206 research outputs found
A Comparison between Relativistic and Semi-Relativistic Treatment in the Diquark-Quark Model
In the diquark-quark model of the nucleon including scalar and axialvector
diquarks we compare solutions of the ladder Bethe-Salpeter equation in the
instantaneous Salpeter approximation and in the fully covariant (i.e.
four-dimensional) treatment. We obtain that the binding energy is severly
underestimated in the Salpeter approximation. For the electromagnetic form
factors of the nucleon we find that in both approaches the overall shapes of
the respective form factors are reasonably similar up to
GeV^2. However, the magnetic moments differ substantially as well as results
for the pion-nucleon and the axial coupling of the nucleon.Comment: 12 pages,4 figures, 3 tables; minor modifications in text and tables,
references added, version to be published in Physics Letters
Nucleon form factors and a nonpointlike diquark
Nucleon form factors are calculated on q^2 in [0,3] GeV^2 using an Ansatz for
the nucleon's Fadde'ev amplitude motivated by quark-diquark solutions of the
relativistic Fadde'ev equation. Only the scalar diquark is retained, and it and
the quark are confined. A good description of the data requires a nonpointlike
diquark correlation with an electromagnetic radius of 0.8 r_pi. The composite,
nonpointlike nature of the diquark is crucial. It provides for diquark-breakup
terms that are of greater importance than the diquark photon absorption
contribution.Comment: 5 pages, REVTEX, epsfig, 3 figure
Nucleon Form Factors in a Covariant Diquark-Quark Model
In a model where constituent quarks and diquarks interact through quark
exchange the Bethe-Salpeter equation in ladder approximation for the nucleon is
solved. Quark and diquark confinement is effectively parametrized by choosing
appropriately modified propagators. The coupling to external currents is
implemented via nontrivial vertex functions for quarks and diquarks to ensure
gauge invariance at the constituent level. Nucleon matrix elements are
evaluated in a generalised impulse approximation, and electromagnetic, pionic
and axial form factors are calculated.Comment: 33 Pages, 10 figures, modfied elsart.sty include
Bound states in a constituent quark model
We consider the existence of bound systems consisting of two quarks and two
antiquarks () within the framework of a constituent quark model.
The underlying quark dynamics is described by a linear confinement potential
and an effective interaction which has its origin in instanton
effects of QCD. We calculate the spectra and examine the internal structure of
the states found.Comment: 11 pages, needs epsf.st
Electromagnetic N- transition form factors in a covariant quark-diquark model
The electromagnetic N- transition form factors are calculated in the
framework of a formally covariant constituent diquark model. As a spin-3/2
particle the is assumed to be a bound state of a quark and an
axial-vector diquark. The wave function is obtained from a diquark-quark
Salpeter equation with an instantaneous quark exchange potential. The three
transition form factors are calculated for momentum transfers squared from the
pseudothreshold up to . The magnetic form
factor is in qualitative agreement with experiment. We find very interesting
results for the ratios E2/M1 and C2/M1.Comment: 16 pp, RevTeX, 7 figs, uses eps
Using Long-Duration Static Stretch Training to Counteract Strength and Flexibility Deficits in Moderately Trained Participants
Many sports injuries result in surgery and prolonged periods of immobilization, which may lead to significant atrophy accompanied by loss of maximal strength and range of motion and, therefore, a weak-leg/strong-leg ratio (as an imbalance index â ) lower than 1. Consequently, there are common rehabilitation programs that aim to enhance maximal strength, muscle thickness and flexibility; however, the literature demonstrates existing strength imbalances after weeks of rehabilitation. Since no study has previously been conducted to investigate the effects of long-duration static stretch training to treat muscular imbalances, the present research aims to determine the possibility of counteracting imbalances in maximal strength and range of motion. Thirty-nine athletic participants with significant calf muscle imbalances in maximal strength and range of motion were divided into an intervention group (one-hour daily plantar flexors static stretching of the weaker leg for six weeks) and a control group to evaluate the effects on maximal strength and range of motion with extended and bent knee joint. Results show significant increases in maximal strength (d = 0.84â1.61, p < 0.001â0.005) and range of motion (d = 0.92â1.49, p < 0.001â0.002) following six weeks of static stretching. Group * time effects ( p < 0.001â0.004, ηÂČ = 0.22â0.55) revealed â changes in the intervention group from 0.87 to 1.03 for maximal strength and from 0.92 to 1.11 in range of motion. The results provide evidence for the use of six weeks of daily, one hour stretching to counteract muscular imbalances. Related research in clinical settings after surgery is suggested
A Spectator-Quark-Model for the Photoproduction of Kaons
A simple model for the photoproduction of kaons off protons with a lambda
hyperon in the final state is presented. In a quark model, the interaction is
modelled by the pair-creation of the (anti-) strange quarks in the final state
which recombine with the three quarks of the proton to form the lambda and
kaon. The calculated scattering cross sections for photon energies up to
E_\gamma = 1.9 \; \mbox{GeV} are compared to experiment. The pair-creation
process is found to have a significant contribution to the total cross section.Comment: 16 pages, LaTex, 10 figures included; packed with 'uufiles' utilit
Electromagnetic form factors of the nucleon in a covariant diquark model
We present a simple covariant constituent diquark-quark model for the
nucleon. The nucleon is assumed to be composed of a scalar diquark and a quark
which interact via a quark exchange. Starting from the Bethe-Salpeter equation,
the instantaneous approximation leads to a diquark-quark Salpeter equation. In
the Mandelstam formalism, the electromagnetic form factors of the nucleon are
calculated for momentum transfers up to q^2 = - 3 \; (\mbox{GeV/c})^2. A
remarkable description of the experimental data is obtained. Especially, the
model gives nearly the right values for the proton and (negative) neutron
charge radii, and a qualitative description of the magnetic form factors.Comment: 17 pages, revtex, 8 figures in additional fil
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