Precision Studies in Subatomic Systems

该奖项支持一项实验研究计划，该计划旨在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的限制。该项目研究了三个系统：pi介子、Muon(电子质量更大的表亲)和中子，重点关注它们各自的内在低能特性。这些性质对标准模型的一些基本成分很敏感，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发现超越标准模型的新物理。该项目旨在推进核物理和基本粒子物理方面的基础科学，通过教育新一代STEM劳动力为建设国家的STEM人才做出贡献，同时保持其多样性并挖掘最广泛的可用人才库，最后，开发可能具有更广泛应用的基础研究新工具。拟议的研究延续了正在进行的低能量电弱和强相互作用研究计划。该项目的近期目标是：(A)完成PEN实验的数据分析，精确测量介子(pi-e2和pi-e2g)的电子和辐射电子罕见衰变，由Paul Scherrer研究所(PSI)完成，以及(B)NAB仪器的组装和调试，NAB是精确测量散裂中子源的中子β衰变参数的方案。UVA集团在这两个项目中都发挥着主导作用。随着PEN的工作接近完成，焦点将转移到(C)新的Muon旋磁比的精确测量上，即费米实验室的Muon g-2实验。PEN的目标是对介子电子衰变的分支比达到万分之五的精度，这是目前对轻子普适性(LU)最准确的实验测试。目前，pi-e2衰变测量的精度比SM理论预测的要落后一个数量级以上。SM之外的一些物理场景会导致LU违反。由于中微子物理学的发展，Lu和一般的轻子性质具有额外的意义。NAB是电子-反中微子关联参数“a”的高速率非极化测量，精度为千分之几，也是第一次测量中子衰变参数“b”，即Fierz干扰项。NAB旨在解决Cabibbo-Kobayashi-Maskawa夸克混合矩阵中确定核子轴向量形状因子以及由此确定VUD项的长期不一致问题，并通过利用中子衰变的独特优势，为标准模型的可能扩展提供新的限制，如左右对称模型、轻夸克等，中子衰变是核物理中最基本和理论上理解最好的过程之一。

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) 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 (b) 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). The UVa group plays a leading role in both projects. 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. 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. 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.