Investigations of Hadronic Structure using CB-TAPS at the Mainz Microtron

现代物理学研究的中心问题之一涉及到我们对原子核的组成部分——质子、中子和其他结合它们的粒子（介子）的理解。我们对这些粒子的概念导致了这样一幅图景：这些粒子由更基本的成分组成，夸克和胶子。虽然一个非常好的理论框架（称为量子色动力学，或QCD）能够准确地描述夸克和胶子如何在极高能量下相互作用（或者，当夸克非常接近时），但将QCD应用于较低能量（较大距离）现象是非常困难的。矛盾的是，随着夸克-胶子相互作用的进一步分离，夸克-胶子相互作用的复杂性日益增加，这对它们被限制在核构建块内至关重要，但我们无法进行必要的QCD计算来证实我们的理解。复杂的夸克-胶子相互作用表现在质子和中子所具有的丰富结构中——它们可以被激发成许多不同的“激发态”，具有独特的内部电荷和磁性的空间分布，并且在电场或磁场探测时具有独特的响应。但是，这些观测到的低能结构性质与QCD的基本理论之间的确切联系只是部分被理解。这个研究项目是加拿大三所大学的研究人员合作的继续，他们共同努力利用德国美因茨核物理研究所的实验设施，对强子结构进行精确测量。这些实验的目标是专注于测量特定的强子结构细节，通过与现代QCD启发的模型计算的比较，这些细节显示出特别的希望，可以直接连接到潜在的夸克/胶子动力学，并在“晶格”上计算QCD的解决方案。这个项目的测量结果，以及世界上其他类似的实验室的测量结果，提供了在夸克/胶子动力学和观察到的强子结构之间建立难以捉摸的联系的工具。

One of the central problems of modern physics research concerns our understanding of the building blocks of the atomic nucleus---the protons, neutrons, and other particles (mesons) that bind them. Our conception of these particles has led to the picture that these particles consist of yet more fundamental constituents, the quarks and gluons. While a very good theoretical framework (called Quantum Chromo-Dynamics, or QCD) is able to accurately describe how quarks and gluons interact at extremely high energies (or, equivalently, when the quarks are very close together), it has been very difficult to apply QCD to lower energy (larger distance) phenomena. The paradox is that the increasing complexity of the quark-gluon interaction as they get further apart is critically important to their confinement within the nuclear building blocks, but we cannot perform the QCD calculations necessary to confirm our understanding. The complex quark-gluon interaction is manifested in the rich structure that protons and neutrons have been measured to possess---they can be excited into many different "excited states", have unique spatial distributions of internal electric charge and magnetism, and unique responses when probed by electric or magnetic fields. But the exact connection of these observed low-energy structure properties to the underlying theory of QCD is only partly understood. This research project represents a continuation of the collaboration of researchers at three Canadian universities joining efforts to use the experimental facilities at the Institute for Nuclear Physics in Mainz, Germany, to make precision measurements related to hadron structure. The goal of these experiments is to focus on measuring specific hadron structure details that show particular promise of allowing a direct connection to underlying quark/gluon dynamics through comparison to modern QCD-inspired model calculations, and to solutions to QCD done computationally on a "lattice". Results from measurements in this project, and from others like it at laboratories throughout the world, provide the tools to make the elusive connection between quark/gluon dynamics and the observed hadron structure.