8,137 research outputs found
Generalized parton distributions in a meson cloud model
We present a model calculation of the generalized parton distributions where
the nucleon is described by a quark core surrounded by a mesonic cloud. In the
one-meson approximation, we expand the Fock state of the physical nucleon in a
series involving a bare nucleon and two-particle, meson-baryon, states. We
discuss the role of the different Fock-state components of the nucleon by
deriving a convolution formalism for the unpolarized generalized parton
distributions, and showing predictions at different kinematics.Comment: 7 pages, 3 figures; Invited talk at the Fifth International
Conference on Perspectives in Hadronic Physics, Miramare - Trieste (Italy),
22-26 May 200
A view of the Galactic halo using beryllium as a time scale
Beryllium stellar abundances were suggested to be a good tracer of time in
the early Galaxy. In an investigation of its use as a cosmochronometer, using a
large sample of local halo and thick-disk dwarfs, evidence was found that in a
log(Be/H) vs. [alpha/Fe] diagram the halo stars separate into two components.
One is consistent with predictions of evolutionary models while the other is
chemically indistinguishable from the thick-disk stars. This is interpreted as
a difference in the star formation history of the two components and suggests
that the local halo is not a single uniform population where a clear
age-metallicity relation can be defined.Comment: To appear in Proceedings of the International Astronomical Union, IAU
Symposium, Volume 265, Chemical abundances in the Universe: connecting first
stars to planets, K. Cunha, M. Spite and B. Barbuy, eds. 2 Pages, 2 figure
A class of AM-QFT algorithms for power-of-two FFT
This paper proposes a class of power-of-two FFT (Fast Fourier Transform)
algorithms, called AM-QFT algorithms, that contains the improved QFT (Quick
Fourier Transform), an algorithm recently published, as a special case. The
main idea is to apply the Amplitude Modulation Double Sideband - Suppressed
Carrier (AM DSB-SC) to convert odd-indices signals into even-indices signals,
and to insert this elaboration into the improved QFT algorithm, substituting
the multiplication by secant function. The 8 variants of this class are
obtained by re-elaboration of the AM DSB-SC idea, and by means of duality. As a
result the 8 variants have both the same computational cost and the same memory
requirements than improved QFT. Differently, comparing this class of 8 variants
of AM-QFT algorithm with the split-radix 3add/3mul (one of the most performing
FFT approach appeared in the literature), we obtain the same number of
additions and multiplications, but employing half of the trigonometric
constants. This makes the proposed FFT algorithms interesting and useful for
fixed-point implementations. Some of these variants show advantages versus the
improved QFT. In fact one of this variant slightly enhances the numerical
accuracy of improved QFT, while other four variants use trigonometric constants
that are faster to compute in `on the fly' implementations
Beryllium abundances and the formation of the halo and the thick disk
The single stable isotope of beryllium is a pure product of cosmic-ray
spallation in the ISM. Assuming that the cosmic-rays are globally transported
across the Galaxy, the beryllium production should be a widespread process and
its abundance should be roughly homogeneous in the early-Galaxy at a given
time. Thus, it could be useful as a tracer of time. In an investigation of the
use of Be as a cosmochronometer and of its evolution in the Galaxy, we found
evidence that in a log(Be/H) vs. [alpha/Fe] diagram the halo stars separate
into two components. One is consistent with predictions of evolutionary models
while the other is chemically indistinguishable from the thick-disk stars. This
is interpreted as a difference in the star formation history of the two
components and suggests that the local halo is not a single uniform population
where a clear age-metallicity relation can be defined. We also found evidence
that the star formation rate was lower in the outer regions of the thick disk,
pointing towards an inside-out formation.Comment: 6 pages, 5 figures, To appear in the Proceedings of IAU Symp. 268 -
Light Elements in the Universe (C. Charbonnel, M. Tosi, F. Primas, C.
Chiappini, eds
Nucleon spin densities in a light-front constituent quark model
The first moment of longitudinal and transverse spin densities of quarks in
the nucleon are calculated in a light-front constituent quark model for the
different cases of quark and nucleon polarization. Significant distortions are
found for the transverse spin densities. In particular the Sivers function is
predicted with opposite sign for up and down quarks and the Boer-Mulders
function is predicted large and negative for both up and down quarks, in
agreement with lattice calculations. Quite a different spin distribution is
obtained for up and down quarks in the cases of quarks and proton transversely
or longitudinally polarized in the same direction.Comment: one reference added; to be published in Phys. Lett.
Models for TMDs and numerical methods
We study the connection between the quark orbital angular momentum and the
pretzelosity transverse-momentum dependent parton distribution function. We
discuss the origin of this relation in quark models, identifying as key
ingredient for its validity the assumption of spherical symmetry for the
nucleon in its rest frame. Finally we show that the individual quark
contributions to the orbital angular momentum obtained from this relation can
not be interpreted as the intrinsic contributions, but include the contribution
from the transverse centre of momentum which cancels out only in the total
orbital angular momentum.Comment: 43 pages, 8 figures; proceedings of International School of Physics
"Enrico Fermi", Course CLXXX - "Three-dimensional Partonic Structure of the
Nucleon", 28 June - 8 July 2011, Varenna (Italy
Dispersion Theory in Electromagnetic Interactions
We review various applications of dispersion relations (DRs) to the
electromagnetic structure of hadrons. We discuss the way DRs allow one to
extract information on hadron structure constants by connecting information
from complementary scattering processes. We consider the real and virtual
Compton scattering processes off the proton, and summarize recent advances in
the DR analysis of experimental data to extract the proton polarizabilities, in
comparison with alternative studies based on chiral effective field theories.
We discuss a multipole analysis of real Compton scattering data, along with a
DR fit of the energy-dependent dynamical polarizabilities. Furthermore, we
review new sum rules for the double-virtual Compton scattering process off the
proton, which allow for model independent relations between polarizabilities in
real and virtual Compton scattering, and moments of nucleon structure
functions. The information on the double-virtual Compton scattering is used to
predict and constrain the polarizability corrections to muonic hydrogen
spectroscopy.Comment: preprint version of a review to appear in Ann. Rev. Nucl. Part. Sci.
68 (2018
On the Origin of Model Relations among Transverse-Momentum Dependent Parton Distributions
Transverse-momentum dependent parton distributions (TMDs) are studied in the
framework of quark models. In particular, quark-model relations among TMDs are
reviewed, elucidating their physical origin in terms of the quark-spin
structure in the nucleon. The formal aspects of the derivation of these
relations are complemented with explicit examples, emphasizing how and to which
extent the conditions which lead to relations among TMDs are implemented in
different classes of quark models.Comment: 38 pages, 3 figures, 3 table
Electroweak structure of the nucleon, meson cloud and light-cone wavefunctions
The meson-cloud model of the nucleon consisting of a system of three valence
quarks surrounded by a meson cloud is applied to study the electroweak
structure of the proton and neutron. Light-cone wavefunctions are derived for
the dressed nucleon as pictured to be part of the time a bare nucleon and part
of the time a baryon-meson system. Configurations are considered where the
baryon can be a nucleon or a \Delta and the meson can be a pion as well as a
vector meson such as the \rho or the \omega. An overall good description of the
electroweak form factors is obtained. The contribution of the meson cloud is
small and only significant at low Q^2. Mixed-symmetry S'-wave components in the
wavefunction are most important to reproduce the neutron electric form factor.
Charge and magnetization densities are deduced as a function of both the radial
distance from the nucleon center and the transverse distance with respect to
the direction of the three-momentum transfer. In the latter case a central
negative charge is found for the neutron. The up and down quark distributions
associated with the Fourier transform of the axial form factor have opposite
sign, with the consequence that the probability to find an up (down) quark with
positive helicity is maximal when it is (anti)aligned with the proton helicity.Comment: references updated and typos in figure 2 corrected; to be published
in Phys. Rev.
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