, and 5. We first motivate legitimacy of such pattern
as fundamental fields as they emerge in the decomposition of triple fermion
constructs into Lorentz representations. We then study the baryon realization
of RS fields as composite systems by means of the quark version of the U(4)
symmetric diatomic rovibron model. In using the U(4)/ O(4)/ O(3)/ O(2)
reduction chain, we are able to reproduce quantum numbers and mass splittings
of the above resonance assemblies. We present the essentials of the four
dimensional angular momentum algebra and construct electromagnetic tensor
operators. The predictive power of the model is illustrated by ratios of
reduced probabilities concerning electric de-excitations of various resonances
to the nucleon.Comment: Phys. Rev. D (in press, 2001
Production Processes as a Tool to Study Parameterizations of Quark Confinement
We introduce diquarks as separable correlations in the two-quark Green's
function to facilitate the description of baryons as relativistic three-quark
bound states. These states then emerge as solutions of Bethe-Salpeter equations
for quarks and diquarks that interact via quark exchange. When solving these
equations we consider various dressing functions for the free quark and diquark
propagators that prohibit the existence of corresponding asymptotic states and
thus effectively parameterize confinement. We study the implications of
qualitatively different dressing functions on the model predictions for the
masses of the octet baryons as well as the electromagnetic and strong form
factors of the nucleon. For different dressing functions we in particular
compare the predictions for kaon photoproduction, , and
associated strangeness production, with experimental data.
This leads to conclusions on the permissibility of different dressing
functions.Comment: 43 pages, Latex, 28 eps files included via epsfig; version to be
published in Physical Review
Mesons in a Poincare Covariant Bethe-Salpeter Approach
We develop a covariant approach to describe the low-lying scalar,
pseudoscalar, vector and axialvector mesons as quark-antiquark bound states.
This approach is based on an effective interaction modeling of the
non--perturbative structure of the gluon propagator that enters the quark
Schwinger-Dyson and meson Bethe-Salpeter equations. We consistently treat these
integral equations by precisely implementing the quark propagator functions
that solve the Schwinger-Dyson equations into the Bethe-Salpeter equations in
the relevant kinematical region. We extract the meson masses and compute the
pion and kaon decay constants. We obtain a quantitatively correct description
for pions, kaons and vector mesons while the calculated spectra of scalar and
axialvector mesons suggest that their structure is more complex than being
quark-antiquark bound states.Comment: 18 pages LaTeX, 5 figures; some changes in the presentation, new
results on axial vector mesons in enlarged mixing scheme; version to be
published in Physical Review
- âŠ