Probing the Electromagnetic Structure of Hadrons at Jefferson Lab

在过去的一个世纪里，对物质结构的理解发生了一场革命，特别是对原子核的理解。1932年，中子的发现使人们接受了这样一种观点，即原子核是由称为中子和质子的基本粒子（或“核子”）组成的，它们通过称为介子的较轻粒子（其中介子最轻）的交换相互作用；此外，在20世纪70年代，实验结果清楚地表明，即使是这些核子，也分别是由更基本的粒子——夸克和胶子构成的。现代核物理研究人员现在正在努力获得对这些潜在的夸克和胶子之间相互作用的清晰认识，以及这些相互作用如何引起核子的内部结构特性。虽然存在一个非常好的理论框架（称为量子色动力学，或QCD），可以准确地描述夸克和胶子如何在极高的能量下相互作用（或者，当夸克非常接近时），一个不幸的现实是，这个QCD理论很难在较低的能量（或更大的距离）下解决/解释，因为夸克/胶子的力随着它们离得更远而变得越来越复杂。然而，夸克和胶子的这种复杂的相互作用是由核子所具有的丰富结构所表现出来的；虽然很明显，这种核子结构是由其内部带电夸克的运动产生的，但所观察到的核子低能结构与QCD基础理论的确切联系仍然是现代物理学研究的核心问题之一。本研究计划将利用美国杰佛逊实验室升级后的电子束设备，对质子（和介子）结构进行精确测量。这个项目最大的努力将是设计和建造一个新的闪烁光纤“坐标探测器”，用于提供质子弹性散射电子的精确位置和角度测量，以努力将质子的“形式”（或结构）的研究推向迄今为止达到的最极端极限。

During the past century, a revolution has occurred in the understanding of the structure of matter - specifically, in understanding the atomic nucleus. In 1932, the discovery of the neutron brought acceptance to the idea that the atomic nucleus is composed of elementary particles called neutrons and protons (or "nucleons") which interact via exchanges of lighter particles called mesons (with the pion being the lightest); further, during the 1970's, experimental results made it clear that even these nucleons are individually made of more fundamental particles called quarks and gluons. Modern nuclear physics researchers now strive to gain clear insight into understanding the interactions between these underlying quarks and gluons, and how these interactions give rise to the internal structure properties of nucleons. While a very good theoretical framework (called Quantum Chromo Dynammics, or QCD) exists which can accurately describe how quarks and gluons interact at extremely high energies (or, equivalently, when the quarks are very close together), an unfortunate reality is that this theory of QCD is very difficult to solve/interpret at lower energies (or, at larger distances) because the quark/gluon force gets increasingly complicated as they get further apart. However, this complicated interaction of quarks and gluons is manifest by the rich structure which nucleons have been measured to possess; while it's clear such nucleon structure results from the motion of its internal electrically-charged quarks, the exact connection of the observed low-energy structure of nucleons to the underlying theory of QCD remains one of the central problems of modern physics research. This research proposal will exploit the upgraded electron beam facility at Jefferson Laboratory in the USA to make precision measurements of proton (and pion) structure. The biggest effort in this project will be the design and construction of a new scintillating-fiber "coordinate detector" to be used for providing precision position and angle measurements of elastically-scattered electrons from the proton in efforts to push studies of the proton's "form" (or structure) to the most extreme limits ever reached.