2,440 research outputs found
An interacting quark-diquark model of baryons
A simple quark-diquark model of baryons with direct and exchange interactions
is constructed. Spectrum and form factors are calculated and compared with
experimental data. Advantages and disadvantages of the model are discussed.Comment: 13 pages, 3 eps-figures, accepted by Phys.Rev. C Rapid Communication
Soft-core meson-baryon interactions. II. and scattering
The potential includes the t-channel exchanges of the scalar-mesons
and f_0, vector-meson , tensor-mesons f_2 and f_2' and the
Pomeron as well as the s- and u-channel exchanges of the nucleon N and the
resonances , Roper and S_{11}. These resonances are not generated
dynamically. We consider them as, at least partially, genuine three-quark
states and we treat them in the same way as the nucleon. The latter two
resonances were needed to find the proper behavior of the phase shifts at
higher energies in the corresponding partial waves. The soft-core -model
gives an excellent fit to the empirical S- and P-wave phase shifts up
to T_{lab}=600 MeV. Also the scattering lengths have been reproduced well and
the soft-pion theorems for low-energy scattering are satisfied. The
soft-core model for the interaction is an SU_f(3)-extension of the
soft-core -model. The potential includes the t-channel exchanges
of the scalar-mesons a_0, and f_0, vector-mesons , and
, tensor-mesons a_2, f_2 and f_2' and the Pomeron as well as u-channel
exchanges of the hyperons and . The fit to the empirical S-, P- and D-wave phase shifts up to T_{lab}=600 MeV is reasonable and
certainly reflects the present state of the art. Since the various
phase shift analyses are not very consistent, also scattering observables are
compared with the soft-core -model. A good agreement for the total and
differential cross sections as well as the polarizations is found.Comment: 24 pages, 20 PostScript figures, revtex4, submitted to Phys. Rev.
Pertinent Dirac structure for QCD sum rules of meson-baryon coupling constants
Using general baryon interpolating fields for
without derivative, we study QCD sum rules for meson-baryon couplings and their
dependence on Dirac structures for the two-point correlation function with a
meson i\int d^4x e^{iqx} \bra 0|{\rm T}[J_B(x)\bar{J}_B(0)] |{\cal M}(p)\ket.
Three distinct Dirac structures are compared: ,
i\gamma_5\fslash{p}, and structures.
From the dependence of the OPE on general baryon interpolating fields, we
propose criteria for choosing an appropriate Dirac structure for the coupling
sum rules. The sum rules satisfy the
criteria while the sum rules beyond the chiral limit do not. For
the i\gamma_5\fslash{p} sum rules, the large continuum contributions prohibit
reliable prediction for the couplings. Thus, the structure seems pertinent for realistic predictions. In the SU(3) limit,
we identify the OPE terms responsible for the ratio. We then study the
dependence of the ratio on the baryon interpolating fields. We conclude the
ratio for appropriate choice of the interpolating fields.Comment: To be published in Phys.Rev.C ; 21 pages,8 figures, revtex ;
references are adde
Consequences of limited sediment supply for long-term evolution of offshore tidal sand waves, a 3D model perspective
Field data show that offshore tidal sand waves in areas where sediment supply is limited have different characteristics (shape and dimensions) compared with their counterparts in areas with sufficient sediment supply. So far, only the initial formation of tidal sand waves on a sediment-starved shelf has been studied with a 2DV model that ignores variations along the crests. In this study, a 3D non-linear morphodynamic model is used to investigate the effects of sediment availability on the long-term evolution of offshore tidal sand waves. Overall, the simulated sand waves have characteristics that resemble those of observed sand waves. The mature sand waves that develop in the case of limited sediment supply (i.e., thickness of erodible sediment layer is smaller than the height of sand waves) are more three-dimensional, i.e., having isolated and more irregular crestlines compared with those in the case of sufficient supply. With decreasing sediment supply, sand waves have larger spacings between successive crests, smaller heights and they migrate faster. These differences in the characteristics of the sand waves start to occur once the hard bed underneath the erodible sediment layer is exposed
Nucleon-Nucleon interaction, charge symmetry breaking and renormalization
We study the interplay between charge symmetry breaking and renormalization
in the NN system for s-waves. We find a set of universality relations which
disentangle explicitly the known long distance dynamics from low energy
parameters and extend them to the Coulomb case. We analyze within such an
approach the One-Boson-Exchange potential and the theoretical conditions which
allow to relate the proton-neutron, proton-proton and neutron-neutron
scattering observables without the introduction of extra new parameters and
providing good phenomenological success.Comment: 15 pages, 6 figure
Baryon-Baryon Interactions
After a short survey of some topics of interest in the study of baryon-baryon
scattering, the recent Nijmegen energy dependent partial wave analysis (PWA) of
the nucleon-nucleon data is reviewed. In this PWA the energy range for both pp
and np is now 0 < Tlab < 350 MeV and a chi^2_{d.o.f.}=1.08 was reached. The
implications for the pion-nucleon coupling constants are discussed. Comments
are made with respect to recent discussions around this coupling constant in
the literature. In the second part, we briefly sketch the picture of the baryon
in several, more or less QCD-based, quark-models that have been rather
prominent in the literature. Inspired by these pictures we constructed a new
soft-core model for the nucleon-nucleon interaction and present the first
results of this model in a chi^2 -fit to the new multi-energy Nijmegen PWA.
With this new model we succeeded in narrowing the gap between theory and
experiment at low energies. For the energies Tlab = 25-320 MeV we reached a
record low chi^2_{p.d.p.} = 1.16. We finish the paper with some conclusions and
an outlook describing the extension of the new model to baryon-baryon
scattering.Comment: 12 pages LaTeX and one postscript figure included. Invited talk
presented at the XIVth European Conference of Few-Body Problems in Physics,
Amsterdam, August 23-28, 199
Soft two-meson-exchange nucleon-nucleon potentials. II. One-pair and two-pair diagrams
Two-meson-exchange nucleon-nucleon potentials are derived where either one or
both nucleons contains a pair vertex. Physically, the meson-pair vertices are
meant to describe in an effective way (part of) the effects of heavy-meson
exchange and meson-nucleon resonances. {}From the point of view of ``duality,''
these two kinds of contribution are roughly equivalent. The various
possibilities for meson pairs coupling to the nucleon are inspired by the
chiral-invariant phenomenological Lagrangians that have appeared in the
literature. The coupling constants are fixed using the linear model.
We show that the inclusion of these two-meson exchanges gives a significant
improvement over a potential model including only the standard one-boson
exchanges.Comment: 21 pages RevTeX, 7 postscript figures; revised version as to appear
in Phys. Rev.
The Lambda-Lambda Interaction and ^{6}_{Lambda Lambda}He
An OBE potential model for the ^{1}S_0 S = -2 interaction is analyzed with
emphasis on the role of coupling between the Lambda Lambda, N Xi, and Sigma
Sigma channels. Singlet scalar exchange, an approximation to two-pion exchange,
is significant in all channels; surprisingly, the one-pion exchange component
is almost negligible. The size of the channel coupling as a function of the
overall strength of the OBE model potential is examined. Implications of the
analysis for the binding energy of ^{6}_{Lambda Lambda}He are considered; the
new experimental datum may suggest a consistency between the extracted Lambda
Lambda matrix element and the relation implied by SU(3) among OBE baryon-baryon
interactions. \\Comment: 4 pages brief report to Physical Review
An isotopic effect in phi photoproduction at a few GeV
A distinct isotopic effect in phi photoproduction at 2-5 GeV region is
identified by examining the production amplitudes due to Pomeron-exchange and
meson-exchange mechanisms. This effect is mainly caused by the pi-eta
interference constrained by SU(3) symmetry and the isotopic structure of the
gamma NN coupling in the direct phi-radiation amplitude. It can be tested
experimentally by measuring differences in the polarization observables between
the gamma-p and gamma-n reactions.Comment: 11 pages, 6 figure
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