340 research outputs found
Approximate treatment of electron Coulomb distortion in quasielastic (e,e') reactions
In this paper we address the adequacy of various approximate methods of
including Coulomb distortion effects in (e,e') reactions by comparing to an
exact treatment using Dirac-Coulomb distorted waves. In particular, we examine
approximate methods and analyses of (e,e') reactions developed by Traini et al.
using a high energy approximation of the distorted waves and phase shifts due
to Lenz and Rosenfelder. This approximation has been used in the separation of
longitudinal and transverse structure functions in a number of (e,e')
experiments including the newly published 208Pb(e,e') data from Saclay. We find
that the assumptions used by Traini and others are not valid for typical (e,e')
experiments on medium and heavy nuclei, and hence the extracted structure
functions based on this formalism are not reliable. We describe an improved
approximation which is also based on the high energy approximation of Lenz and
Rosenfelder and the analyses of Knoll and compare our results to the Saclay
data. At each step of our analyses we compare our approximate results to the
exact distorted wave results and can therefore quantify the errors made by our
approximations. We find that for light nuclei, we can get an excellent
treatment of Coulomb distortion effects on (e,e') reactions just by using a
good approximation to the distorted waves, but for medium and heavy nuclei
simple additional ad hoc factors need to be included. We describe an explicit
procedure for using our approximate analyses to extract so-called longitudinal
and transverse structure functions from (e,e') reactions in the quasielastic
region.Comment: 30 pages, 8 figures, 16 reference
Three-dimensional architecture and mechanical properties of bovine bone mixed with autologous platelet liquid, blood, or physiological water: An in vitro study
In recent years, several techniques and material options have been investigated and developed for bone defect repair and regeneration. The progress in studies of composite graft materials and autologous platelet-derived growth factors for bone regeneration in dentistry and their biological and biomechanical properties has improved clinical strategies and results. The aim of this study was to evaluate the three-dimensional architecture and mechanical properties of three different combinations of composite bovine graft, adding autologous platelet liquid (APL), blood, or physiological water. One experimental group for each combination of biomaterials was created. In particular, in Group I, the bovine graft was mixed with APL; in Group II, it was mixed with blood, and in Group III, the biomaterial graft was combined with physiological water. Then, the composite biomaterials were evaluated by scanning electron microscopy (SEM), and a compression-loading test was conducted. The evaluation showed a statistical significance (p < 0.01) of the elastic regime of deformation resistance, in which the combination of APL with bone graft resulted in an 875% increase in the mechanical resistance. The protocol of APL mixed with bovine bone graft produced a composite sticky graft block that was capable of increasing the mechanical properties in order to improve its clinical use in the treatment of the maxillary bone defects
A human clinical and histomorphometrical study on different resorbable and non-resorbable bone substitutes used in post-extractive sites:Preliminary results
Double Parton Distributions in Light-Front Constituent Quark Models
Double parton distribution functions (dPDF), accessible in high energy proton-proton and proton-nucleus collisions, encode information on how partons inside a proton are correlated among each other and could represent a tool to explore the 3D proton structure. In recent papers, double parton correlations have been studied in the valence quark region, by means of constituent quark models. This framework allows to understand clearly the dynamical origin of the correlations and to establish which, among the features of the results, are model independent. Recent relevant results, obtained in a relativistic light-front scheme, able to overcome some drawbacks of previous calculations, such as the poor support, will be presented. Peculiar transverse momentum correlations, generated by the correct treatment of the boosts, are obtained. The role of spin correlations will be also shown. In this covariant approach, the symmetries of the dPDFs are unambiguously reproduced. The study of the QCD evolution of the model results has been performed in the valence sector, showing that, in some cases, the effect of evolution does not cancel that of correlations
A human clinical and histomorphometrical study on different resorbable and non-resorbable bone substitutes used in post-extractive sites:Preliminary results
Nonperturbative versus perturbative effects in generalized parton distributions
Generalized parton distributions (GPDs) are studied at the hadronic
(nonperturbative) scale within different assumptions based on a relativistic
constituent quark model. In particular, by means of a meson-cloud model we
investigate the role of nonperturbative antiquark degrees of freedom and the
valence quark contribution. A QCD evolution of the obtained GPDs is used to add
perturbative effects and to investigate the GPDs' sensitivity to the
nonperturbative ingredients of the calculation at larger (experimental) scale.Comment: 17 pages, 10 figures; submitted to Phys. Rev.
Meson exchange and nucleon polarizabilities in the quark model
Modifications to the nucleon electric polarizability induced by pion and
sigma exchange in the q-q potentials are studied by means of sum rule
techniques within a non-relativistic quark model. Contributions from meson
exchange interactions are found to be small and in general reduce the quark
core polarizability for a number of hybrid and one-boson-exchange q-q models.
These results can be explained by the constraints that the baryonic spectrum
impose on the short range behavior of the mesonic interactions.Comment: 11 pages, 1 figure added, expanded discussio
Healing properties of implants inserted concomitantly with anorganic bovine bone. A histomorphometric human study.
Eikonal analysis of Coulomb distortion in quasi-elastic electron scattering
An eikonal expansion is used to provide systematic corrections to the eikonal
approximation through order , where is the wave number. Electron
wave functions are obtained for the Dirac equation with a Coulomb potential.
They are used to investigate distorted-wave matrix elements for quasi-elastic
electron scattering from a nucleus. A form of effective-momentum approximation
is obtained using trajectory-dependent eikonal phases and focusing factors.
Fixing the Coulomb distortion effects at the center of the nucleus, the
often-used ema approximation is recovered. Comparisons of these approximations
are made with full calculations using the electron eikonal wave functions. The
ema results are found to agree well with the full calculations.Comment: 12 pages, 6 Postscript figure
Medium modifications of nucleon electromagnetic form factors
We use the Nambu-Jona-Lasinio model as an effective quark theory to
investigate the medium modifications of the nucleon electromagnetic form
factors. By using the equation of state of nuclear matter derived in this
model, we discuss the results based on the naive quark-scalar diquark picture,
the effects of finite diquark size, and the meson cloud around the constituent
quarks. We apply this description to the longitudinal response function for
quasielastic electron scattering. RPA correlations, based on the
nucleon-nucleon interaction derived in the same model, are also taken into
account in the calculation of the response function.Comment: 46 pages, 16 figure
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