Measurement of Deeply Virtual Compton Scattering with a Polarized-Proton Target

Abstract

The longitudinal target-spin asymmetry $A_{\mathrm{UL}}$ for the exclusive electroproduction of high-energy photons was measured for the first time in $e\overrightarrow{p}\to e^{\prime }p\gamma$. The data have been accumulated at JLab with the CLAS spectrometer using 5.7 GeV electrons and a longitudinally polarized ${\mathrm{NH}}_3$ target. A significant azimuthal angular dependence was observed, resulting from the interference of the deeply virtual Compton scattering and Bethe-Heitler processes. The amplitude of the $\sin \varphi$ moment is $0.252\pm {0.042}^{\mathrm{stat}}\pm {0.020}^{\mathrm{sys}}$. Theoretical calculations are in good agreement with the magnitude and the kinematic dependence of the target-spin asymmetry, which is sensitive to the generalized parton distributions $\tilde{H}$ and $H$.