Exploring the Nucleon with Electromagnetic Probes

这个项目将解决我们目前在理解质子方面的一些挑战：质子半径之谜，即对半径的高精度缪子氢测量与目前可接受的电子测量值有七个标准偏差；以及质子标量和自旋极化率的测量。最近对质子标量极化率的理论计算描述了数据和基于色散关系的分析，但产生的磁极化率是色散分析的两倍。在许多理论框架内可以预测自旋的极化率，但直到最近，对这些性质的实验测量是不可行的。我们将进行一系列单极化和双极化的康普顿散射测量，以提供标量极化率的高质量提取，并提供世界上第一个独立提取所有四个自旋极化率的方法。测量计划将在A2合作的框架内在德国美因茨的Mainzer MIkrotron加速器进行，其中PI是极化率计划的主导PI。瑞士Paul Scherer Institute(PSI)的Muon散射实验(MUSE)将首次通过Muon散射获得质子半径，实现与当前电子散射数据相当的精度水平。这种电子束是介子、介子和电子/正电子的混合物，将在荷电状态和电子两种状态下使用，并将采用平行的介子散射数据。这将允许从质子和µ子弹性散射中提取半径，以达到与目前在电子散射实验中获得的精度相当的水平。在这样做的过程中，它提供了对轻子普适性的彻底测试，即与从散射数据中提取半径相关的双光子交换修正，并访问了假设发生标准模型物理之外的可能区域。双光子交换修正将在北卡罗来纳州三角大学核实验室(TUNL)的高强度伽马源(HIGS)的互补双轻子光产生实验中进一步测试。-我们与之相互作用的所有可见物质都是由原子组成的，而原子又由中子、质子和电子组成。我们知道，核子：质子和中子本身都有子结构：称为夸克的成分。由于这些夸克是如此强烈地结合在一起，我们不可能为了检查质子的内部结构和结合而将它们分开。相反，我们必须形成它们如何结合在一起的理论，并基于这些理论做出预测，我们可以通过实验测试这些理论，以便了解我们日常生活的这些组成部分。这有点像查看乐高模型的目录，并试图从目录中可能的配置推断出积木的形状和粘合机制。在这个方案中，我们将研究质子的两个性质：它的半径和极化率。通过这样做，我们的目标是更好地了解宇宙的这一基本组成部分，并通过它更好地了解当前核物理理论模型的优势和局限性。这些模型影响了许多现代技术，例如那些涉及核能或放射治疗的技术，并决定了星星在天空中的燃烧方式。本科生和研究生参与国际研究提供了极好的物理和国际关系方面的培训机会。一位年轻女性PI的领导有望鼓励更多来自代表性不足群体的个人考虑在自然科学领域工作。

This project will address some of the current challenges in our understanding of the proton: the proton radius puzzle, whereby the highly precise muonic hydrogen measurement of the radius is seven standard deviations from the currently accepted value from electron-based measurements; and measurement of the scalar and spin polarizabilities of the proton. Recent theoretical calculations of the proton scalar polarizabilities describe the data as well as dispersion-relation-based analyses, but produce a magnetic polarizability twice that of the dispersion analyses. The spin polarizabilities are predicted within many theoretical frameworks, but, until recently, experimental measurement of these properties was unfeasible. We will perform a series of singly- and doubly-polarized Compton scattering measurements to provide a high quality extraction of the scalar polarizabilities, and a world-first independent extraction of all four spin polarizabilities. The measurement program will be carried out within the framework of the A2 Collaboration at the MAinzer MIkrotron accelerator in Mainz, Germany, where the PI is a lead PI on the polarizability program.The MUon Scattering Experiment (MUSE) at the Paul Scherer Institute (PSI) in Switzerland will access the proton radius for the first time via muon scattering, achieving a level of precision comparable with the current electron scattering data. The beam, a mixture of muons, pions and electrons / positrons, will be employed in both charge states and electron and muon scattering data will be taken in parallel. This will allow for an extraction of the radius from both proton and muon elastic scattering to a level of precision comparable with that currently obtained in electron scattering experiments. In so doing, it provides a thorough test of lepton universality, two-photon-exchange corrections relevant to the extraction of the radius from the scattering data and accesses regions where possible beyond standard model physics is postulated to occur. The two photon exchange corrections will be further tested in a complementary dilepton photoproduction experiment at the High Intensity Gamma Source (HIGS) at the Triangle Universities Nuclear Laboratory (TUNL) in North Carolina.--------------All visible matter with which we interact is formed of atoms, which are in turn composed of neutrons, protons and electrons. We know that the nucleons: the protons and neutrons themselves have substructure: components known as quarks. As these quarks are so strongly bound together, it is impossible for us to pull them apart in order to examine the internal structure and binding of the proton. Instead we have to form theories of how they might be bound together, and make predictions based on those theories which we can test experimentally in order to gain understanding of these building blocks of our everyday lives. It is a little like looking at a catalogue of Lego models and trying to infer the shape and binding mechanisms of the bricks from the possible configurations in the catalogue. We will investigate two properties of the proton in this proposal: its radius and polarizabilities. In so doing, we aim to gain a better understanding of this fundamental building block of the universe and, though it, to better understand the strengths and limitations of current theoretical models of nuclear physics. These models influence many modern technologies such as those involved in nuclear power or radiotherapy treatment, and determine how the stars burn in the sky. The involvement of undergraduate and graduate research students in international research offers excellent training opportunities both in physics and in international relationships. The leadership of a young female PI will hopefully encourage more individuals from underrepresented groups to consider a career in the natural sciences.