Nucleons (protons and neutrons) are the building blocks of atomic nuclei and are responsible for more than 99% of the visible matter in the Universe. Despite decades of efforts in studying its internal structure, there are still a number of puzzles surrounding the proton such as its spin and charge radius. Accurate knowledge about the proton charge radius is not only essential for understanding how QCD works in the nonperturbative region but also important for bound state QED calculations of atomic energy levels. It also has an impact on the Rydberg constant, one of the most precisely measured fundamental constants in nature. This review examines the latest situation concerning the proton charge radius in light of the new experimental results from both atomic hydrogen spectroscopy and electron-scattering measurements, with particular focus on the latter. Theoretical backgrounds and recent developments concerning the determination of the proton charge radius using different experimental techniques are also presented. Upcoming experiments are discussed, and the deuteron charge radius puzzle is mentioned at the end.

中文翻译：

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质子电荷半径

核子（质子和中子）是原子核的组成部分，负责宇宙中 99% 以上的可见物质。尽管在研究其内部结构方面进行了数十年的努力，但质子周围仍然存在许多难题，例如其自旋和电荷半径。关于质子电荷半径的准确知识不仅对于理解 QCD 如何在非微扰区域中工作至关重要，而且对于原子能级的束缚态 QED 计算也很重要。它还对里德堡常数产生影响，里德堡常数是自然界中测量最精确的基本常数之一。本综述根据原子氢光谱和电子散射测量的新实验结果，研究了有关质子电荷半径的最新情况，特别关注后者。还介绍了有关使用不同实验技术确定质子电荷半径的理论背景和最新进展。讨论了即将进行的实验，最后提到了氘核电荷半径之谜。