The spin structure of the nucleon
The spin structure of the nucleon
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DOI：
10.1088/1361-6633/ab0b8f
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发表时间：
2018-07
影响因子：
18.1
通讯作者：
A. Deur;S. Brodsky;G. F. de Téramond
中科院分区：
文献类型：
--
作者：
A. Deur;S. Brodsky;G. F. de Téramond
我们回顾了目前对质子和中子的自旋结构的理解，质子和中子是原子核的基本组成部分，统称为核子。核子自旋场为检验强相互作用的规范理论量子色动力学（QCD）提供了一个关键窗口，因为它涉及强子结构的基本方面，可以在实验中详细探测，特别是极化目标上的深度非弹性轻子散射。量子色动力学最初是在非偏振光束与靶的高能深度非弹性轻子散射中被探索的。随着时间的推移，人们的兴趣从测试微扰QCD转移到阐明核子结构本身。事实上，强子的自旋自由度为微扰和非微扰QCD动力学提供了一个必要的和详细的验证。基于部分子模型的直觉，核子自旋最初被认为主要来自其夸克成分的自旋。然而，最初的实验表明，这种预期是不正确的。现在很清楚，核子物理要复杂得多，涉及夸克轨道角动量以及胶子和海夸克的贡献。因此，核子自旋结构仍然是QCD研究中最活跃的一个方面，其中包括诸如广义部分子分布（GPD）和横向动量分布（TMD）等重要进展。弹性和非弹性轻子-质子散射以及光吸收实验为研究非微扰QCD提供了多种途径。基本求和规则--如极化核子的极化光吸收的比约肯求和规则--也在非微扰域中。这一认识引发了一个强有力的计划，将强相互作用的低能有效强子描述与QCD的基本夸克和胶子自由度联系起来。这也导致了量子色动力学的格点规范理论模拟的进展，以及基于AdS/CFT或规范/引力对应的全息量子色动力学思想的发展，这是一种新的方法，为量子色动力学提供了一个有根据的半经典近似。任何基于QCD的核子自旋和动力学模型也必须成功地解释观测到的强子光谱。强子谱的解析计算是QCD研究的一个长期目标，现在已经利用光阵面全息术和超共形量子力学实现了，这是一种与核子自旋研究结果一致的形式。我们开始这篇评论与核子结构的一般和自旋结构的唯象描述，旨在从事非专业读者。接下来，我们讨论了在高能量的核子自旋结构，包括主题，如狄拉克的前面的形式和光前量子化，提供了一个独立的框架，相对论描述的强子结构和动力学，自旋和规则的推导，并直接连接到QCD拉格朗日。然后，我们讨论了实验和理论的进展，在非微扰领域，特别是光前全息QCD和超共形量子力学的发展，他们的预测核子的自旋内容，计算PDF和强子质量。
We review the present understanding of the spin structure of protons and neutrons, the fundamental building blocks of nuclei collectively known as nucleons. The field of nucleon spin provides a critical window for testing Quantum Chromodynamics (QCD), the gauge theory of the strong interactions, since it involves fundamental aspects of hadron structure which can be probed in detail in experiments, particularly deep inelastic lepton scattering on polarized targets. QCD was initially probed in high energy deep inelastic lepton scattering with unpolarized beams and targets. With time, interest shifted from testing perturbative QCD to illuminating the nucleon structure itself. In fact, the spin degrees of freedom of hadrons provide an essential and detailed verification of both perturbative and nonperturbative QCD dynamics. Nucleon spin was initially thought of coming mostly from the spin of its quark constituents, based on intuition from the parton model. However, the first experiments showed that this expectation was incorrect. It is now clear that nucleon physics is much more complex, involving quark orbital angular momenta as well as gluonic and sea quark contributions. Thus, the nucleon spin structure remains a most active aspect of QCD research, involving important advances such as the developments of generalized parton distributions (GPD) and transverse momentum distributions (TMD). Elastic and inelastic lepton-proton scattering, as well as photoabsorption experiments provide various ways to investigate non-perturbative QCD. Fundamental sum rules—such as the Bjorken sum rule for polarized photoabsorption on polarized nucleons—are also in the non-perturbative domain. This realization triggered a vigorous program to link the low energy effective hadronic description of the strong interactions to fundamental quarks and gluon degrees of freedom of QCD. This has also led to advances in lattice gauge theory simulations of QCD and to the development of holographic QCD ideas based on the AdS/CFT or gauge/gravity correspondence, a novel approach providing a well-founded semiclassical approximation to QCD. Any QCD-based model of the nucleon’s spin and dynamics must also successfully account for the observed spectroscopy of hadrons. Analytic calculations of the hadron spectrum, a long sought goal of QCD research, have now being realized using light-front holography and superconformal quantum mechanics, a formalism consistent with the results from nucleon spin studies. We begin this review with a phenomenological description of nucleon structure in general and of its spin structure in particular, aimed to engage non-specialist readers. Next, we discuss the nucleon spin structure at high energy, including topics such as Dirac’s front form and light-front quantization which provide a frame-independent, relativistic description of hadron structure and dynamics, the derivation of spin sum rules, and a direct connection to the QCD Lagrangian. We then discuss experimental and theoretical advances in the nonperturbative domain—in particular the development of light-front holographic QCD and superconformal quantum mechanics, their predictions for the spin content of nucleons, the computation of PDFs and of hadron masses.