Precision studies of the electromagnetic structure of the nucleaon and light nuclei

在过去的一个世纪中，对物质结构的理解发生了一场革命。  我们已经深入研究原子内部包含的无数复杂性——道尔顿在 1805 年提出“不可分割”——从分子结构到核中心周围的电子轨道，再到原子核本身的结构（本研究提案的兴趣所在）。  1932 年，中子的发现使人们接受了原子核由称为中子和质子的基本粒子组成的观点。  此外，在 20 世纪 70 年代，实验结果清楚地表明，即使是这些中子和质子也是由更小、更基本的粒子（称为夸克和胶子）单独构成的。  现代核物理研究人员现在努力清楚地了解原子核内中子和质子之间的详细相互作用，以便了解这些相互作用与夸克和胶子的基本结构的关系。  此外，还需要付出相当大的努力来理解这些夸克和胶子本身之间的相互作用，以及这些相互作用如何产生质子和中子的内部结构特性。该研究项目利用了先进的技术，使在美国托马斯·杰斐逊国家加速器设施（“杰斐逊实验室”）使用中能电子束对质子和轻原子核的详细结构测量成为可能。  通过测量液态氢电子散射中的偏振可观测值，我们正在解决一个具体问题，即精确确定质子的特定激发态（所谓的“罗珀共振”）是否代表质子的“呼吸模式”激发，其中内部夸克有效地相互进出……或者这种状态是否源于质子内夸克和介子的复杂相互作用。  类似的方法将用于氘的分解，以研究低能状态，其中这种简单原子核的完善的核子/介子理论应该是准确的，但似乎难以描述现有数据。

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 (where lies the interest of this research proposal).  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.  Further, during the 1970's, experimental results made it clear that even these neutrons and protons are individually made of smaller, more fundamental, particles called quarks and gluons.  Modern nuclear physics researchers now strive to gain clear insight into the detailed interactions between neutrons and protons within nuclei in order to understand the relation of these interactions to the underlying structure of quarks and gluons.  Further, considerable effort is also pointed toward understanding the interaction between these quarks and gluons themselves, and how these interactions give rise to the internal structure properties of protons and neutrons. This research project exploits the technological advances which make possible detailed structure measurements of the proton and light nuclei using intermediate energy electron beams at the Thomas Jefferson National Accelerator Facility ("Jefferson Lab") in the USA.  One specific question being addressed, through measurement of polarization observables in the scattering of electrons from liquid hydrogen, is a precise determination of whether a particular excited state of the proton (the so-called "Roper resonance") represents a "breathing mode" excitation of the proton, in which the internal quarks effectively slosh in-and-out against each other ... or whether this state arises from a complicated interaction of the quarks and mesons within the proton.  Similar methods will be used in the disintegration of deuterium to study a low-energy regime where well-developed nucleon/meson theories of this simple nucleus should be accurate, but seem to have difficulty describing existing data.