Covariant extension of the GPD overlap representation at low Fock states

We present a novel approach to compute generalized parton distributions within the lightfront wave function overlap framework. We show how to systematically extend generalized parton distributions computed within the DGLAP region to the ERBL one, fulfilling at the same time both the polynomiality and positivity conditions. We exemplify our method using pion lightfront wave functions inspired by recent results of non-perturbative continuum techniques and algebraic nucleon lightfront wave functions. We also test the robustness of our algorithm on reggeized phenomenological parameterizations. This approach paves the way to a better understanding of the nucleon structure from non-perturbative techniques and to a unification of generalized parton distributions and transverse momentum dependent parton distribution functions phenomenology through lightfront wave functions.     

Probing quark-distribution amplitudes through generalized parton distributions at large momentum transfer

In the large momentum transfer limit, generalized parton distributions can be calculated through a QCD factorization theorem which involves perturbatively calculable hard kernels and light-cone parton distribution amplitudes of hadrons. We illustrate this through the H(q)(x,xi,t) distribution for the pion and the proton, presenting the hard kernels at leading order. As a result, experimental data on the generalized parton distributions in this regime can be used to determine the functional form of the parton distribution amplitudes which has thus far been quite challenging to obtain. Our result can also be used as a constraint in phenomenological generalized parton distribution parametrizations.     

Pion production by fragmentation of weak currents in Weinberg theory

The Feynman quark parton model is used to study inclusive single-pion production by the fragmentation of the neutral and charged weak currents of Weinberg theory. The structure functions for the neutral current-induced reactions are related to those for the electromagnetic and charged weak current processes. Analogues of inclusive deep-inelastic sum rules are derived. The ratios of neutral current to charged current cross sections for semi-inclusive pion production are studied. In the approximation of neglect of “core” partons, these ratios are given in terms of average pion multiplicities in the charged current-induced reaction. We finally specialise to the quark parton distributions of McElhaney and Tuan to calculate these ratios as functions of the Weinberg angle.     

Semi-inclusive neutrino scattering and pion structure functions

We show that a study of the processes

can provide information on the pion structure functions. Using a realistic parton distribution for the pion we calculate cross sections and relevant distributions for these processes. The results show that such measurements are experimentally accessible.     

Parton Distribution Functions from Lattice QCD

We present recent results on the first moments of parton distributions using gauge configurations generated with two degenerate flavors of light twisted mass quarks with pion mass fixed approximately to its physical value. We also present a first study of the vector parton distribution function using a twisted mass ensemble at pion mass of 373 MeV.     

Generalized vector form factors of the pion in a chiral quark model

Generalized vector form factors of the pion, related to the moments of the generalized parton distribution functions, are evaluated in the Nambu-Jona-Lasinio model with the Pauli-Villars regularization. The lowest moments (the electromagnetic and the gravitational form factors) are compared to recent lattice data, with fair agreement. Predictions for higher-order moments are also made. Relevant features of the generalized form factors in the chiral quark models are highlighted and the role of the QCD evolution for the higher-order GFFs is stressed.     

The generalized parton distribution of the pion in a relativistic Bethe-Salpeter quark model

We present a Poincaré covariant calculation of the generalized parton distribution of the pion. Results for different values of the kinematical parameters are shown and discussed.     

Factorization breaking in dijet photoproduction with a leading neutron

The production of dijets with a leading neutron in ep interactions at HERA is calculated in leading order and next-to-leading order of perturbative QCD using a pion-exchange model. Differential cross sections for deep-inelastic scattering and photoproduction are presented as a function of several kinematic variables. By comparing the theoretical predictions for DIS dijets to recent H1 data, the pion flux factor together with the parton distribution functions of the pion is determined. The dijet cross sections in photoproduction show factorization breaking if compared to the H1 photoproduction data. The suppression factor is S=0.48 (0.64) for resolved (global) suppression. PACS 12.38.Bx; 13.60.-r     

Meson polarized distribution function and mass dependence of the nucleon parton densities

The polarized distribution functions of mesons,including pion,kaon and eta,using the proton structure function,are calculated.We are looking for a relationship between the polarized distribution of mesons and the polarized structure of nucleons.We show that the meson polarized parton distributions leads to zero total spin for the concerned mesons,considering the orbital angular momentum of quarks and gluons inside the meson.Two separate Monte Carlo algorithms are applied to compute the polarized parton distributions of the kaon.Via the mass dependence of quark distributions,the distribution function of the eta meson is obtained.A new method by which the polarized sea quark distributions of protons are evolved separately-which cannot be performed easily using the standard solution of DGLAP equations-is introduced.The mass dependence of these distributions is obtained,using the renormalization group equation which makes their evolutions more precise.Comparison between the evolved distributions and the available experimental data validates the suggested solutions for separated evolutions.     

The distribution of partons in a hadron

The distribution of partons in the dual model of Nielsen and Susskind is derived and some simple explanations of the relation between Regge behavior form factors, pion production and the parton distribution are explained in qualitative terms. A possible modification of this distribution is introduced in order to ‘flatten’ the pomeron.     

Implication of the overlap representation for modelling generalized parton distributions

Based on a field theoretically inspired model of light-cone wave functions, we derive valence-like generalized parton distributions and their double distributions from the wave function overlap in the parton number conserved s-channel. The parton number changing contributions in the t-channel are restored from duality. In our construction constraints of positivity and polynomiality are simultaneously satisfied and it also implies a model dependent relation between generalized parton distributions and transverse momentum dependent parton distribution functions. The model predicts that the t  -behavior of resulting hadronic amplitudes depends on the Bjorken variable xBj$x_{\mathrm{Bj}}$. We also propose an improved ansatz for double distributions that embeds this property.     

Structure Function of the Bound Proton in the Dressed-quark Exchange Formalism

The idea of the dressed quark scenario, in the context of the quark exchange model has been used to obtain the bound parton distribution functions as well as the bound proton structure function. The Glück, Reya, and Vogt (GRV)’s parton distribution functions, where they have a good agreement with the experimental data in the whole range of (x, Q²)-region, have been used to extract the amount of the momentum carried by each parton inside the proton; therefore, by using these values, the different parton densities inside the constituent quark have been calculated. To extract the parton distribution functions in the bound proton, the phenomenological constituent quark exchange formalism and the nuclear convolution theorem have been used. By considering these densities, the structure function of the bound proton, \({F_{2}^{P}}(x,Q^{2})\), inside the light nuclei like helium-3 or tritium has been calculated at low- and high-Q² values. The correctness of the calculations has been tested through computing the difference of the proton-neutron structure functions as well as the ratio of the neutron to the proton structure functions, in which they are well known from experimental data. The results of the present study are in an agreement with both the available and relevant experimental data and the theoretical predictions.

     

Recent results from the HERMES experiment

Results are presented from the Hermes experiment which uses semi-inclusive deep inelastic lepton scattering to study the flavor structure of the nucleon. Data have been accumulated for pion and kaon double spin asymmetries, single-spin azimuthal asymmetries for meson electroproduction, deep virtual Compton scattering (DVCS), and meson multiplicities. These results provide information on the properties of the strange sea in the proton, constraints on transverse momentum dependent quark parton distributions, and demonstrate the promise of DVCS for isolating the total angular momentum carried by the quarks in the proton.     

Next-to-leading-order calculation in kT factorization

We explain the framework for calculating next-to-leading-order (NLO) corrections to exclusive processes in the kT factorization theorem, taking πγ→γ T as an example. Partons off-shell by k2T are considered in both the quark diagrams from full QCD and the effective diagrams for the pion wave function. The gauge dependences in the above two sets of diagrams cancel, when deriving the kT-dependent hard kernel as their difference. The light-cone singularities in the kT-dependent pion wave function are regularized by rotating the Wilson lines away from the light cone. Both the large double logarithms In2 kT and ln2x, x being a parton momentum fraction, arise from the loop correction to the virtual photon vertex, the former being absorbed into the pion wave function, and the latter into a jet function.     

Pion distributions in inelastic neutrino interactions

Using data on semi-inclusive pion electroproduction as input we calculate in the framework of the quark parton model the distribution of pions in the current fragmentation region for inelastic neutrino interactions. Results for the π⁺/π^? asymmetry are presented for both charged and neutral currents. These provide stringent tests of the parton fragmentation hypothesis and the Weinberg-Salam gauge model.     

Deeply virtual Compton scattering on a virtual pion target

We study deeply virtual Compton scattering on a virtual pion that is emitted by a proton. Using a range of models for the generalized parton distributions of the pion, we evaluate the cross section, as well as the beam spin and beam charge asymmetries in the leading-twist approximation. Studying Compton scattering on the pion in suitable kinematics puts high demands on both beam energy and luminosity, and we find that the corresponding requirements will first be met after the energy upgrade at Jefferson Laboratory. As a by-product of our study, we construct a parameterization of pion generalized parton distributions that has a non-trivial interplay between the x and t dependence and is in good agreement with form factor data and lattice calculations.     

Final state interactions and the transverse structure of the pion using non-perturbative eikonal methods

In the factorized picture of semi-inclusive hadronic processes the naive time-reversal odd parton distributions exist by virtue of the gauge link which renders it color gauge invariant. The link characterizes the dynamical effect of initial/final-state interactions of the active parton due soft gluon exchanges with the target remnant. Though these interactions are non-perturbative, studies of final-state interaction have been approximated by perturbative one-gluon approximation in Abelian models. We include higher-order gluonic contributions from the gauge link by applying non-perturbative eikonal methods incorporating color degrees of freedom in a calculation of the Boer–Mulders function of the pion. Using this framework we explore under what conditions the Boer–Mulders function can be described in terms of factorization of final state interactions and a spatial distribution in impact parameter space.     

Valence and sea contributions to the nucleon spin

The first moments of the polarized valence parton distribution functions (PDFs) truncated to the wide Bjorken x region 0.004<x<0.7 are directly (without any fitting procedure) extracted in the next to leading order (NLO) QCD from both COMPASS and HERMES data on pion production in polarized semi-inclusive DIS (SIDIS) experiments. The COMPASS and HERMES data are combined in two ways and two scenarios for the fragmentation functions (FFs) are considered. Two procedures are proposed for an estimation of light sea quark contributions to the proton spin. Both lead to the conclusion that these contributions are compatible with zero within the errors.     

1/N calculation of structure functions

We calculate the quark and pion structure functions in CP^n?1 models with quarks using 1/N expansions. We find that the quark structure function, in the lowest order in 1/N, is just the free parton result with a modified coupling constant. The pion structure function resembles the phenomelogically successful fit but contains correction terms which modulate the function.