π介子：标准模型中的一个谜
The pion: an enigma within the Standard Model
摘要：
胶子和夸克发现近50年后，我们才刚刚开始了解QCD如何构建原子核的基本砖块：中子，质子和束缚它们的π介子。QCD的特征是两种涌现现象：禁闭和动力学手征对称性破缺（DCSB）。它们在π介子的性质中表现得非常有力。反过来，π介子性质表明，禁闭和DCSB是密切相关的。π介子既是南武-戈德斯通玻色子，又是夸克-反夸克束缚态，在自然界中是独一无二的。因此，理解它的性质对于揭示标准模型的基本特征至关重要。我们描述了在这个方向上的实验进展，使用电磁探针，突出带电π形状因子的数据，在过去的十年中取得的精度的改进，和中性π过渡形状因子的新成果。两者都对现有的π介子结构概念提出了挑战。我们还为这些经验进展提供了一个理论背景，首先解释DCSB如何工作，以保证π介子是不自然的轻;但也，尽管如此，确保π介子是揭示产生几乎所有强子质量的机制的关键。我们的讨论统一了带电π弹性和中性π跃迁的形状因子，和π的扭曲-2部分子分布振幅。它还表明如何研究的带电K介子形状因子可以提供显着的贡献。重要的是，最近对π形状因子的大$Q^2$行为的预测可以通过JLab 12计划的实验进行测试。这些实验将把精确的带电π形状因子数据扩展到动量，这些动量有可能用于验证QCD中的因子分解定理，揭示非微扰域和微扰域之间的过渡，从而达到长期推动强子粒子物理学的目标。
英文摘要：
Almost 50 years after the discovery of gluons & quarks, we are only just beginning to understand how QCD builds the basic bricks for nuclei: neutrons, protons, and the pions that bind them. QCD is characterised by two emergent phenomena: confinement & dynamical chiral symmetry breaking (DCSB). They are expressed with great force in the character of the pion. In turn, pion properties suggest that confinement & DCSB are closely connected. As both a Nambu-Goldstone boson and a quark-antiquark bound-state, the pion is unique in Nature. Developing an understanding of its properties is thus critical to revealing basic features of the Standard Model. We describe experimental progress in this direction, made using electromagnetic probes, highlighting both improvements in the precision of charged-pion form factor data, achieved in the past decade, and new results on the neutral-pion transition form factor. Both challenge existing notions of pion structure. We also provide a theoretical context for these empirical advances, first explaining how DCSB works to guarantee that the pion is unnaturally light; but also, nevertheless, ensures the pion is key to revealing the mechanisms that generate nearly all the mass of hadrons. Our discussion unifies the charged-pion elastic and neutral-pion transition form factors, and the pion's twist-2 parton distribution amplitude. It also indicates how studies of the charged-kaon form factor can provide significant contributions. Importantly, recent predictions for the large-$Q^2$ behaviour of the pion form factor can be tested by experiments planned at JLab 12. Those experiments will extend precise charged-pion form factor data to momenta that can potentially serve in validating factorisation theorems in QCD, exposing the transition between the nonperturbative and perturbative domains, and thereby reaching a goal that has long driven hadro-particle physics.
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