Studies of hadronic structure using electromagnetic probes

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、中子和其他束缚它们的粒子(介子)--的理解。值得注意的发现表明，这些粒子由更基本的成分--夸克和胶子组成。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在极高能量下(或相当于当夸克非常接近时)如何相互作用，但将QCD应用于较低能量(较长距离)的现象一直是非常困难的。我们提高理解的障碍之一是缺乏高质量的数据，特别是深度排他性反应，这些数据提供了QCD内部运作的最清晰的图景。杰斐逊实验室以其连续、高光度的电子束和良好的粒子识别和可重现的系统学探测器使这些研究成为可能。1.P+和K+弹性形状因子(FP，FK)：两个最轻介子的内部结构由它们的形状因子参数化，是需要理解的临界强子结构量。P+形状因子为我们提供了直接观测QCD从长距离强禁闭到短距离渐近自由的最大希望。K+，其中d夸克被一个S夸克取代，提供了一个重要的第二个研究案例。它的目标是：(I)建立从大运动学范围内的电产生数据中提取介子形状因子的可靠性，(Ii)获得高质量的形状因子测量到高动量转移。K+数据是在2018-19年获得的，而p+数据是在2019年获得的，大部分将在2021-2023年获得。瑞金纳的四名博士生正在分析这些数据。2.独特的U通道物理途径：后角介子的电产生预计将提供对核子波函数的三夸克加海分量的独特途径。我们开创了一项新技术，证明了在这个以前从未探索过的地区进行高质量测量的可行性。我们即将进行的实验将需要？，？和f数据的目标：(I)确定多个反应通道的后角横截面峰的存在；(Ii)在较大的运动范围内提取分离的横截面；以及(Iii)确定共线因式分解方案是否有效。3.广义部分子分布研究(GPDs)：Solid(螺线管大强度装置)将允许在GPDs上获得新的数据，这是提供对核子夸克和胶子结构的全面描述的“宇宙对象”。Regina的学生将参与2022-25年计划的固体探测器建造和测试，并为我们探测已知最贫穷的GPD-E-tilde的实验做出贡献。资金被要求用于杰斐逊实验室的实地工作，博士生和PDF工资支持。

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. Notable discoveries have indicated 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 (longer distance) phenomena. One of the obstacles to our improved comprehension has been a lack of quality data, particularly for the Deep Exclusive Reactions, which provide the clearest picture of the inner workings of QCD. Jefferson Lab enables these studies with its continuous, high luminosity electron beams, and detectors with good particle identification and reproducible systematics. 1. p+ and K+ Elastic Form Factors (Fp, FK): The internal structures of the two lightest mesons are parametrized by their form factors, and are critical hadronic structure quantities to understand. The p+ form factor offers our best hope of directly observing QCD's transition from strong-confinement at long-distance scales to asymptotic-freedom at short-distances. The K+, where the d quark is replaced with an s quark, provides a vital second study case.  Objectives: (i) establish the reliability of the extraction of the meson form factors from electroproduction data over a broad kinematic range, (ii) obtain high quality form factor measurements to high momentum transfer. The K+ data were acquired in 2018-19, while the p+ data were partly acquired in 2019, with the bulk to be acquired in 2021, 2023. The data are being analyzed by four Regina PhD students. 2. Unique Access to u-Channel Physics: Backward-angle meson electroproduction is anticipated to offer unique access to the three quark plus sea component of the nucleon wave function. We pioneered a new technique, which demonstrated the feasibility of high quality measurements in this previously unexplored region. Our upcoming experiment will take ?, ? and f data with goals: (i) establish existence of a backward-angle cross section peak for multiple reaction channels; (ii) extract the separated cross sections over a wide kinematic range; and (iii) establish whether the collinear factorization scheme is valid. 3. Study of Generalized Parton Distributions (GPDs):  SoLID (Solenoidal Large Intensity Device) will allow new data to be acquired on GPDs, "universal objects" providing a comprehensive description of the quark and gluon structure of the nucleon. Regina students will participate in SoLID detector construction and testing, planned for 2022-25, and contribute to our experiment probing the poorest known GPD, E-tilde. Funds are requested for Jefferson Lab fieldwork, PhD student and PDF salary support.