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 $-\xi<x<\xi$ 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 $L^u$ and $L^d$ 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 $L^u - L^d$ 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$_{\rm H}$), 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 60$\mu$m/25$\mu$m 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$_{\rm H}$) 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$_{\rm H}$, 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