Abstract
Deeply virtual exclusive reactions provide a unique opportunity to probe the complex internal structure of the nucleon. They allow to access information about the correlations between parton transverse spatial and longitudinal momentum distributions from experimental observables. Dedicated experiments to study Deeply Virtual Compton Scattering (DVCS) and Deeply Virtual Meson Production (DVMP) have been carried out at Jefferson Lab using continuous electron beam with energies up to 6 GeV. Unpolarized cross sections, beam, target and double spin asymmetries have been measured for DVCS as well as for π 0 exclusive electroproduction. The data from Hall B provide a wide kinematic coverage with Q 2 =1-4.5 GeV 2 , xB=0.1-0.5, and −t up to 2 GeV 2 . Hall A data have limited kinematic range partially overlapping with Hall B kinematics but provide a high accuracy measurements. Scaling tests of the DVCS cross sections provide solid evidence of twist-2 dominance, which makes chiral-even GPDs accessible even at modest Q 2 . We will discuss the interpretation of these data in terms of Generalized Parton Distributions (GPDs) model. Successful description of the recent CLAS π 0 exclusive production data within the framework of the GPD-based model provides a unique opportunity to access the chiral-odd GPDs.
Highlights
In recent years, parton distribution functions have been generalized to contain information on the longitudinal and on the transverse distributions of partons in a fast moving hadron
The Generalized Parton Distributions (GPDs) [1,2,3] add important piece of information that is missing in one-dimensional parton densities, in particular, distribution of partons in the plane transverse to the direction of motion
In Bjorken scaling regime, the scattering amplitude of hard exclusive processes, such as Deeply Virtual Compton Scattering (DVCS) and Deeply Virtual Meson Production (DVMP), factorizes into a hard scattering part and soft part described by Generalized Parton Distributions
Summary
Parton distribution functions have been generalized to contain information on the longitudinal and on the transverse distributions of partons in a fast moving hadron. In Bjorken scaling regime, the scattering amplitude of hard exclusive processes, such as Deeply Virtual Compton Scattering (DVCS) and Deeply Virtual Meson Production (DVMP), factorizes into a hard scattering part and soft part described by Generalized Parton Distributions This opens the prospect of using exclusive meson electroproduction processes as a means to systematically explore the quark structure of the nucleon. Hall-B at Jefferson Lab measured surprisingly large π0 beam spin asymmetry (BSA) values that strongly suggests substantial contributions from transversely polarized photons [12], otherwise BSA should be zero or very small These contributions can be calculated within a handbag approach as the convolutions of leading-twist chiral-odd GPDs with a twist-3 meson distribution amplitude [13,14,15]. The variable x is integrated over and can not be accessed experimentally
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