8,711 research outputs found
Impact-parameter dependence of the generalized parton distribution of the pion in chiral quark models
We compute the off-forward diagonal non-singlet generalized parton
distribution of the pion in two distinct chiral quark models: the
Nambu-Jona-Lasinio model with the Pauli-Villars regulator and the Spectral
Quark Model. The analysis is carried out in the impact-parameter space.
Leading-order perturbative QCD evolution is carried out via the inverse Mellin
transform in the index space. The model predictions agree very reasonably with
the recent results from transverse-lattice calculations, reproducing
qualitatively both the Bjorken-x and the impact-parameter dependence of the
data.Comment: 13 pages, 3 figures, to appear in Phys. Lett.
The generalized parton distributions of the nucleon in the NJL model based on the Faddeev approach
We study the generalized parton distributions, including the helicity-flip
ones, using Nambu-Jona-Lasinio model based on a relativistic Faddeev approach
with `static approximation'. Sum rules relating the generalized parton
distributions to nucleon electromagnetic form factors are satisfied. Moreover,
quark-antiquark contributions in the region are non-vanishing. Our
results are qualitatively similar to those calculated with Radyushkin's double
distribution ansatz using forward parton distribution functions calculated in
the NJL model as inputs.Comment: 35 pages, 15 figure
Pion-photon Transition Distribution Amplitudes in the Spectral Quark Model
The vector and axial pion-photon transition distribution amplitudes are
analyzed in the Spectral Quark Model. We proceed by the evaluation of double
distributions through the use of a manifestly covariant calculation based on
the alpha representation of propagators. As a result polynomiality is
incorporated and calculations become rather simple. Explicit formulas, holding
at the low-energy quark-model scale, are obtained. The corresponding form
factors for the anomalous decay pi0 -> gamma gamma* and the radiative pion
decays are also evaluated and confronted with the data.Comment: 13 pages, 3 figure
Generalized vector form factors of the pion in a chiral quark model
Generalized vector form factors of the pion, related to the moments of the
generalized parton distribution functions, are evaluated in the
Nambu--Jona-Lasinio model with the Pauli-Villars regularization. The lowest
moments (the electromagnetic and the gravitational form factors) are compared
to recent lattice data, with fair agreement. Predictions for higher-order
moments are also made. Relevant features of the generalized form factors in the
chiral quark models are highlighted and the role of the QCD evolution for the
higher-order GFFs is stressed.Comment: Dedicated to the memory of Manoj K. Banerjee, to appear in a special
issue of the Indian Journal of Physics, 6 pages, 4 figure
Get my pizza right: Repairing missing is-a relations in ALC ontologies (extended version)
With the increased use of ontologies in semantically-enabled applications,
the issue of debugging defects in ontologies has become increasingly important.
These defects can lead to wrong or incomplete results for the applications.
Debugging consists of the phases of detection and repairing. In this paper we
focus on the repairing phase of a particular kind of defects, i.e. the missing
relations in the is-a hierarchy. Previous work has dealt with the case of
taxonomies. In this work we extend the scope to deal with ALC ontologies that
can be represented using acyclic terminologies. We present algorithms and
discuss a system
A Model for the Pion Structure Function
The pion structure function is investigated in a simple model, where pion and
constituent quark fields are coupled through the simplest pseudoscalar
coupling. The imaginary part of the forward gamma* pi-> gamma* pi scattering
amplitude is evaluated and related to the structure functions. It is shown that
the introduction of non-perturbative effects, linked to the size of the pion
and preserving gauge invariance, allows a connection with the quark
distribution. It is predicted that higher-twist terms become negligible for Q2
larger than about 2 GeV2 and that quarks in the pion have a momentum fraction
smaller than in the proton case.Comment: 14 pages, 6 figures, LaTeX, elsart clas
The role of orbital angular momentum in the proton spin
The orbital angular momenta and of up and down quarks in the
proton are estimated as functions of the energy scale as model-independently as
possible, on the basis of Ji's angular momentum sum rule. This analysis
indicates that is large and negative even at low energy scale of
nonperturbative QCD, in contrast to Thomas' similar analysis based on the
refined cloudy bag model. We pursuit the origin of this apparent discrepancy
and suggest that it may have a connection with the fundamental question of how
to define quark orbital angular momenta in QCD.Comment: 14 pages, 3 figures, 1 table A slightly extended version to appear in
Eur. Phys. J.
In situ measurements and thermo-mechanical simulation of Ti–6Al–4V laser solid forming processes
Residual stresses and distortions are two technical obstacles for popularizing the additive manufacturing (AM) technology. The evolution of the stresses in AM components during the thermal cycles of the metal depositing process is not yet clear, and more accurate in situ measurements are necessary to calibrate and validate the numerical tools developed for its simulation. In this work a fully coupled thermo-mechanical analysis to simulate the laser solid forming (LSF) process is carried out. At the same time, an exhaustive experimental campaign is launched to measure the temperature evolution at different locations, as well as the distortions and both the stress and strain fields. The thermal and mechanical responses of single-wall coupons under different process parameters are recorded and compared with the numerical models. Good agreement between the numerical results and the experimental measurements is obtained. Sensitivity analysis demonstrates that the AM process is significantly affected by the laser power and the feeding rate, while poorly influenced by the scanning speed.Peer ReviewedPostprint (author's final draft
Nuclear obscuration and scattering in Seyfert 2 galaxies
We study the relation between gaseous absorbing column density (N),
infrared colors and detectability of the broad lines in a large sample of
Seyfert 2 galaxies(Sy2s). We confirm that Sy2s without polarized broad lines
tend to have cooler 60m/25m colors; this correlation was previously
ascribed to the effect of obscuration towards the nuclear region.
We find some evidence that Sy2s without polarized broad lines have larger
absorbing column density (N) and that a fraction of them are
characterized by dust lanes crossing their nuclei.
However, we find that the IR colors do not correlate with N, in
disagreement with the obscuration scenario.
Also, Sy2s without polarized broad lines follow the same radio-FIR relation
as normal and starburst galaxies, at variance with Sy2s with polarized broad
lines. These results indicate that the lack of broad lines in the polarized
spectrum of Sy2s is mostly due to the contribution/dilution from the host
galaxy or from a circumnuclear starburst, though at a lower extent the
obscuration toward the nuclear region also plays a role.Comment: 6 pages, 4 figures, to appear in A&A
Orobanche centaurina Bertol. the correct name for O. kochii F.W. Schultz (Orobanchaceae)
Después de estudiar el material original de Orobanche centaurina Bertol. (Orobanchaceae) depositado en BOLO, una olvidada especie descrita de Massa (Italia central) parásita de Centaurea paniculata L., y compararlo con el tipo de O. kochii F.W. Schultz y con los tipos de otros táxones actualmente considerados inónimos de éste, estimamos que el nombre de Bertoloni es el correcto y prioritario para esta especi
- …