Exploring the Nucleon with Electromagnetic Probes

该项目将解决目前我们对质子理解中的一些挑战：质子半径之谜，即高度精确的介子氢半径测量值与目前接受的基于电子的测量值相差7个标准差；测量质子的标量和自旋极化率。最近质子标量极化率的理论计算描述了数据以及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。自旋极化率在许多理论框架中都有预测，但直到最近，对这些性质的实验测量都是不可行的。我们将进行一系列单极化和双极化康普顿散射测量，以提供高质量的标量极化率提取，以及世界上第一次独立提取所有四种自旋极化率。测量计划将在德国美因茨的美因策尔微加速器的A2协作框架内进行，PI是偏振性计划的主要PI。瑞士保罗谢勒研究所（PSI）的μ子散射实验（MUSE）将首次通过μ子散射获得质子半径，达到与当前电子散射数据相当的精度水平。这束由介子、介子和电子/正电子混合而成的光束将被用于两种电荷状态，电子和介子的散射数据将被并行地获取。这将允许从质子和介子弹性散射中提取半径，其精度可与目前在电子散射实验中获得的精度相媲美。在这样做的过程中，它提供了对轻子普适性的彻底测试，与从散射数据中提取半径相关的双光子交换修正，并访问可能超出标准模型物理假定发生的区域。这两个光子交换修正将在北卡罗来纳州三角大学核实验室（TUNL）的高强度伽玛源（HIGS）的互补双轻子光产生实验中进一步测试。--------------所有与我们相互作用的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，也就是夸克。由于这些夸克紧密地结合在一起，我们不可能把它们分开来研究质子的内部结构和结合。相反，我们必须形成关于它们如何结合在一起的理论，并根据这些理论做出预测，我们可以通过实验来验证这些理论，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绑定机制。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理解这个宇宙的基本组成部分，并通过它更好地理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，例如涉及核能或放射治疗的技术，并决定了恒星如何在天空中燃烧。本科生和研究生参与国际研究为物理学和国际关系提供了极好的培训机会。一位年轻的女性PI的领导将有望鼓励更多来自代表性不足的群体的个人考虑从事自然科学事业。

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.