Deeply virtual Compton scattering off Helium nuclei with positron beams

Positron initiated deeply virtual Compton scattering (DVCS) off $$^4$$ 4 He and $$^3$$ 3 He nuclei is described. The way the so-called $$d-$$ d - term could be obtained from the real part of the relevant Compton form factor is summarized, and the importance and novelty of this measurement is discussed. The measurements addressed for $$^3$$ 3 He targets could be very useful even in a standard unpolarized target setup, measuring beam spin and beam charge asymmetries only. The unpolarized beam charge asymmetries for DVCS off $$^3$$ 3 He and $$^4$$ 4 He are also estimated, at JLab kinematics and, for $$^4$$ 4 He, also at a configuration typical at the future Electron–Ion Collider. Incoherent DVCS processes, in particular the ones with tagging the internal target by measuring slow recoiling nuclei, and the unique possibility offered by positron beams for the investigation of Compton form factors of higher twist, are also briefly addressed.

Investigating GPDs in the framework of the double distribution model

In this paper, we construct the generalized parton distribution (GPD) in terms of the kinematical variables [Formula: see text], [Formula: see text], [Formula: see text], using the double distribution model. By employing these functions, we could extract some quantities which makes it possible to gain a three-dimensional insight into the nucleon structure function at the parton level. The main objective of GPDs is to combine and generalize the concepts of ordinary parton distributions and form factors. They also provide an exclusive framework to describe the nucleons in terms of quarks and gluons. Here, we first calculate, in the Double Distribution model, the GPD based on the usual parton distributions arising from the GRV and CTEQ phenomenological models. Obtaining quarks and gluons angular momenta from the GPD, we would be able to calculate the scattering observables which are related to spin asymmetries of the produced quarkonium. These quantities are represented by [Formula: see text] and [Formula: see text]. We also calculate the Pauli and Dirac form factors in deeply virtual Compton scattering. Finally, in order to compare our results with the existing experimental data, we use the difference of the polarized cross-section for an initial longitudinal leptonic beam and unpolarized target particles [Formula: see text]. In all cases, our obtained results are in good agreement with the available experimental data.

FIRST MEASUREMENTS OF THE ρ3 SPIN DENSITY MATRIX ELEMENTS IN γp → pω USING CLAS AT JLAB

In an effort towards a "complete" experiment for the ω meson, we present studies from an experiment with an unpolarized target and a circularly polarized photon beam (g1c), carried out using the CEBAF Large Acceptance Spectrometer (CLAS) at Jefferson Lab. The experiment was analyzed using an extended maximum likelihood fit with partial wave amplitudes. New likelihood functions were calculated to account for the polarization of the photon beam. Both circular and linear polarizations are explored. The results of these fits are then used to project out the spin density matrix for the ω. First measurements of the ρ3 spin density matrix elements will be presented using this method.