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

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 $H$.

Figure 1

Feynman diagrams for DVCS (a) and Bethe-Heitler processes (b) contributing to the amplitude of $ep\to ep\gamma$ scattering.

Figure 2

Missing mass squared distribution of the system ($ep$) for reaction $e\overrightarrow{p}\to e^{\prime }p\gamma$. Panels (a) and (b) show the $M_X^2$ spectrum before and after the cut ${\theta }_{\gamma X}<2°$. The ${}^{12}\mathrm{C}$ data (stars) are normalized to the ${}^{15}\mathrm{NH}_3$ data (solid line) using the negative tail of the $M_X^2$. The two dashed lines show the final cut on $M_X^2$ to select single photon production.

Figure 3

Azimuthal angle $\varphi$ dependence of the measured target-spin asymmetry for photons (a) and ${\pi }^0$ (b). The solid curves represent the fitted function $\alpha \sin \varphi +\beta \sin 2\varphi$ with parameters $\alpha =0.240\pm 0.042$ and $\beta =-0.087\pm 0.045$ (a), and $\alpha =0.109\pm 0.056$ and $\beta =-0.319\pm 0.061$ (b). In (a), the photon events are still contaminated by ${\pi }^0$ events.

Figure 4

The azimuthal angle $\varphi$ dependence of the target-spin asymmetry for exclusive electroproduction of photons after subtraction of the ${\pi }^0$ background. The dashed curve is the full model prediction using the $\xi$-dependent GPD parametrization [9] ($b_{\mathrm{val}}=b_{\mathrm{sea}}=1$, and $E=\tilde{E}=0$) based on MRST02 unpolarized PDFs [20] and polarized PDFs [21] for the twist-2 terms, and higher twists included in those terms. The dotted curve shows the asymmetry when $\tilde{H}=0$. The solid curve is described in the text.

Figure 5

The left panel shows the $-t$ dependence of the $\sin \varphi$ moment of $A_{\mathrm{UL}}$ for exclusive electroproduction of photons, while the right shows the $\xi$ dependence. Curves as in Fig. 4.