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.     

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     

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.     

Valence Double Parton Distributions of the Nucleon in a Simple Model

Valence double parton distribution functions of the nucleon are evaluated in the framework of a simple model, where the conservation of the longitudinal momentum is taken into account. The leading-order DGLAP QCD evolution from the low quark-model scale to higher renormalization scales is carried out via the Mellin moments of the distributions. Results of the valence quark correlation function show that in general the double distributions cannot be approximated as a product of the single-particle distributions.     

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.     

Impact Parameter-Dependent Parton Distributions for a Relativistic Composite System

We investigate the impact parameter-dependent parton distributions for a relativistic composite system in light-front framework. We express them in terms of overlaps of light-cone wave functions for a self-consistent two-body spin- state, namely an electron dressed with a photon in QED. The pdf’s are distorted in the transverse space for transverse polarization of the state at one-loop level.     

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.     

Wigner Distributions in Light-Front Quark Models

We discuss the quark Wigner distributions which represent the quantum-mechanical analogues of the classical phase-space distributions. These functions can be obtained through a Fourier transform in the transverse space of the generalized transverse momentum dependent parton distributions, which encode the most general one-body information of partons in momentum space. In particular, we present a study within light-front quark models. The quark orbital angular momentum is also obtained from the phase-space average of the orbital angular momentum operator weighted with the Wigner distribution of unpolarized quark in a longitudinally polarized nucleon. The corresponding results calculated within different light-front quark models are compared with alternative definitions of the quark orbital angular momentum as given in terms of generalized parton distributions and transverse momentum dependent parton distributions.     

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.     

Generalized Parton Distributions of 3He and the Neutron Orbital Structure

The two leading twist, quark helicity conserving generalized parton distributions (GPDs) of ³He, accessible, for example, in coherent deeply virtual Compton scattering (DVCS), are calculated in impulse approximation (IA). Their sum, at low momentum transfer, is found to be largely dominated by the neutron contribution, so that ³He is very promising for the extraction of the neutron information. Anyway, such an extraction could be not trivial. A technique, able to take into account the nuclear effects included in the IA analysis in the extraction procedure, even at moderate values of the momentum transfer, is proposed. Coherent DVCS arises therefore as a crucial experiment to access, for the first time, the neutron GPDs and the orbital angular momentum of the partons in the neutron.     

Dynamical correlations in the Dual Parton Model

The effect of multiple ‘chain’ contributions to particle production, which appears in the Dual Parton Model at collider energies, is investigated in an exploratory model, in which other sources of correlations are largely neglected. Attention is given to the question of KNO scaling for the multiplicities observed in restricted or unrestricted rapidity ranges. The rapidity distribution associated with a fixed number of particles within a suitably chosen interval, is found to be sensitive to details of the mechanism of multiple chain production.     

Extended Quark Potential Model from Random Phase Approximation

The quark potential model is extended to include the sea quark excitation using the random phase approx-imation. The effective quark interaction preserves the important QCD properties - chiral symmetry and confinementsimultaneously. A primary qualitative analysis shows that the π meson as a well-known typical Goldstone boson andthe other mesons made up of valence qq quark pair such as the ρ meson can also be described in this extended quarkpotential model.     

Mesons in the Constituent Quark Model

The quark-antiquark （q^-q） spectrum is studied by solving the Schrōdinger equation in the framework of nonrelativistic constituent quark model. An overall good fit to the experimental data of meson is obtained. The interactions between quark and antiquark consist of quadratic colour confinement-exchange, one-gluon-exchange, and Goldstone-boson-exchange potentials.     

Tetraneutron in the Chiral Quark Model

The tetraneutron state is studied in the framework of the chiral quark model with tetrahedron configuration. The universal attraction property of σ-meson exchange leads to a strong attraction in the effective potential of the system. It is possible to form a bound state. For comparison, the naive quark model is also employed to carry out the calculation. A weak attraction, which is too weak to bind the system, is obtained.     

Sum Rules in the Quark Model

With the help of various sum rules we look into the spin-isospin excitations in nuclei within a framework of the quark model. The effect of Pauli correlations is explicitly taken care of. In agreement with the experiments we find negligible amount of M1 strength in heavy nuclei. This is explained in terms of the Fermi gas model.