Probing electromagnetic structure of the nucleon using polarization 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。我们已经很好地进入了对原子内部包含的无数复杂性的研究--从分子结构到围绕原子核中心的电子轨道，再到原子核本身的结构，道尔顿在1805年提出了这一概念，认为原子内部的复杂性是不可分的。1932年，中子的发现使人们接受了原子核是由称为中子和质子(或称核子)的基本粒子组成的观点；此外，在20世纪70年代的S时期，实验结果表明，即使是这些核子也是由更基本的称为夸克和胶子的粒子单独组成的。现代核物理研究人员现在努力了解这些潜在的夸克和胶子之间的相互作用，以及这些相互作用是如何引起核子内部结构性质的。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在极高能量下(或者相当于当夸克非常接近时)是如何相互作用的，但不幸的是，这个QCD理论在较低能量(或更远的距离)下很难求解/解释，因为随着夸克/胶子之间的距离越来越远，夸克/胶子力变得越来越复杂。然而，夸克和胶子的这种复杂的相互作用从核子被测量到的丰富的结构中可见一斑；虽然很明显这种核子结构是其内部带电夸克运动的结果，但所观察到的核子低能结构与QCD基本理论的确切联系仍然是现代物理研究的中心问题之一。这项研究计划将利用美国杰斐逊实验室现有的60亿伏特电子束对低能质子结构进行精确测量，并有助于设计/建造新的闪烁光纤跟踪探测器，用于研究如何在杰斐逊实验室进入建造阶段将其电子束能量增加到120亿伏特时，推动这些质子结构测量的能量。

During the past century, a revolution has occurred in the understanding of the structure of matter. We have moved well into examination of the myriad of complexity contained inside atoms - proposed as "indivisible" in 1805 by Dalton - from molecular structures, to electron orbitals around a nuclear center, to the structure of the atomic nucleus itself. 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"); 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 existing 6-billion-Volt electron beam at Jefferson Laboratory in the USA to make precision measurements of low-energy proton structure, as well as contribute to designing/constructing new scintillating-fiber tracking detectors to be used in studies to push these proton structure measurements up in energy as Jefferson Lab enters a construction phase to increase its electron beam energy to 12-billion-Volts.