轻核上的深度虚康普顿散射
Spectator-Tagged Deeply Virtual Compton Scattering on Light Nuclei
摘要：
质子中夸克和胶子的三维图像将通过深度虚康普顿散射来揭示，而一维图像中一个至关重要的难题仍然存在，即EMC效应的起源。非相干核DVCS，即核内核子的DVCS，可以揭示束缚核子的三维部分子结构，为EMC效应提供新的解释。然而，费米运动的打击核子，离壳效应和最终状态的相互作用（FSI）复杂的部分子水平的解释。我们在这里提出了一个测量的不相干DVCS的反冲旁观者系统（核A-1）的标签，系统地控制核效应。通过观察者标记的DVCS，一个完全检测到的最终状态提供了一个独特的机会，系统地研究这些核效应，并干净地观察核子夸克分布的可能修改。我们建议测量$^4$ He和氘靶上的DVCS束流自旋不对称性（BSA）。反应$^4$ He $(e,e^{\prime}γ\,p\,^3$ H $)$与一个完全检测到的最终状态具有罕见的能力，同时量化FSI，测量初始核子动量，并提供一个敏感的探针，在部分子水平上的其他核效应。在一个（准自由）中子上的DVCS BSA将通过用氘靶标记旁观质子来测量。类似地，束缚中子测量检测离开$^4$ He靶的旁观者$^3$ He。这两个观测值将允许中子偏离前向EMC效应的独立测量。
英文摘要：
The three-dimensional picture of quarks and gluons in the proton is set to be revealed through Deeply virtual Compton scattering while a critically important puzzle in the one-dimensional picture remains, namely, the origins of the EMC effect. Incoherent nuclear DVCS, i.e. DVCS on a nucleon inside a nucleus, can reveal the 3D partonic structure of the bound nucleon and shed a new light on the EMC effect. However, the Fermi motion of the struck nucleon, off-shell effects and final-state interactions (FSIs) complicate this parton level interpretation. We propose here a measurement of incoherent DVCS with a tagging of the recoiling spectator system (nucleus A-1) to systematically control nuclear effects. Through spectator-tagged DVCS, a fully detected final state presents a unique opportunity to systematically study these nuclear effects and cleanly observe possible modification of the nucleon's quark distributions. We propose to measure the DVCS beam-spin asymmetries (BSAs) on $^4$He and deuterium targets. The reaction $^4$He$(e,e^{\prime}γ\,p\,^3$H$)$ with a fully detected final state has the rare ability to simultaneously quantify FSIs, measure initial nucleon momentum, and provide a sensitive probe to other nuclear effects at the parton level. The DVCS BSA on a (quasi-free) neutron will be measured by tagging a spectator proton with a deuteron target. Similarly, a bound neutron measurement detects a spectator $^3$He off a $^4$He target. These two observables will allow for a self-contained measurement of the neutron off-forward EMC Effect.
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