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

在过去的世纪里，对物质结构的认识发生了一场革命。 我们已经深入研究了原子内部包含的无数复杂性--道尔顿在1805年提出了"不可分割"的概念--从分子结构到围绕核中心的电子轨道，再到原子核本身的结构（这是本研究计划的兴趣所在）。 1932年，中子的发现使人们接受了原子核是由基本粒子中子和质子组成的观点。 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、更基本的夸克和胶子粒子组成的。 现代核物理研究人员现在努力获得对原子核内中子和质子之间的详细相互作用的清晰洞察，以了解这些相互作用与夸克和胶子的潜在结构的关系。 此外，相当大的努力也指向了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引起质子和中子的内部结构特性。这项研究项目利用了美国托马斯杰斐逊国家加速器设施（"杰斐逊实验室"）的先进技术，使利用中能电子束对质子和轻核进行详细结构测量成为可能。 一个具体的问题正在解决，通过测量极化可观的散射电子从液态氢，是一个精确的确定是否一个特定的激发态的质子（所谓的"罗珀共振"）代表了"呼吸模式"激发的质子，其中内部夸克有效地晃动在和对彼此。或者这种状态是否是由质子中夸克和介子的复杂相互作用产生的。 类似的方法将用于氘的衰变，以研究低能区，在那里，这种简单核的发达核子/介子理论应该是准确的，但似乎很难描述现有的数据。

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.