Precision Studies in Subatomic Systems

该奖项支持一项实验研究计划，以探索我们对某些亚原子粒子及其在最小长度尺度上相互作用的知识极限。该项目研究了三个系统：π介子，μ子（电子的一个更大的“表亲”）和中子，专注于它们中每一个的内在低能特性。这些性质对标准模型的一些基本组成部分很敏感，标准模型描述了已知基本粒子的数量和基本特征。因此，确定这些性质有可能揭示标准模型之外的新物理。该项目旨在推进核和基本粒子物理学的基础科学，通过教育新一代STEM劳动力的成员，为建设国家的STEM人才做出贡献，同时关注其多样性并挖掘最广泛的可用人才库，最后，开发基础研究的新工具，并可能有更广泛的应用。拟议的研究将继续进行一项正在进行的低能量电弱和强相互作用的研究计划。该项目的近期目标是：（a）为Nab组装和交付使用仪器，这是一个精确测量散斑中子源中子β衰变参数的方案;（B）PEN实验的数据分析结论，PEN实验是对π介子的电子和辐射电子稀有衰变的精确测量（pi-e2和pi-e2 g），在保罗·谢勒研究所（PSI）完成;随着PEN的工作接近完成，重点将转移到（c）μ子旋磁比的新精确测量，费米实验室的μ子g-2实验。 Na B是对电子-反中微子关联参数a的高速率非极化测量，其精度为千分之几，并且是对中子衰变参数B的第一次测量，即Fierz干涉项。Nab旨在解决在确定核子轴向矢量形状因子以及Cabibbo-Kobayashi-Maskawa夸克混合矩阵中的Vud项时持续存在的不一致性，并为标准模型的可能扩展提供新的限制，例如左右对称模型，leptoquark等，利用中子衰变的独特优势，中子衰变是核物理学中最基本和理论上最好理解的过程之一。PEN的目标是使π介子电子衰变的分支比达到万分之五的精度，这是对轻子普适性（LU）最精确的实验测试。目前，pi-e2衰变测量的精度落后于SM理论预测超过一个数量级。SM之外的许多物理场景将导致LU违规。由于中微子物理学的发展，LU和轻子的性质具有额外的意义。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值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 a program of experimental research to explore the limits of our knowledge of certain subatomic particles, and of their interactions at the smallest length scale. The project investigates three systems: the pi meson, the muon (a more massive "cousin" of the electron), and the neutron, focusing on the intrinsic low-energy properties of each of them. Those properties are sensitive to some of the basic components of the Standard Model, which describes the number and basic characteristics of the known elementary particles. Determining these properties thus has the potential to uncover new physics beyond the Standard Model. The project aims to advance basic science in nuclear and elementary particle physics, contribute to building the nation's STEM talent by educating members of the new generation of the STEM workforce, while keeping in focus its diversity and tapping the broadest available talent pool, and, finally, to develop new tools in fundamental research with possible broader applications. The proposed research continues an ongoing program of study of the electroweak and strong interactions at low energies. The immediate goals of the project are: (a) assembly and commissioning of the apparatus for Nab, a program of precise measurements of the neutron beta decay parameters at the Spallation Neutron Source (SNS); (b) conclusion of the data analysis for the PEN experiment, a precise measurement of the electronic and radiative electronic rare decays of the pion (pi-e2 and pi-e2g), completed at the Paul Scherrer Institute (PSI); and as the work on PEN nears completion, focus will move to (c) the new precise measurement of the muon gyromagnetic ratio, the muon g-2 experiment at Fermilab. Nab is a highly rate unpolarized measurement of the electron-antineutrino correlation parameter "a" with accuracy of a few parts in 1000, and the first measurement of the neutron decay parameter "b", the Fierz interference term. Nab aims to resolve the persistent inconsistencies in the determination of the nucleon axial vector form factor and, consequently, of the Vud term in the Cabibbo-Kobayashi-Maskawa quark mixing matrix, and to provide new limits on possible extensions of the Standard Model, such as left-right symmetric models, leptoquarks, etc., by exploiting the unique advantages of neutron decay, one of the most basic and theoretically best understood processes in nuclear physics. The goal of PEN is to reach the precision of 5 parts in 10,000 for the branching ratio of the electronic decay of the pion, which is the most accurate experimental test of lepton universality (LU) available. At present, precision of the pi-e2 decay measurements lags behind the SM theoretical predictions by more than an order of magnitude. A number of physics scenarios outside the SM would lead to LU violation. LU, and lepton properties in general, carry added significance due to developments in neutrino physics.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.