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是关于我们对原子核的组成部分--质子、中子和其他粒子（介子）的理解。 我们对这些粒子的概念导致了这样一种图景，即这些粒子是由更基本的成分夸克和胶子组成的。 虽然一个非常好的理论框架（称为量子色动学，或QCD）能够准确地描述夸克和胶子在极高能量下（或者，等价地，当夸克非常接近时）如何相互作用，但很难将QCD应用于较低能量（较大距离）的现象。 矛盾的是，随着夸克-胶子相互作用的进一步分离，它们之间的相互作用越来越复杂，这对它们被限制在核结构单元中至关重要，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--它们可以被激发到许多不同的“激发态”，具有独特的内部电荷和磁性的空间分布，以及在电场或磁场探测时的独特响应。 但是，这些观测到的低能结构性质与QCD基础理论的确切联系只被部分理解。 这一研究项目是加拿大三所大学的研究人员继续合作，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，这些细节显示出通过与现代QCD启发的模型计算的比较，以及与在“晶格”上计算的QCD解决方案直接连接到底层夸克/胶子动力学的特别希望。 这个项目的测量结果，以及世界各地实验室中其他类似项目的测量结果，提供了在夸克/胶子动力学和观测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.