Precision Studies in Subatomic Systems

该奖项支持弗吉尼亚大学小组的一项实验研究计划，该计划旨在探索我们对某些亚原子粒子及其相互作用的最小长度尺度知识的限制。该项目研究了三个系统：皮介子、介子(电子质量更大的表亲)和中子。该研究计划侧重于它们各自固有的低能量特性，即对标准模型(SM)的一些基本组件敏感的特性。SM描述了所有已知(到目前为止)的基本粒子及其基本特征。足够精确地确定这些性质是否与SM预测不同，有可能发现新的物理过程和粒子。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，专注于其多样性，并挖掘最广泛的可用人才库，为建设国家的STEM人才做出贡献。最后，该项目旨在开发基础研究的新工具，可能会有更广泛的应用。拟议的研究继续正在进行的研究计划的电场弱和强相互作用在低能量。该项目的近期目标是：(A)在橡树岭国家实验室的散裂中子源进行NAB实验的测量，这是一个精确测量中子β衰变参数的计划；(B)参加对Muon旋磁比的新的精确测量和数据分析，即费米实验室的Muon g-2实验；以及(C)完成PEN实验的数据分析，这是对介子的电子和辐射电子罕见衰变的精确测量。NAB是电子-反中微子关联参数“a”的高速非极化测量，精度为千分之几，并精确测量中子衰变参数“b”，即Fierz干扰项。NAB的目的是解决在确定核子轴向矢量形状因子以及由此确定Cabibbo-Kobayashi-Maskawa夸克混合矩阵中的VUD项方面的长期不一致问题。这最终将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，例如左右对称模型、轻夸克等。中子衰变是核物理学中最基本、理论上最了解的过程之一。Muon g-2实验的重点是对极其精确地预测的电弱过程进行强子修正，这些过程决定了带电轻子与真空的相互作用，这是一个具有发现Beyond-SM物理最高潜力的研究课题。Muon g-2实验预计将在获奖期间完成数据采集，并将建立在广泛报道的Run-1结果的基础上。PEN的目标是使介子电子衰变的分支比达到万分之五的精度，这为轻子普适性(LU)提供了最准确的实验测试。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。由于中微子物理的发展，Lu和整个轻子性质具有额外的意义。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 a program of experimental research by the University of Virginia group 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. The research program is focused on the intrinsic low-energy properties of each of them, properties that are sensitive to some of the basic components of the Standard Model (SM). The SM describes all known (to date) elementary particles and their basic characteristics. Determining these properties precisely enough to establish whether they differ from SM predictions has the potential to uncover new physical processes and particles. 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, by focusing on its diversity, and tapping the broadest available talent pool. Finally, the project aims 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) carrying out the measurements of the Nab experiment, a program of precise measurements of the neutron beta decay parameters at the Spallation Neutron Source (SNS), Oak Ridge National Laboratory; (b) taking part in the new precise measurement and data analysis of the muon gyromagnetic ratio, the Muon g-2 experiment at Fermilab, and (c) conclusion of the data analysis for the PEN experiment, a precise measurement of the electronic and radiative electronic rare decays of the pion. Nab is a high-rate unpolarized measurement of the electron-antineutrino correlation parameter "a" with accuracy of a few parts in 1000, and a precise 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. This will ultimately 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 Muon g-2 experiment is focused on the hadronic corrections to the extremely precisely predicted electroweak processes that determine the interaction of charged leptons with the vacuum, a research subject with one of the highest potentials for discovery of beyond-SM physics. The Muon g-2 experiment is projected to complete data taking during the award period, and will build on the widely reported result from Run-1. 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 provides 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.