Measurement of deeply virtual Compton scattering off Helium-4 with CLAS at Jefferson Lab

Abstract

We report on the measurement of the beam spin asymmetry in the deeply virtual Compton scattering off ${}^4$He using the CEBAF Large Acceptance Spectrometer (CLAS) at Jefferson Lab using a 6 GeV longitudinally polarized electron beam incident on a pressurized ${}^4$He 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 ${}^4$He nuclei and an inner calorimeter to extend the photon detection acceptance at forward angles. Our results confirm the theoretically predicted enhancement of the coherent ($e^4$He$\to e^{\prime }$${}^4$He${}^{\prime }{\gamma }^{\prime }$) beam spin asymmetries compared to those observed on the free proton, while the incoherent ($e^4$He$\to e^{\prime }$p${}^{\prime }{\gamma }^{\prime }$X${}^{\prime }$) asymmetries exhibit a 30$\%$ suppression. From the coherent data, we were able to extract, in a model-independent way, the real and imaginary parts of the only ${}^4$He Compton form factor, ${\mathcal{H}}_{\mathcal{A}}$, leading the way toward 3D imaging of the partonic structure of nuclei.

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