Symmetries and structure of skewed and double distributions

Extending the concept of parton densities onto nonforward matrix elements of quark and gluon light-cone operators, one can use two types of nonperturbative functions: double distributions (DDs) f(x,\alpha;t), F(x,y;t) and skewed (off&nonforward) parton distributions (SPDs) H(x,\xi;t), F_\zeta(X,t). We treat DDs as primary objects producing SPDs after integration. We emphasize the role of DDs in understanding interplay between X (x) and \zeta (\xi) dependences of SPDs.In particular, the use of DDs is crucial to secure the polynomiality condition: Nth moments of SPDs are Nth degree polynomials in the relevant skewedness parameter \zeta or \xi. We propose simple ansaetze for DDs having correct spectral and symmetry properties and derive model expressions for SPDs satisfying all known constraints. Finally, we argue that for small skewedness, one can obtain SPDs from the usual parton densities by averaging the latter with an appropriate weight over the region [X-\zeta,X] (or [x - \xi, x + \xi]).Comment: 10 pages, Latex, 3 figure

Compton scattering and nonforward parton distributions

The hard exclusive electroproduction processes provide new information about hadronic structure accumulated in nonforward parton distributions. The NFPD's are universal hybrid functions having the properties of parton densities, hadronic form factors and distribution amplitudes. They give a unified description of various hard exclusive and inclusive reactions. The basic supplier of information about nonforward parton distributions is deeply virtual Compton scattering which offers a remarkable example of Bjorken scaling phenomena in exclusive processes. Wide-angle real Compton scattering is an ideal tool to test angle-dependent scaling laws characteristic for soft overlap mechanism. Hard meson electroproduction is the best candidate to see pQCD hard gluon exchange in exclusive reactions.Comment: 11 pages, Latex, 6 figures; Contribution to proceedings of the workshop ``Physics and instrumentation with 6-12 GeV beams'', Jefferson Lab, June 15-18, 199

Modeling Nucleon Generalized Parton Distributions

We discuss building models for nucleon generalized parton distributions (GPDs) H and E that are based on the formalism of double distributions (DDs). We found that the usual "DD+D-term" construction should be amended by an extra term, \xi E^1_+(x,\xi) built from the \alpha/\beta moment of the DD e(\beta,\alpha) that generates GPD E(x,\xi). Unlike the D-term, this function has support in the whole -1 \leq x \leq 1 region, and in general does not vanish at the border points |x|=\xi.Comment: Minor fixes, version to be published in PR

Nucleon Form Factors from Generalized Parton Distributions

We discuss the links between Generalized Parton Distributions (GPDs) and elastic nucleon form factors. These links, in the form of sum rules, represent powerful constraints on parametrizations of GPDs. A Regge parametrization for GPDs at small momentum transfer, is extended to the large momentum transfer region and it is found to describe the basic features of proton and neutron electromagnetic form factor data. This parametrization is used to estimate the quark contribution to the nucleon spin.Comment: 18 pages, 5 figures, replaced with published versio