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

TL;DR

This paper reports the first measurement of beam spin asymmetry in deeply virtual Compton scattering off helium-4, revealing enhanced coherent asymmetries and enabling extraction of the helium-4 Compton form factor for nuclear structure insights.

Contribution

It introduces a novel experimental setup with upgraded CLAS to measure DVCS off helium-4 and extracts the helium-4 Compton form factor in a model-independent manner.

Findings

Confirmed enhancement of coherent beam spin asymmetries on helium-4.

Observed 30% suppression of incoherent asymmetries.

Extracted the real and imaginary parts of the helium-4 Compton form factor.

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'$$^4$He$'\gamma'$) beam spin asymmetries compared to those observed on the free proton, while the incoherent ($e^4$He$~\to~e'$p$'\gamma'$X$'$) asymmetries exhibit a 30$\%$ suppression. From the coherent data, we were able to extract, in a model-independent way, the real and imaginary parts of the only $^4$He Compton form factor, $\cal H_A$, leading the way toward 3D imaging of the partonic structure of nuclei.