Deeply virtual Compton scattering off nuclei

Abstract

We consider the hard leptoproduction of a photon off nuclei up to spin-1. As a new result we present here the general azimuthal angular dependence of the differential cross section for a spin-1 target. Its twist-two Fourier coefficients of the interference and squared deeply virtual Compton scattering amplitude are evaluated in leading order approximation of perturbation theory in terms of generalized parton distributions, while the pure Bethe-Heitler cross section is exactly calculated in terms of electromagnetic form factors. Relying on a simple model for the nucleon generalized parton distribution H, which describes the existing DVCS data for a proton target, we estimate the size of unpolarized cross sections, beam and longitudinal target spin as well as unpolarized charge asymmetries for present fixed target experiments with nuclei. These estimates are confronted with preliminary HERMES data for deuterium and neon.

Measurement of azimuthal asymmetries associated with deeply virtual Compton scattering on an unpolarized deuterium target

“…Both H 3 and H 5 are associated with the 5% D-wave component of the deuteron wave function in terms of nucleons [34]. H 3 is related to isoscalar currents and probes the binding forces in the deuteron, and H 5 involves a tensor term [32,35], the analog of which has no relationship to any local current due to Lorentz invariance.…”

“…For a target of atomic mass number A, the cross section for the hard exclusive leptoproduction of real photons is given by [35,36] …”

“…The Fourier coefficients c I n and s I n in Eq. 9 can be expressed as linear combinations of Compton Form Factors F (ξ, t) (CFFs) [35], which in turn are convolutions of the corresponding GPDs F q (x, ξ, t) with the hard scattering coefficient functions C ∓ q [11,12,13]:…”

Angular momentum sum rule for spin-one hadronic systems

“…The question of whether the quarks' OAM can actually be measured for a deuteron target is therefore mandatory. While observables were presented in [24] that contain several deuteron GPDs, none of them is sensitive to H 2 . Here we suggest the measurement of the deuteron target transverse spin asymmetry, A U T , which we derive in terms of GPDs as,…”

Generalized parton distributions of nuclei

“…Since nuclei consist of protons and neutrons, models of nuclear GPDs require proton and neutron GPDs as input [1,2,3]. Therefore, measurements of deeply virtual Compton scattering (DVCS) with nuclear targets in the coherent (the target nucleus stays intact) and incoherent (the target nucleus breaks up) regimes provide additional constraints on the nucleon GPDs, which are complimentary to the information extracted from DVCS on a free proton.…”