M. Hattawy et al. (CLAS Collaboration), Phys. Rev. Lett. 119, 202004 (2017)

A wealth of information on the structure of matter lies in the correlations between the momentum and spatial distributions of the fundamental constituents of matter : quarks and gluons. These correlations are accessible via the study of generalized parton distributions (GPDs), which we probe with hard exclusive scattering reactions. Deeply-virtual Compton scattering (DVCS) reaction is the preferred probe of GPDs and has been used to study the proton in a number of experiments at HERA, CERN, and Jefferson Lab (JLab), yielding the first 3D partonic images of the proton. New groundbreaking measurements of exclusive DVCS from the ⁴He nucleus are a critical step towards providing similar 3D pictures of the quark structure of nuclei, and provide a new approach to understanding the modification of protons and neutrons within the dense environment of a nucleus. Studies of ⁴He are particularly important, as their partonic structure is encoded within a single GPD, simplifying their extraction and interpretation.

Our new measurements provided the first beam-spin asymmetry data on exclusive DVCS from ⁴He using a highly-polarized electron beam from the Continuous Electron Beam Facility (CEBAF) at JLab. Scattering from a pressurized ⁴He target was measured in the CEBAF Large Acceptance Spectrometer (CLAS). Isolating exclusive scattering where the nucleus is not broken up in the high-energy collision requires detection of the recoil ⁴He nucleus. The recoil nucleus was detected using a new radial time projection chamber to ensure that this rare scattering process was cleanly identified. From fitting the azimuthal dependence of the beam-spin asymmetry (A_LU), shown in Fig. 1, the real and imaginary parts of the ⁴He Compton form factor (CFF), which directly sample the nuclear GPD, were extracted in a model-independent way, presented in Fig.2. 

These data, while limited in statistics, proved the experimental feasibility of measuring such nuclear exclusive reactions and led the way to the approval of a next generation nuclear measurements. The future measurements will be carried out using the new CLAS12 spectrometer and the upgraded CEBAF 12 GeV electron beam. In addition to DVCS on ⁴He, deeply-virtual meson production of φ-meson will be measured, giving us the chance to compare directly the quark and the gluon 3D tomography of the ⁴He nucleus.

FIG. 1 : A_LU as a function of azimuthal angle φ. Results are presented for different Q² bins (top panel), x_B bins (middle panel), and t bins (bottom panel). The error bars represent the statistical uncertainties. The gray bands represent the systematic uncertainties, including the normalization uncertainties. The red curves are the results of fits with the real and the imaginary parts of the ⁴He CFF

as the free parameters. FIG. 2 : The leading-twist model-independent extraction of the imaginary (top panel) and real (bottom panel) parts of the ⁴He CFF (H_A), as a function of Q² (left panel), x B (middle panel), and t (right panel). The full red curves are the calculations based on a convolution model. The black-dashed curves are calculations from the same model using different GPDs for the nucleons (VGG model). The blue long-dashed curve on the top-right plot is from an off-shell model.