More than three decades ago, the European Muon Collaboration published a surprising result on the spin structure of the proton: the spins of its three quark components account for only a small part of the spin of the proton. Ever since, theoretical and experimental progress has been made in understanding the origins of the proton spin. In this Review, we discuss what has been learned so far, what is still missing and what could be learned from the upcoming experiments, including the Jefferson Lab 12 GeV upgrade and the proposed Electron-Ion Collider. In particular, we focus on first-principles calculations and experimental measurements of the total gluon helicity ΔG, and the quark and gluon orbital angular momenta.

三十多年前，欧洲Muon协作组织发表了有关质子自旋结构的令人惊讶的结果：三个夸克成分的自旋仅占质子自旋的一小部分。从那时起，在了解质子自旋的起源方面已取得了理论和实验上的进步。在这篇评论中，我们讨论了到目前为止所学到的东西，仍然缺少的东西以及可以从即将进行的实验中学到的东西，包括杰斐逊实验室12 GeV升级和拟议的电子对撞机。特别是，我们专注于总胶子螺旋度ΔG以及夸克和胶子轨道角动量的第一性原理计算和实验测量。