168 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
Lo humano en riesgo: la educaciΓ³n frente a la globalizaciΓ³n
Los especialistas ponen al descubierto los principales desafΓos polΓtico-educativos, teΓ³ricos y prΓ‘cticos del discurso sobre la educaciΓ³n y plantean la necesidad de que esta se abra para dar acceso a la integraciΓ³n de la persona y a la orientaciΓ³n de una sociedad configurada por valores humanos fundamentales, esto es, la bΓΊsqueda de una educaciΓ³n en verdad humana en la era de la globalizaciΓ³n.ITESO, A.C
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
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
Mutations at positions 186 and 194 in the HA gene of the 2009 H1N1 pandemic influenza virus improve replication in cell culture and eggs
Obtaining suitable seed viruses for influenza vaccines poses a challenge for public health authorities and manufacturers. We used reverse genetics to generate vaccine seed-compatible viruses from the 2009 pandemic swine-origin influenza virus. Comparison of viruses recovered with variations in residues 186 and 194 (based on the H3 numbering system) of the viral hemagglutinin showed that these viruses differed with respect to their ability to grow in eggs and cultured cells. Thus, we have demonstrated that molecular cloning of members of a quasispecies can help in selection of seed viruses for vaccine manufacture
A transient homotypic interaction model for the influenza A virus NS1 protein effector domain
Influenza A virus NS1 protein is a multifunctional virulence factor consisting of an RNA binding domain (RBD), a short linker, an effector domain (ED), and a C-terminal 'tail'. Although poorly understood, NS1 multimerization may autoregulate its actions. While RBD dimerization seems functionally conserved, two possible apo ED dimers have been proposed (helix-helix and strand-strand). Here, we analyze all available RBD, ED, and full-length NS1 structures, including four novel crystal structures obtained using EDs from divergent human and avian viruses, as well as two forms of a monomeric ED mutant. The data reveal the helix-helix interface as the only strictly conserved ED homodimeric contact. Furthermore, a mutant NS1 unable to form the helix-helix dimer is compromised in its ability to bind dsRNA efficiently, implying that ED multimerization influences RBD activity. Our bioinformatical work also suggests that the helix-helix interface is variable and transient, thereby allowing two ED monomers to twist relative to one another and possibly separate. In this regard, we found a mAb that recognizes NS1 via a residue completely buried within the ED helix-helix interface, and which may help highlight potential different conformational populations of NS1 (putatively termed 'helix-closed' and 'helix-open') in virus-infected cells. 'Helix-closed' conformations appear to enhance dsRNA binding, and 'helix-open' conformations allow otherwise inaccessible interactions with host factors. Our data support a new model of NS1 regulation in which the RBD remains dimeric throughout infection, while the ED switches between several quaternary states in order to expand its functional space. Such a concept may be applicable to other small multifunctional proteins
Sensorimotor Experience Influences Recovery of Forelimb Abilities but Not Tissue Loss after Focal Cortical Compression in Adult Rats
Sensorimotor activity has been shown to play a key role in functional outcome after extensive brain damage. This study was aimed at assessing the influence of sensorimotor experience through subject-environment interactions on the time course of both lesion and gliosis volumes as well as on the recovery of forelimb sensorimotor abilities following focal cortical injury. The lesion consisted of a cortical compression targeting the forepaw representational area within the primary somatosensory cortex of adult rats. After the cortical lesion, rats were randomly subjected to various postlesion conditions: unilateral C5βC6 dorsal root transection depriving the contralateral cortex from forepaw somatosensory inputs, standard housing or an enriched environment promoting sensorimotor experience and social interactions. Behavioral tests were used to assess forelimb placement during locomotion, forelimb-use asymmetry, and forepaw tactile sensitivity. For each group, the time course of tissue loss was described and the gliosis volume over the first postoperative month was evaluated using an unbiased stereological method. Consistent with previous studies, recovery of behavioral abilities was found to depend on post-injury experience. Indeed, increased sensorimotor activity initiated early in an enriched environment induced a rapid and more complete behavioral recovery compared with standard housing. In contrast, severe deprivation of peripheral sensory inputs led to a delayed and only partial sensorimotor recovery. The dorsal rhizotomy was found to increase the perilesional gliosis in comparison to standard or enriched environments. These findings provide further evidence that early sensory experience has a beneficial influence on the onset and time course of functional recovery after focal brain injury
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