158 research outputs found
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
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 covariant diquark-quark model of the nucleon in the Salpeter approach
We develop a rather simple, formally covariant quark-diquark model of the
nucleon. The nucleon is treated as a bound state of a constituent quark and a
diquark interacting via a quark exchange. We include both scalar and
axial-vector diquarks. The underlying Bethe-Salpeter equation is transformed
into a pair of coupled Salpeter equations. The electromagnetic form factors of
the nucleon are calculated in the Mandelstam formalism. We obtain a very good
description of all electromagnetic form factors for momentum transfers up to -3
(GeV/c)^2.Comment: 17 pages, REVTeX, 10 figures (ps and eps) include
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
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
A hybrid approach to CMB lensing reconstruction on all-sky intensity maps
Based on realistic simulations, we propose an hybrid method to reconstruct
the lensing potential power spectrum, directly on PLANCK-like CMB frequency
maps. It implies using a large galactic mask and dealing with a strong
inhomogeneous noise. For l < 100, we show that a full-sky inpainting method,
already described in a previous work, still allows a minimal variance
reconstruction, with a bias that must be accounted for by a Monte-Carlo method,
but that does not couple to the deflection field. For l>100 we develop a method
based on tiling the cut-sky with local 10x10 degrees overlapping tangent planes
(referred to in the following as "patches"). It requires to solve various
issues concerning their size/position, non-periodic boundaries and irregularly
sampled data after the sphere-to-plane projection. We show how the leading
noise term of the quadratic lensing estimator applied onto an apodized patch
can still be taken directly from the data. To not loose spatial accuracy, we
developed a tool that allows the fast determination of the complex Fourier
series coefficients from a bi-dimensional irregularly sampled dataset, without
performing an interpolation. We show that the multi-patch approach allows the
lensing power spectrum reconstruction with a very small bias, thanks to
avoiding the galactic mask and lowering the noise inhomogeneities, while still
having almost a minimal variance. The data quality can be assessed at each
stage and simple bi-dimensional spectra build, which allows the control of
local systematic errors.Comment: A&A version. Mostly english correction
Two distinct populations of doublecortin-positive cells in the perilesional zone of cortical infarcts
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 C5–C6 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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