(e,e'N) and (e,e'NN) experiments at NIKHEF, Mainz and JLab and open problems in one- and two-nucleon emission

A critical review is presented of recent data on direct one- and two-nucleon emission obtained in electron scattering at NIKHEF, Mainz and JLab. In the case of (e,e$'$p) reactions attention is focussed on extracting spectroscopic factors, looking at medium effects on the bound nucleon form factors, and investigating nuclear transparency. The possibility of obtaining information on nucleon-nucleon correlations in (e,e$'$pp) and (e,e$'$pn) reactions is also discussed.Comment: 14 pages, LaTeX, four eps figures, style file included. Invited talk at the International Workshop on Nuclear Many-Body Problem and Sub-Nucleonic Degrees of Freedom in Nuclei, Jilin University, Changchun, China, July 2-8, 200

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

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.

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

Wigner distributions and quark orbital angular momentum

We discuss the quark phase-space or Wigner distributions of the nucleon which combine in a single picture all the information contained in the generalized parton distributions and the transverse-momentum dependent parton distributions. In particular, we present results for the distribution of unpolarized quarks in a longitudinally polarized nucleon obtained in a light-front constituent quark model. We show how the quark orbital angular momentum can be extracted from the Wigner distributions and compare it with alternative definitions.Comment: 8 pages, 2 figures, 1 table, contribution to the proceedings of the QCD Evolution workshop 2012, May 14-17, JLa

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

Chiral-Odd Generalized Parton Distributions and Transversity in Light-Front Constituent Quark Models

We present the general framework to calculate chiral-odd generalized parton distributions in the overlap representation using the Fock-state decomposition in the transverse spin basis. 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. Numerical results are obtained by applying the formalism to the case of light-cone wavefunctions of constituent quark models.Comment: 8 pages, 3 figures. Invited talk at the "Advanced Studies Institute on SYMMETRIES AND SPIN" (SPIN-Praha-2006), 19-26 July, 2006, Prague. To be pubblished in Czechoslovak Journal of Physics. Some references adde

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.

Semi-inclusive structure functions in the spectator model

We establish the relationship between distribution and fragmentation functions and the structure functions appearing in the cross section of polarized 1-particle inclusive deep-inelastic scattering. We present spectator model evaluations of these structure functions focusing on the case of an outgoing spin-1/2 baryon. Distribution functions obtained in the spectator model are known to fairly agree at low energy scales with global parameterizations extracted, for instance, from totally inclusive DIS data. Therefore, we expect it to give good hints on the functional dependence of the structure functions on the scaling variables x(Bjorken), z and on the transverse momentum of the observed outgoing hadron, P_{h\perp}. Presently, this dependence is not very well known, but experiments are planned in the near future.Comment: 19 pages, 16 figures, submitted to Eur. Phys. J.