B. Pasquini

Transverse momentum dependent parton distributions in a light-cone quark model

The leading twist transverse momentum dependent parton distributions (TMDs) are studied in a light-cone description of the nucleon where the Fock expansion is truncated to consider only valence quarks. General analytic expressions are derived in terms of the six amplitudes needed to describe the three-quark sector of the nucleon light-cone wave function. Numerical calculations for the T-even TMDs are presented in a light-cone constituent quark model, and the role of the so-called pretzelosity is investigated to produce a nonspherical shape of the nucleon.

Pion generalized parton distributions with covariant and light-front constituent quark models

We investigate the model dependence of no-helicity flip generalized parton distribution of the pion upon different approaches for the quark-hadron and quark-photon vertices, in the spacelike region. In order to obtain information on contributions from both the valence and the nonvalence regions, we compare results for spacelike momentum transfers obtained from (i) an analytic covariant model with a bare quark-photon vertex, (ii) a light-front approach with a quark-photon vertex dressed through a microscopic vector-meson model, and (iii) a light-front approach based on the relativistic Hamiltonian dynamics. Our comparisons lead us to infer the same dynamical mechanism, the one-gluon-exchange dominance at short distances, as a source of both the electromagnetic form factor at large momentum transfer and the parton distribution close to the end points. The expected collinear behavior of the generalized parton distributions at high-momentum transfer, i.e. a maximum for x{approx}1, is also illustrated, independently of the different approaches. Finally, a comparison with recent lattice calculations of the gravitational form factors is presented.

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.

Azimuthal spin asymmetries in light-cone constituent quark models

We present results for all leading-twist azimuthal spin asymmetries in semi-inclusive lepton-nucleon deep-inelastic scattering due to T-even transverse-momentum dependent parton distribution functions on the basis of a light-cone constituent quark model. Attention is paid to discuss the range of applicability of the model, especially with regard to the scale dependence of the observables and the transverse-momentum dependence of the distributions. We find good agreement with available experimental data and present predictions to be further tested by future CLAS, COMPASS, and HERMES data.

Twist-2 generalized transverse-momentum dependent parton distributions and the spin/orbital structure of the nucleon

K. Kanazawa, C. Lorcé, A. Metz, B. Pasquini, M. Schlegel 1 Graduate School of Science and Technology, Niigata University, Ikarashi, Niigata 950-2181, Japan Department of Physics, Barton Hall, Temple University, Philadelphia, PA 19122, USA 3 IPNO, Université Paris-Sud, CNRS/IN2P3, 91406 Orsay, France and IFPA, AGO Department, Université de Liège, Sart-Tilman, 4000 Liège, Belgium Dipartimento di Fisica, Università degli Studi di Pavia, and Istituto Nazionale di Fisica Nucleare, Sezione di Pavia, I-27100 Pavia, Italy Institute for Theoretical Physics, Tübingen University, Auf der Morgenstelle 14, D-72076 Tübingen, Germany

Light-Front Interpretation of Proton Generalized Polarizabilities

We extend the recently developed formalism to extract light-front quark charge densities from nucleon form factor data to the deformations of these quark charge densities when applying an external electric field. We show that the resulting induced polarizations can be extracted from proton generalized polarizabilities. The available data for the generalized electric polarizability of the proton yield a pronounced structure in its induced polarization at large transverse distances, which will be pinned down by forthcoming high precision virtual Compton scattering experiments.

Generalized parton distributions of the pion in a covariant Bethe-Salpeter model and light-front models

The generalized parton distributions of the pion are studied within different light-front approaches for the quark-hadron and quark-photon vertices, exploring different kinematical regions in both the valence and non-valence sector. Moments of the generalized parton distributions which enter the definition of generalized form factors are also compared with recent lattice calculations.

TMDs and Azimuthal Spin Asymmetries in a Light‐Cone Quark Model

The main properties of the leading‐twist transverse momentum dependent parton distributions in a light‐cone constituent quark model of the nucleon are reviewed, with focus on the role of the spin‐spin and spin‐orbit correlations of quarks. Results for azimuthal single spin asymmetries in semi‐inclusive deep inelastic scattering are also discussed.

Extraction of proton form factors in the timelike region from single-polarized e + e − → p → p ¯ events

We have performed numerical simulations of the single-polarized e{sup +}e{sup -}{yields}p-vectorp process in kinematic conditions under discussion for a possible upgrade of the existing DAFNE facility. By fitting the cross section and spin asymmetry angular distributions with typical Born expressions, we can study the conditions for extracting information on moduli and phases of the proton electromagnetic form factors in the timelike region, which are poorly known and whose preliminary data show puzzling features. We have explored also non-Born contributions by introducing a further component in the angular fit, which is related to two-photon exchange diagrams. Using a dipole parametrization, we show that these corrections can be identified if larger than 5% of the Born contribution; we also explore the conditions for extracting information on the phase and, consequently, on the relative weight between their real and imaginary parts, which are presently unknown.

LIGHT-FRONT DENSITIES FOR TRANSVERSELY POLARIZED HADRONS

We discuss the recent interpretation of quark distribution functions in the plane transverse to the light-cone direction. Such a mapping is model independent and allows one to build multidimensional pictures of the hadron and to develop a semi-classical intuition of the quark dynamics. We comment briefly the results obtained from the Form Factors of the nucleon. A generalization to a target with arbitrary spin led to a set of preferred values for the electromagnetic coupling characterizing structureless particles. Generalized polarizabilities can also be interpreted in that frame as the distortion of the charge densities due to an external electromagnetic field. Finally, we present preliminary results for the Generalized Transverse-Momentum dependent Distributions which encode in principle the most complete information about quark distributions.