Measurement of deeply virtual Compton scattering off He 4 with the CEBAF Large Acceptance Spectrometer at Jefferson Lab

We report on the measurement of the beam spin asymmetry in the deeply virtual Compton scattering off He4 using the CEBAF Large Acceptance Spectrometer (CLAS) at Jefferson Lab using a 6 GeV longitudinally polarized electron beam incident on a pressurized He4 gaseous target. We detail the method used to ensure the exclusivity of the measured reactions, in particular the upgrade of CLAS with a radial time projection chamber to detect the low-energy recoiling He4 nuclei and an inner calorimeter to extend the photon detection acceptance at forward angles. Our results confirm the theoretically predicted enhancement of the coherent (eHe4→e′He4′γ′) beam spin asymmetries compared to those observed on the free proton, while the incoherent (eHe4→e′p′γ′X′) asymmetries exhibit a 30% suppression. From the coherent data, we were able to extract, in a model-independent way, the real and imaginary parts of the only He4 Compton form factor, HA, leading the way toward 3D imaging of the partonic structure of nuclei