2,255 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
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.
Nuclear Compton scattering in the -resonance region with polarized photons
Nuclear Compton scattering in the -resonance region is reconsidered
within the framework of the -hole model. The different role of the
resonant and non-resonant contributions to the transition amplitudes is
discussed and their effect is investigated by comparing the results of
calculation with recent data also taken with polarized photons.Comment: 22 pages, LaTeX, 7 figures, postscript files available from the
Author
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
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
Testing the LCDM model (and more) with the time evolution of the redshift
With the many ambitious proposals afoot for new generations of very large
telescopes, along with spectrographs of unprecedented resolution, there arises
the real possibility that the time evolution of the cosmological redshift may,
in the not too distant future, prove to be a useful tool rather than merely a
theoretical curiosity. Here I contrast this approach with the standard
cosmological procedure based on the luminosity (or any other well-defined)
distance. I then show that such observations would not only provide a direct
measure of all the associated cosmological parameters of the LCDM model, but
would also provide wide-ranging internal consistency checks. Further, in a more
general context, I show that without introducing further time derivatives of
the redshift one could in fact map out the dark energy equation of state should
the LCDM model fail. A consideration of brane-world scenarios and interacting
dark energy models serves to emphasize the fact that the usefulness of such
observations would not be restricted to high redshifts.Comment: In final form as to appear in Physical Review D. 12 pages 6 figure
The spin structure of the nucleon in light-cone quark models
The quark spin densities related to generalized parton distributions in
impact-parameter space and to transverse-momentum dependent parton
distributions are reviewed within a light-cone quark model, with focus on the
role of the different spin-spin and spin-orbit correlations of quarks.Results
for azimuthal spin asymmetries in semi-inclusive deep-inelastic scattering due
to T-even transverse-momentum dependent parton distributions are also
discussed.Comment: proceedings of the workshop "Recent Advances in Perturbative QCD and
Hadronic Physics", 20-25 July 2009, ECT*, Trento (Italy), in Honor of Prof.
Anatoly Efremov's 75th Birthday Celebration; also based on the talk given at
the workshop "Transverse Partonic Structure of Hadrons", 21-26 June 2009,
Yerevan (Armenia); to appear in Mod. Phys. Lett.
Generalized Polarizabilities in a Constituent Quark Model
We discuss low-energy virtual Compton scattering off the proton within the
framework of a nonrelativistic constituent quark model. A simple interpretation
of the spin-averaged generalized polarizabilities is given in terms of the
induced electric polarization (and magnetization). Explicit predictions for the
generalized polarizabilities obtained from a multipole expansion are presented
for the Isgur-Karl model and are compared with results of various models.Comment: 10 pages, Latex, 2 figures, uses ws-p8-50x6-00.cls, talk given at
NSTAR2001, Workshop on "The Physics of Excited Nucleons," Mainz, Germany,
March 7-10, 200
Chiral-odd generalized parton distributions in constituent quark models
We derive the overlap representation of chiral-odd generalized parton
distributions using the Fock-state decomposition in the transverse-spin basis.
This formalism is applied to the case of light-cone wave functions in a
constituent quark model. Numerical results for the four chiral-odd generalized
parton distributions at the hadronic scale are shown in different kinematics.
In the forward limit we derive the transversity distribution, the tensor charge
and the angular momentum sum rule for quarks with transverse polarization in an
unpolarized nucleon.Comment: 23 pages, 7 figures; one reference added and one formula corrected;
conclusions and discussions unchange
Chiral-Odd Generalized Parton Distributions, Transversity and Double Transverse-Spin Asymmetry in Drell-Yan Dilepton Production
Within the framework of light-cone quantization we derive the overlap
representation of generalized parton distributions for transversely polarized
quarks using the Fock-state decomposition in the transverse-spin basis. We
apply this formalism to the case of light-cone wave functions in a constituent
quark model giving numerical results for the four chiral-odd generalized parton
distributions in a region where they describe the emission and reabsorption of
a quark by the nucleon. With the transversity distribution obtained in the
forward limit of the generalized distribution, we provide some predictions for
the double transverse-spin asymmetry in Drell-Yan dilepton production in the
kinematics of the PAX experiment.Comment: 8 pages, 4 figures. To appear in the Proceedings of the "XI
Conference on Problems in Theoretical Nuclear Physics", Cortona, Italy, 11-14
October 200
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