2,086 research outputs found
Longitudinal and Transverse Parton Momentum Distributions for Hadrons within Relativistic Constituent Quark Models
Longitudinal and transverse parton distributions for pion and nucleon are
calculated from hadron vertexes obtained by a study of form factors within
relativistic quark models. The relevance of the one-gluon-exchange dominance at
short range for the behavior of the form factors at large momentum transfer and
of the parton distributions at the end points is stressed.Comment: 5 pages, 4 figs. Proceedings of Hadron 09, to appear in AIP
Conference Proceeding
Probing the parton content of the nucleon
The parton content of the nucleon is explored within a meson-cloud model
developed to derive light-cone wave functions for the physical nucleon. The
model is here applied to study electromagnetic form factors, distribution
amplitudes and nucleon-to-meson transition distribution amplitudes.Comment: 10 pages, 6 figures; proceedings of the workshop "Recent Advances in
Perturbative QCD and Hadronic Physics" in Honor of Prof. Anatoly Efremov's
75th Birthday Celebration; to appear in Mod. Phys. Lett.
The Nuclear Shell Model Toward the Drip Lines
We describe the "islands of inversion" that occur when approaching the
neutron drip line around the magic numbers N=20, N=28 and N=40 in the framework
of the Interacting Shell Model in very large valence spaces. We explain these
configuration inversions (and the associated shape transitions) as the result
of the competition between the spherical mean field (monopole) which favors
magicity and the correlations (multipole) which favor deformed intruder states.
We also show that the N=20 and N=28 islands are in reallity a single one, which
for the Magnesium isotopes is limited by N=18 and N=32.Comment: Proceedings of the Nordic Conference in Nuclear Physics 2011,
Stockholm, to appear in Physica Script
Beam normal spin asymmetry for the process
We calculate the single spin asymmetry for the
process, for an electron beam polarized normal to the scattering plane. Such
single spin asymmetries vanish in the one-photon exchange approximation, and
are directly proportional to the absorptive part of a two-photon exchange
amplitude. As the intermediate state in such two-photon exchange process is on
its mass shell, the asymmetry allows one to access for the first time the
on-shell as well as electromagnetic
transitions. We present the general formalism to describe the beam normal spin asymmetry, and provide a numerical estimate of its
value using the nucleon, , , and
intermediate states. We compare our results with the first data from the
Qweak@JLab experiment and give predictions for the A4@MAMI experiment.Comment: 18 pages, 9 figure
Shell-model phenomenology of low-momentum interactions
The first detailed comparison of the low-momentum interaction V_{low k} with
G matrices is presented. We use overlaps to measure quantitatively the
similarity of shell-model matrix elements for different cutoffs and oscillator
frequencies. Over a wide range, all sets of V_{low k} matrix elements can be
approximately obtained from a universal set by a simple scaling. In an
oscillator mean-field approach, V_{low k} reproduces satisfactorily many
features of the single-particle and single-hole spectra on closed-shell nuclei,
in particular through remarkably good splittings between spin-orbit partners on
top of harmonic oscillator closures. The main deficiencies of pure two-nucleon
interactions are associated with binding energies and with the failure to
ensure magicity for the extruder-intruder closures. Here, calculations
including three-nucleon interactions are most needed. V_{low k} makes it
possible to define directly a meaningful unperturbed monopole Hamiltonian, for
which the inclusion of three-nucleon forces is tractable.Comment: 5 pages, 4 figures, minor additions, to appear as Rapid Comm. in
Phys. Rev.
Testing LCDM with the Growth Function \delta(a): Current Constraints
We have compiled a dataset consisting of 22 datapoints at a redshift range
(0.15,3.8) which can be used to constrain the linear perturbation growth rate
f=\frac{d\ln\delta}{d\ln a}. Five of these data-points constrain directly the
growth rate f through either redshift distortions or change of the power
spectrum with redshift. The rest of the datapoints constrain f indirectly
through the rms mass fluctuation \sigma_8(z) inferred from Ly-\alpha at various
redshifts. Our analysis tests the consistency of the LCDM model and leads to a
constraint of the Wang-Steinhardt growth index \gamma (defined from
f=\Omega_m^\gamma) as \gamma=0.67^{+0.20}_{-0.17}. This result is clearly
consistent at with the value \gamma={6/11}=0.55 predicted by LCDM. A
first order expansion of the index \gamma in redshift space leads to similar
results.We also apply our analysis on a new null test of LCDM which is similar
to the one recently proposed by Chiba and Nakamura (arXiv:0708.3877) but does
not involve derivatives of the expansion rate . This also leads to the
fact that LCDM provides an excellent fit to the current linear growth data.Comment: 7 pages, 4 figures. Added comments on the data of Table I (after eq.
(2.16)). Corrected a typo on eq. (2.15). The mathematica files with the
numerical analysis of this study may be found at
http://nesseris.physics.uoi.gr/growth/growth.ht
Atom interferometry with Bose-Einstein condensates in a double-well potential
A trapped-atom interferometer was demonstrated using gaseous Bose-Einstein
condensates coherently split by deforming an optical single-well potential into
a double-well potential. The relative phase between the two condensates was
determined from the spatial phase of the matter wave interference pattern
formed upon releasing the condensates from the separated potential wells.
Coherent phase evolution was observed for condensates held separated by 13
m for up to 5 ms and was controlled by applying ac Stark shift potentials
to either of the two separated condensates.Comment: 4 pages, 4 figure
Distillation of Bose-Einstein condensates in a double-well potential
Bose-Einstein condensates of sodium atoms, prepared in an optical dipole
trap, were distilled into a second empty dipole trap adjacent to the first one.
The distillation was driven by thermal atoms spilling over the potential
barrier separating the two wells and then forming a new condensate. This
process serves as a model system for metastability in condensates, provides a
test for quantum kinetic theories of condensate formation, and also represents
a novel technique for creating or replenishing condensates in new locations
The Two-Body Random Ensemble in Nuclei
Combining analytical and numerical methods, we investigate properties of the
two-body random ensemble (TBRE). We compare the TBRE with the Gaussian
orthogonal ensemble of random matrices. Using the geometric properties of the
nuclear shell model, we discuss the information content of nuclear spectra, and
gain insight in the difficulties encountered when fitting the effective
interaction. We exhibit the existence of correlations between spectral widths
pertaining to different quantum numbers. Using these results, we deduce the
preponderance of spin-zero ground states in the TBRE. We demonstrate the
existence of correlations between spectra with different quantum numbers and/or
in different nuclei.Comment: 16 pages, 13 figure
Convergence of Wnt signalling on the HNF4a-driven transcription in controlling liver zonation
BACKGROUND & AIMS:
In each hepatocyte, the specific repertoire of gene expression is influenced by its exact location along the portocentrovenular axis of the hepatic lobule and provides a reason for the liver functions compartmentalization defined "metabolic zonation." So far, few molecular players controlling genetic programs of periportal (PP) and perivenular (PV) hepatocytes have been identified; the elucidation of zonation mechanisms remains a challenge for experimental hepatology. Recently, a key role in induction and maintenance of the hepatocyte heterogeneity has been ascribed to Wnt/beta-catenin pathway. We sought to clarify how this wide-ranging stimulus integrates with hepatocyte specificity.
METHODS:
Reverse transcriptase polymerase chain reaction (RT-PCR) allowed the transcriptional profiling of hepatocytes derived from in vitro differentiation of liver stem cells. The GSK3beta inhibitor 6-bromoindirubin-3'-oxime (BIO) was used for beta-catenin stabilization. Co-immunoprecipitations were used to study biochemical protein interactions while ChIP assays allowed the in vivo inspection of PV and PP genes regulatory regions.
RESULTS:
We found that spontaneous differentiation of liver stem cells gives rise to PP hepatocytes that, after Wnt pathway activation, switch into PV hepatocytes. Next, we showed that the Wnt downstream player LEF1 interacts with the liver-enriched transcriptional factor HNF4alpha. Finally, we unveiled that the BIO induced activation of PV genes correlates with LEF1 binding to both its own and HNF4alpha consensus, and the repression of PP genes correlates with HNF4alpha displacement from its own consensus.
CONCLUSION:
Our data show a direct and hitherto unknown convergence of the canonical Wnt signaling on the HNF4alpha-driven transcription providing evidences of a mechanism controlling liver zonated gene expression
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