4,374 research outputs found
Light-front CQM calculations of baryon electromagnetic form factors
The parameter-free predictions for the and electromagnetic transition form factors, obtained within our
light-front constituent quark model using eigenfunctions of a baryon mass
operator which includes a large amount of configuration mixing, are reported.
The effects due to small components in the baryon wave functions, such as S'-
and D-wave, are also investigated.Comment: to appear in the Proceedings of the International Workshop on Hadron
Dynamics with the new DAPHNE and CEBAF facilities, Frascati, Italy, 11-14
November 199
New results from an extensive aging test on bakelite Resistive Plate Chambers
We present recent results of an extensive aging test, performed at the CERN
Gamma Irradiation Facility on two single--gap RPC prototypes, developed for the
LHCb Muon System. With a method based on a model describing the behaviour of an
RPC under high particle flux conditions, we have periodically measured the
electrode resistance R of the two RPC prototypes over three years: we observe a
large spontaneous increase of R with time, from the initial value of about 2
MOhm to more than 250 MOhm. A corresponding degradation of the RPC rate
capabilities, from more than 3 kHz/cm2 to less than 0.15 kHz/cm2 is also found.Comment: 6 pages, 7 figures, presented at Siena 2002, 8th Topical Seminar on
Innovative Particle and Radiation Detectors 21-24 October 2002, Siena, Ital
A light-front description of electromagnetic form factors for hadrons
A review of the hadron electromagnetic form factors obtained in a light-front
constituent quark model, based on the eigenfunctions of a mass operator, is
presented. The relevance of different components in the q-q interaction for the
description of hadron experimental form factors is analysed.Comment: 6 pages, Latex, 3 Postscript figures included. Proceedings of
"Nucleon 99", Frascati, June 1999. To appear in Nucl. Phys.
Extracellular vesicle-induced differentiation of neural stem progenitor cells
Neural stem progenitor cells (NSPCs) from E13.5 mouse embryos can be maintained in culture under proliferating conditions. Upon growth-factor removal, they may differentiate toward either neuronal or glial phenotypes or both. Exosomes are small extracellular vesicles that are part of the cell secretome; they may contain and deliver both proteins and genetic material and thus play a role in cell–cell communication, guide axonal growth, modulate synaptic activity and regulate peripheral nerve regeneration. In this work, we were interested in determining whether NSPCs and their progeny can produce and secrete extracellular vesicles (EVs) and if their content can affect cell differentiation. Our results indicate that cultured NSPCs produce and secrete EVs both under proliferating conditions and after differentiation. Treatment of proliferating NSPCs with EVs derived from differentiated NSPCs triggers cell differentiation in a dose-dependent manner, as demonstrated by glial-and neuronal-marker expression
Gaucher Disease and Myelofibrosis: A Combined Disease or a Misdiagnosis?
Background: Gaucher disease (GD) and primary myelofibrosis (PMF) share similar clinical and laboratory features, such as cytopenia, hepatosplenomegaly, and marrow fibrosis, often resulting in a misdiagnosis. Case Report: We report here the case of a young woman with hepatosplenomegaly, leukopenia, and thrombocytopenia. Based on bone marrow (BM) findings and on liver biopsy showing extramedullary hematopoiesis, an initial diagnosis of PMF was formulated. The patient refused stem cell transplantation from an HLA-identical sibling. Low-dose melphalan was given, without any improvement. Two years later, a BM evaluation showed Gaucher cells. Low glucocerebrosidase and high chitotriosidase levels were indicative for GD. Molecular analysis revealed N370S/complex I mutations. Enzyme replacement therapy with imiglucerase was commenced, resulting in clinical and hematological improvements. Due to an unexpected and persistent organomegaly, PMF combined with GD were suspected. JAK2V617F, JAK2 exon 12, MPL, calreticulin, and exon 9 mutations were negative, and BM examination showed no marrow fibrosis. PMF was excluded. Twenty years after starting treatment, the peripheral cell count and liver size were normal, whereas splenomegaly persisted. Conclusion: In order to avoid a misdiagnosis, a diagnostic algorithm for patients with hepatosplenomegaly combined with cytopenia is suggested
Radiative and transition form factors in a light-front constituent quark model
The form factors of the and radiative transitions are
evaluated within a light-front constituent quark model, using for the first
time the eigenfunctions of a light-front mass operator reproducing the meson
mass spectrum and including phenomenological Dirac and Pauli quark form factors
in the one-body electromagnetic current operator. The sensitivity of the
transition form factors both to the meson wave functions and to the constituent
quark form factors is illustrated. It is shown that the measurement of the and radiative transitions could help in discriminating among
various models of the meson structure.Comment: 9 pages, latex, 4 figures available as separate .uu file, to appear
in Phys. Lett.
Pair term in the Electromagnetic Current within the Front-Form Dynamics: Spin-0 Case
The frame and scale dependence of the pair-term contribution to the
electromagnetic form factor of a spin-zero composite system of two-fermions is
studied within the Light Front. The form factor is evaluated from the
plus-component of the current in the Breit frame, using for the first time a
nonconstant, symmetric ansatz for the Bethe-Salpeter amplitude. The frame
dependence is analyzed by allowing a nonvanishing plus component of the
momentum transfer, while the dynamical scale is set by the masses of the
constituents and by mass and size of the composite system. A transverse
momentum distribution, associated with the Bethe-Salpeter amplitude, is
introduced which allows to define strongly and weakly relativistic systems. In
particular, for strongly relativistic systems, the pair term vanishes for the
Drell-Yan condition, while is dominant for momentum transfer along the
light-front direction. For a weakly relativistic system, fitted to the deuteron
scale, the pair term is negligible up to momentum transfers of 1(GeV/c). A
comparison with results obtained within the Front-Form Hamiltonian dynamics
with a fixed number of constituents is also presented.Comment: 25 pages + 7 figures; axodraw.sty included. To appear in Nucl. Phys.
Nucleon generalized polarizabilities within a relativistic Constituent Quark Model
Nucleon generalized polarizabilities are investigated within a relativistic
framework, defining such quantities through a Lorentz covariant multipole
expansion of the amplitude for virtual Compton scattering. The key physical
ingredients in the calculation of the nucleon polarizabilities are the Lorentz
invariant reduced matrix elements of the electromagnetic transition current,
which can be evaluated from off-energy-shell helicity amplitudes. The evolution
of the proton paramagnetic polarizability, , as a function of
the virtual-photon three-momentum transfer is explicitly evaluated within
a relativistic constituent quark model by adopting transition form factors
obtained in the light-front formalism. The discussion is focussed on the role
played by the effects due to the relativistic approach and to the transition
form factors, derived within different models.Comment: 14 pages and three figures (included), to appear in Phys. Rev. C (May
1998
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