203 research outputs found
Color Magnetic Corrections to Quark Model Valence Distributions
We calculate order color magnetic corrections to the valence quark
distributions of the proton using the Los Alamos Model Potential wavefunctions.
The spin-spin interaction breaks the model SU(4) symmetry, providing a natural
mechanism for the difference between the up and down distributions. For a value
of sufficient to produce the mass splitting, we find up
and down quark distributions in reasonable agreement with experiment.Comment: 25 Pages, LA-UR-93-132
Charge Symmetry Breaking in the Valence Quark Distributions of the Nucleon
Using a quark model, we study the effect of charge symmetry breaking on the
valence quark distributions of the nucleon. The effect due to quark mass
differences and the Coulomb interaction of the electrically charged quarks is
calculated and, in contrast to recent claims, found to be small. In addition,
we investigate the effect of charge symmetry breaking in the confining
interaction, and in the perturbative evolution equations used to relate the
quark model distributions to experiment. We find that both these effects are
small, and that the strong charge symmetry breaking effect included in the
scalar confining interactions may be distinguishable from that generated by
quark mass differences.Comment: 10 pages, LaTEX, 5 Postscript figure
A Quantum-Mechanical Equivalent-Photon Spectrum for Heavy-Ion Physics
In a previous paper, we calculated the fully quantum-mechanical cross section
for electromagnetic excitation during peripheral heavy-ion collisions. Here, we
examine the sensitivity of that cross section to the detailed structure of the
projectile and target nuclei. At the transition energies relevant to nuclear
physics, we find the cross section to be weakly dependent on the projectile
charge radius, and to be sensitive to only the leading momentum-transfer
dependence of the target transition form factors. We exploit these facts to
derive a quantum-mechanical ``equivalent-photon spectrum'' valid in the
long-wavelength limit. This improved spectrum includes the effects of
projectile size, the finite longitudinal momentum transfer required by
kinematics, and the response of the target nucleus to the off-shell photon.Comment: 19 pages, 5 figure
Flavor and Charge Symmetry in the Parton Distributions of the Nucleon
Recent calculations of charge symmetry violation(CSV) in the valence quark
distributions of the nucleon have revealed that the dominant symmetry breaking
contribution comes from the mass associated with the spectator quark
system.Assuming that the change in the spectator mass can be treated
perturbatively, we derive a model independent expression for the shift in the
parton distributions of the nucleon. This result is used to derive a relation
between the charge and flavor asymmetric contributions to the valence quark
distributions in the proton, and to calculate CSV contributions to the nucleon
sea. The CSV contribution to the Gottfried sum rule is also estimated, and
found to be small
Влияние анатомических реконструкций связок на комплекс голеностопного сустава: исследование in vitro
The three-dimensional kinematics of both the ankle and the subtalar joints was examined after cutting and subsequent repairing the lateral ligaments using three anatomical procedures: direct repair, tendon graft and carbon-fibre implant. All procedures restored the original kinematics of the subtalar joint, except the plantar/dorsiflexion. For the talocrural joint, the tendon graft and the carbon fibre implant left a minimal laxity for the inversion/eversion and internal/external rotation movements. The direct repair procedure restored the physiologic kinematics almost completely and gave the best results, allowing to restore almost completely the physiologic kinematics. Each procedure respected the insertion points and the directions of the original ligaments. However, the different results for the direct repair and the other two anatomical procedures show that this condition alone is not sufficient to restore the kinematics of the talocrural and subtalar joints perfectly. None of the procedures caused a movement restriction. Thus, we recommend the direct repair of the ligaments as the method of choice. If the quality or the conditions of the ligaments do not allow a direct repair, we recommend to use another anatomical reconstruction.Представлены результаты экспериментального биомеханического исследования трёхмерной кинематики подтаранного и голеностопного суставов после рассечения латерального связочного аппарата, а также после трёх различных операций на связках: 1) непосредственно сухожильного шва; 2) с использованием свободного аутотрансплантата из сухожилия m. peroneus brevis; 3) пластики с применением алллотрансплантата, содержащего углеродистую ткань. Анатомические реконструктивные вмешательства на латеральном связочном аппарате голеностопного сустава, которые учитывают естественные точки прикрепления связок, а также их физиологическое протяжение, не привели к какому-либо ограничению естественной амплитуды движений стопы. Таким образом, при помощи анатомической реконструкции связок удалось достичь нормальной свободы движения в подтаранном и голеностопном суставах. При этом лучшие результаты здесь показал непосредственно шов, или же костная реинсерция. Ограничения движения в подтаранном и голеностопном суставах не наблюдалось. Непосредственный шов, или же костная реинсерция связок, предлагается в качестве метода выбора при лечении механически обусловленной хронической нестабильности голеностопного сустава. Если состояние остатков связочного аппарата не позволяет выполнить непосредственно шов связок, следует применять иной анатомический метод
Gravitational Collapse of Phantom Fluid in (2+1)-Dimensions
This investigation is devoted to the solutions of Einstein's field equations
for a circularly symmetric anisotropic fluid, with kinematic self-similarity of
the first kind, in -dimensional spacetimes. In the case where the radial
pressure vanishes, we show that there exists a solution of the equations that
represents the gravitational collapse of an anisotropic fluid, and this
collapse will eventually form a black hole, even when it is constituted by the
phantom energy.Comment: 10 page
Gpr126 functions in schwann cells to control differentiation and myelination via G-protein activation
The myelin sheath surrounding axons ensures that nerve impulses travel quickly and efficiently, allowing for the proper function of the vertebrate nervous system. We previously showed that the adhesion G-protein-coupled receptor (aGPCR) Gpr126 is essential for peripheral nervous system myelination, although the molecular mechanisms by which Gpr126 functions were incompletely understood. aGPCRs are a significantly understudied protein class, and it was unknown whether Gpr126 couples to G-proteins. Here, we analyze Dhh(Cre);Gpr126(fl/fl) conditional mutants, and show that Gpr126 functions in Schwann cells (SCs) for radial sorting of axons and myelination. Furthermore, we demonstrate that elevation of cAMP levels or protein kinase A activation suppresses myelin defects in Gpr126 mouse mutants and that cAMP levels are reduced in conditional Gpr126 mutant peripheral nerve. Finally, we show that GPR126 directly increases cAMP by coupling to heterotrimeric G-proteins. Together, these data support a model in which Gpr126 functions in SCs for proper development and myelination and provide evidence that these functions are mediated via G-protein-signaling pathways
Nucleon Structure Functions from a Chiral Soliton in the Infinite Momentum Frame
We study the frame dependence of nucleon structure functions obtained within
a chiral soliton model for the nucleon. Employing light cone coordinates and
introducing collective coordinates together with their conjugate momenta,
translational invariance of the solitonic quark fields (which describe the
nucleon as a localized object) is restored. This formulation allows us to
perform a Lorentz boost to the infinite momentum frame of the nucleon. The
major result is that the Lorentz contraction associated with this boost causes
the leading twist contribution to the structure functions to properly vanish
when the Bjorken variable exceeds unity. Furthermore we demonstrate that
for structure functions calculated in the valence quark approximation to the
Nambu--Jona--Lasinio chiral soliton model the Lorentz contraction also has
significant effects on the structure functions for moderate values of the
Bjorken variable .Comment: 16 pages, 1 figure, revised version to be published in Int. J. Mod.
Phys.
Reconstruction Mechanism of FCC Transition-Metal (001) Surfaces
The reconstruction mechanism of (001) fcc transition metal surfaces is
investigated using a full-potential all-electron electronic structure method
within density-functional theory. Total-energy supercell calculations confirm
the experimental finding that a close-packed quasi-hexagonal overlayer
reconstruction is possible for the late 5-metals Ir, Pt, and Au, while it is
disfavoured in the isovalent 4 metals (Rh, Pd, Ag). The reconstructive
behaviour is driven by the tensile surface stress of the unreconstructed
surfaces; the stress is significantly larger in the 5 metals than in 4
ones, and only in the former case it overcomes the substrate resistance to the
required geometric rearrangement. It is shown that the surface stress for these
systems is due to charge depletion from the surface layer, and that the
cause of the 4th-to-5th row stress difference is the importance of relativistic
effects in the 5 series.Comment: RevTeX 3.0, 12 pages, 1 PostScript figure available upon request] 23
May 199
Theoretical analysis of the electronic structure of the stable and metastable c(2x2) phases of Na on Al(001): Comparison with angle-resolved ultra-violet photoemission spectra
Using Kohn-Sham wave functions and their energy levels obtained by
density-functional-theory total-energy calculations, the electronic structure
of the two c(2x2) phases of Na on Al(001) are analysed; namely, the metastable
hollow-site structure formed when adsorption takes place at low temperature,
and the stable substitutional structure appearing when the substrate is heated
thereafter above ca. 180K or when adsorption takes place at room temperature
from the beginning. The experimentally obtained two-dimensional band structures
of the surface states or resonances are well reproduced by the calculations.
With the help of charge density maps it is found that in both phases, two
pronounced bands appear as the result of a characteristic coupling between the
valence-state band of a free c(2x2)-Na monolayer and the
surface-state/resonance band of the Al surfaces; that is, the clean (001)
surface for the metastable phase and the unstable, reconstructed "vacancy"
structure for the stable phase. The higher-lying band, being Na-derived,
remains metallic for the unstable phase, whereas it lies completely above the
Fermi level for the stable phase, leading to the formation of a
surface-state/resonance band-structure resembling the bulk band-structure of an
ionic crystal.Comment: 11 pages, 11 postscript figures, published in Phys. Rev. B 57, 15251
(1998). Other related publications can be found at
http://www.rz-berlin.mpg.de/th/paper.htm
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