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 Δ\Delta-resonance region with polarized photons

Nuclear Compton scattering in the $\Delta$-resonance region is reconsidered within the framework of the $\Delta$-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