We report on the first measurement of the beam-spin asymmetry in the exclusive process of coherent deeply virtual Compton scattering off a nucleus. The experiment uses the 6 GeV electron beam from the Continuous Electron Beam Accelerator Facility (CEBAF) accelerator at Jefferson Lab incident on a pressurized ${}^4\mathrm{He}$ gaseous target placed in front of the CEBAF Large Acceptance Spectrometer (CLAS). The scattered electron is detected by CLAS and the photon by a dedicated electromagnetic calorimeter at forward angles. To ensure the exclusivity of the process, a specially designed radial time projection chamber is used to detect the recoiling ${}^4\mathrm{He}$ nuclei. We measure beam-spin asymmetries larger than those observed on the free proton in the same kinematic domain. From these, we are able to extract, in a model-independent way, the real and imaginary parts of the only ${}^4\mathrm{He}$ Compton form factor, ${\mathcal{H}}_A$. This first measurement of coherent deeply virtual Compton scattering on the ${}^4\mathrm{He}$ nucleus, with a fully exclusive final state via nuclear recoil tagging, leads the way toward 3D imaging of the partonic structure of nuclei.

中文翻译：

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对He4进行深度虚拟康普顿散射的首次独家测量：朝原子核的3D层析成像

我们报告了在相干的深虚拟康普顿散射离核的唯一过程中对光束自旋不对称性的首次测量。该实验使用来自杰斐逊实验室的连续电子束加速器设施（CEBAF）加速器的6 GeV电子束在加压条件下入射${}^4他$气体目标放置在CEBAF大验光谱仪（CLAS）的前面。散射电子由CLAS检测，光子由专用电磁热量计以正向角度检测。为了确保过程的排他性，使用了专门设计的径向时间投影腔来检测反冲${}^4他$核。我们测量的自旋不对称性大于在相同运动域中在自由质子上观察到的自旋性。从这些模型中，我们能够以与模型无关的方式提取唯一的实部和虚部。${}^4他$ 康普顿外形尺寸， ${\mathcal{H}}_{\mathrm{一个}}$。首次测量相干深虚拟康普顿散射${}^4他$ 原子核通过核后坐标记具有完全排他的最终状态，从而引领了原子核部分结构的3D成像。