Precision Experiments at Jefferson Lab to Study Specific Electromagnetic Properties of Hadrons

该奖项支持两个备受瞩目的项目：高精度确定质子电荷半径和测量ETA介子(通过强力相互作用的轻基本粒子之一)衰变为两个光子的概率。可见宇宙主要由质子组成，因此理解质子的性质是当代核物理的重要任务之一。基本对称性，以及最重要的是，这些对称性被破坏的方式，定义了ETA介子的物理性质。这些性质虽然对可见物质的形成非常关键，但却相当小，需要高精度的最先进的实验才能“回顾”我们宇宙发展的早期阶段。根据这一奖项，一名本科生、一名研究生和一名博士后研究员将接受高度多学科的实验核物理培训。这一奖项将扩大STEM中代表人数不足的群体的参与，因为在PI的机构中，大多数学生是非裔美国人。在过去的几十年里，电子-质子散射和传统的氢光谱测量得到了关于质子电荷半径的一致结果。然而，最近用一种新的缪子-氢光谱方法对这个量进行了超高精度的测量，结果比以前所有实验的平均值小了八个标准偏差。这种明显的差异引发了现在所知的“质子半径之谜”。PRAD实验的目标是确定质子电荷半径到0.5%的精度。ETA-2伽马衰变宽度的预测精度将显著改善ETA-部分衰变宽度的其余部分，从而显著影响粒子物理组(PDG)的ETA-介子扇区。因此，它将以一种直接和主要与模型无关的方式关键地改善轻夸克质量比。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

This award supports two high-profile projects: the high precision determination of the proton charge radius and the measurement of the probability for an eta-meson (one of the light elementary particles that interact through the strong force) to decay into two photons. The visible Universe consists mostly of protons; therefore understanding the proton's properties is one of the important tasks of contemporary nuclear physics. Fundamental symmetries and, most importantly, the ways in which these symmetries are broken, define the physical properties of the eta-meson. These properties, although very critical for the formation of the visible matter, are rather small and require high precision state-of-the-art experiments to be able to "look back" at the early stages of the development of our Universe. Under this award one undergraduate, one graduate student and one postdoctoral fellow will be trained in highly multidisciplinary experimental nuclear physics. This award will broaden the participation of under-represented groups in STEM, in that the majority of students at the PI's institution are African American. For the past many decades consistent results on the proton charge radius have been obtained between electron-proton scattering and traditional hydrogen spectroscopic measurements. However, recent ultra-high precision measurements of this quantity performed with a novel muonic-hydrogen spectroscopic method gave results up to eight-standard deviations smaller than the average value from all previous experiments. This clear discrepancy triggered what is now known as the "proton radius puzzle". The goal of the PRad experiment is to determine the proton charge radius to 0.5% precision. The projected accuracy of the eta -- 2 gamma decay width will significantly impact the eta-meson sector of the Particle Physics Group (PDG) by significantly improving the rest of eta-partial decay widths. As a result, it will critically improve the light quark mass ratio in a direct and mostly model independent way.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.