Exploring the Nucleon with Electromagnetic Probes

该项目将解决我们目前对质子的理解所面临的一些挑战：质子半径之谜，其中高度精确的μ介子氢半径测量与目前基于电子的测量值相差7个标准差;以及质子的标量和自旋极化率的测量。 最近的质子标量极化率的理论计算描述的数据以及色散关系为基础的分析，但产生的色散分析的两倍的磁极化。 自旋极化率在许多理论框架内预测，但直到最近，这些属性的实验测量是不可行的。 我们将进行一系列的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的提取，以及世界上第一个独立提取所有四个自旋极化率。 测量计划将在德国美因茨的MAinzer MIkrotron加速器的A2合作框架内进行，其中PI是极化率计划的主要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。 该束是μ子、π子和电子/正电子的混合物，将在两种电荷状态下使用，电子和μ子散射数据将并行采集。 这将允许从质子和μ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，它提供了一个彻底的测试轻子的普适性，双光子交换相关的半径从散射数据的提取校正和访问的地区，可能超出标准模型物理假设发生。 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。 由于这些夸克是如此强烈地束缚在一起，我们不可能为了检查质子的内部结构和束缚而将它们分开。 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检验这些理论，以便了解我们日常生活的这些组成部分。 这有点像看一本乐高模型目录，试图从目录中可能的配置推断砖块的形状和结合机制。 在这个方案中，我们将研究质子的两个性质：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个基本组成部分，并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，如核能或放射治疗技术，并决定了恒星在天空中的燃烧方式。 本科生和研究生研究生参与国际研究提供了极好的培训机会，无论是在物理学和国际关系。 一个年轻的女性PI的领导将有望鼓励更多的人从代表性不足的群体考虑在自然科学的职业生涯。

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.