Photon Generalized Parton Distributions

We present a calculation of the generalized parton distributions of the photon using overlaps of photon light-front wave functions.     

An introduction to the Generalized Parton Distributions

The recently introduced concepts of Generalized Parton Distributions (GPD) are reviewed in an introductory and phenomenological fashion. These distributions provide a rich and unifying picture of the nucleon structure. Their physical meaning is discussed. The GPD are in principle measurable through exclusive deeply virtual production of photons (DVCS) or of mesons (DVMP). Experiments are starting to test the validity of these concepts. First results are discussed and new experimental projects presented, with an emphasis on this program at Jefferson Lab.Received: 30 September 2002, Published online: 22 October 2003PACS: 12.38.-t Quantum chromodynamics - 13.60.-r Photon and charged-lepton interactions with hadrons - 14.20.Dh Properties of protons and neutrons     

Nucleon Generalized Parton Distributions and Holographic Models

Using ideas from light front holography, we discuss the calculation of the nucleon helicity-independent generalized parton distributions of quarks in the zero skewness case.     

Generalized Parton Distributions in Full Lattice QCD

We present recent results on generalized parton distributions from dynamical lattice QCD calculations. Our set of twelve different combinations of couplings and quark masses allows for a preliminary study of the pion mass dependence of the transverse nucleon structure.     

Generalized Parton Distributions and deep virtual Compton scattering

We give a pedagogical introduction to the field of Generalized Parton Distributions and review shortly the experimental situation and perspective for Deep Virtual Compton Scattering.     

Generalized Parton Distributions in Constituent Quark Models

An approach is proposed to calculate Generalized Parton Distributions (GPDs) in a Constituent Quark Model (CQM) scenario. These off-diagonal distributions are obtained from momentum space wave functions to be evaluated in a given non-relativistic or relativized CQM. The general relations linking the twist-two GPDs to the form factors and to the leading-twist quark densities are consistently recovered from our expressions. Results for the leading twist, unpolarized GPD, H, in a simple harmonic-oscillator model, as well as in the Isgur and Karl model, are shown to have the general behavior found in previous estimates. NLO evolution of the obtained distributions, from the low-momentum scale of the model up to the experimental one, is also shown. Further applications of the proposed formalism are addressed. Received: 31 January 2002 / Accepted: 25 November 2002 / Published online: 18 March 2003 RID="a" ID="a"e-mail: sergio.scopetta@pg.infn.it RID="b" ID="b"e-mail: vicente.vento@uv.es Communicated by A. Sch?fer     

Generalized Parton Distributions of the Pion on the Transverse Lattice

The quark-generalized parton distributions of the pion are calculated from light-cone wavefunctions in transverse lattice gauge theory at large N_c. The pion effective size is found to decrease with increasing momentum transfer. An analytic ansatz, consistent with finite bound-state light-cone energy conditions, is given for the light-cone momentum dependence of the wavefunctions. This leads to simple, universal predictions for the behaviour of the distributions near the endpoints, complementing numerical DLCQ data.     

Neutron Transverse-Momentum Distributions and Polarized 3He within Light-Front Hamiltonian Dynamics

The possibility to extract the quark transverse-momentum distributions in the neutron from semi-inclusive deep inelastic electron scattering off polarized ³He is illustrated through an impulse approximation analysis in the Bjorken limit. The generalization of the analysis at finite momentum transfers in a Poincaré covariant framework is outlined. The definition of the light-front spin-dependent spectral function of a J = 1/2 system allows us to show that within the light-front dynamics only three of the six leading twist T-even transverse-momentum distributions are independent.     

Generalized Parton Distributions: Visions,Basics, and Realities

An introductory to generalized parton distributions (GDPs) is given which emphasizes their spectral property and its uses as well as the equivalence of various GDP representations. Furthermore, the status of the theory and phenomenology of hard exclusive processes is shortly reviewed.     

Generalized Parton Distributions from Deeply Virtual Compton Scattering at HERMES

The HERMES Collaboration has recently published a set of (correlated) beam charge, beam spin and target spin asymmetries for the Deeply Virtual Compton Scattering (DVCS) process. This reaction allows in principle to access the Generalized Parton Distributions (GPDs) of the nucleon. We have fitted, in the QCD leading-order and leading-twist handbag approximation, but in a model-independent way, this set of data and we report our results for the extracted Compton form factors. In particular, we are able to extract constrains on the H GPD.     

A fitter code for Deep Virtual Compton Scattering and Generalized Parton Distributions

We have developed a fitting code based on the leading-twist handbag Deep Virtual Compton Scattering (DVCS) amplitude in order to extract Generalized Parton Distribution (GPD) information from DVCS observables in the valence region. In a first stage, with simulations and pseudo-data, we show that the full GPD information can be recovered from experimental data if enough observables are measured. If only some of these observables are measured, valuable information can still be extracted, with certain observables being particularly sensitive to certain GPDs. In a second stage, we make a practical application of this code to the recent DVCS Jefferson Lab Hall-A data from which we can extract numerical constraints for the two H GPD Compton form factors. An erratum to this article can be found at     

Pion Tensor Generalized Parton Distributions in a Covariant Constituent Quark Model

The pion tensor generalized parton distributions are evaluated within a covariant, analytic constituent quark model. The generalized form factors for the first two Mellin moments and the probability densities of polarized quarks in the impact parameter space are derived and compared with lattice QCD and quark model results.     

Transversity Form Factors and Generalized Parton Distributions of the Pion in Chiral Quark Models

The transversity Generalized Parton Distributions (tGPDs) and related transversity form factors of the pion are evaluated in chiral quark models, both local (Nambu–Jona–Lasinio) and nonlocal, involving a momentum-dependent quark mass. The obtained tGPDs satisfy all a priori formal requirements, such as the proper support, normalization, and polynomiality. We evaluate generalized transversity form factors accessible from the recent lattice QCD calculations. These form factors, after the necessary QCD evolution, agree very well with the lattice data, confirming the fact that the spontaneously broken chiral symmetry governs the structure of the pion also in the case of the transversity observables.     

Integrated and Unintegrated Parton Distributions

We compare the twist-2 and twist-4 parton distributions both in integrated and unintegrated parton distributions. We find that when x decreases the difference between the twist-2 and twist-4 parton distributions become obvious except for the valence quark distributions. We hope the future experiments may distinguish whether the twist-4 correction is needed or not and select a reasonable unintegrated parton distribution model.     

Neutron Structure of the Ground State of 7He

JETP Letters - A study of the neutron structure of the ground state of 7He has been performed by means of registration and analysis of the decay channels of the residual nuclei following absorption...     

Neutron beam effects on spin-exchange-polarized 3He

We have observed depolarization effects when high intensity cold neutron beams are incident on alkali-metal spin-exchange-polarized 3He cells used as neutron spin filters. This was first observed as a reduction of the maximum attainable 3He polarization and was attributed to a decrease of alkali-metal polarization, which led us to directly measure alkali-metal polarization and spin relaxation over a range of neutron fluxes at Los Alamos Neutron Science Center and Institute Laue-Langevin. The data reveal a new alkali-metal spin-relaxation mechanism that approximately scales as sqrt[phi_{n}], where phi_{n} is the neutron capture-flux density incident on the cell. This is consistent with an effect proportional to the concentration of electron-ion pairs but is much larger than expected from earlier work.     

QCD Predictions for Spin Dependence of Parton Distributions

We introduce the polarized valon distributions to describe the spin dependence of parton distributions. The polarized valon distributions in the proton and polarized parton distributions inside the valon can help to obtain polarized parton distribution in a proton. In order to be able to obtain the spin contribution of sea quarks, we need to improve the valon model. We employ the Bernstein polynomial averages to obtain the unknown parameters which exist in our calculations. Our results for the polarized proton structure function, xg^p₁, are in good agreement with the experimental data for some values of Q².