Precise Measurement of the Ratio of (V-A) Coupling Constants in Free Neutron Beta Decay and Search for Non-Standard Model Couplings with the Nab and pNab Spectrometers

该基金支持使用橡树岭国家实验室的“Nab”光谱仪测量描述中子衰变的角相关系数。在这个实验中，“a”和“b”系数描述了中子衰变产生的质子和电子。测量的主要目的是测试核与粒子物理学的标准模型，该模型迄今为止在描述基本粒子的大多数性质和行为方面非常成功，但仍然存在一些关键问题，例如宇宙中物质比反物质占优势的原因。因此，超越标准模型物理学的发现将对物理学和宇宙学产生重要影响。Nab光谱仪在橡树岭散裂中子源（SNS）的安装将于2021年夏末完成，调试和数据采集将在提案期间开始并继续进行。PI是Nab协作的项目经理。这笔拨款将资助PI的小组，他们将参与数据采集、探测器模拟、数据分析和质子探测效率的表征。PI坚定地致力于吸引代表性不足的群体的成员。弗吉尼亚大学物理系最近成为APS桥梁项目的新址，PI担任导师。此外，该项目对高等教育也有重要影响：大部分工作是由本科生和研究生完成的，这些学生将为他们未来的职业生涯获得宝贵的技能和经验。标准模型对中子衰变过程的描述中固有的冗余性允许对模型的有效性进行灵敏的检查。其中一个预期的观测值（中微子-电子相关系数“a”）将允许提取Cabbibo-Kobayashi-Maskawa （CKM）矩阵的左上方元素𝑢𝑑，以测试矩阵的统一性。最近对世界数据的重新评估表明，CKM矩阵的统一性被违反了，因此Nab实验的结果是热切期待的。另一个可观测值（Fierz术语“b”）允许我们以一种互补的方式测试低能量下的标量或张量相互作用，其灵敏度与LHC的预期相竞争。目前的《核科学长期计划》专门用一段话来说明这一努力（见《长期计划》，第75页）。该奖项反映了美国国家科学基金会的法定使命，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 grant supports the measurement of angular correlation coefficients describing neutron beta decay using the “Nab” spectrometer at Oak Ridge National Laboratory. In this experiment the "a" and "b" coefficients describe the proton and electron resulting from the decay of the neutron. The main purpose of the measurements is to test the Standard Model of Nuclear and Particle Physics, which has been extremely successful in describing most of the properties and behavior of the elementary particles to date but leaves open critical questions such as the reason for the dominance of matter over anti-matter in the universe. A discovery of Beyond the Standard Model physics would thus have important implications for Physics and Cosmology. Installation of the Nab spectrometer at the Spallation Neutron Source (SNS) at Oak Ridge will be finished by the end of summer 2021, and commissioning and data taking will commence and continue for the duration of the proposal period. The PI is the project manager for the Nab collaboration. This grant will fund the PI’s group, and they will participate in data taking, detector simulation, data analysis, and characterization of the proton detection efficiency. The PI is strongly committed to attracting members of underrepresented groups. The University of Virginia physics department recently became a new site for the APS bridge program, and the PI is serving as a mentor. Furthermore, the project has a valuable impact on higher education: most of the work is done by students at undergraduate and graduate level, and these students will earn valuable skills and experiences for their future careers.The redundancy inherent in the Standard Model description of the neutron beta decay process allows for sensitive checks of the model’s validity. One of the expected observables (the neutrino-electron-correlation coefficient “a”) will allow an extraction of 𝑉𝑢𝑑, the upper left element of the Cabbibo-Kobayashi-Maskawa (CKM) matrix, in order to test the unitarity of the matrix. A recent re-evaluation of the world’s data suggests that CKM matrix unitarity is violated and so the results of the Nab experiment are eagerly anticipated. The other observable (The Fierz term “b”) allows us to test for scalar or tensor interactions at low energies, in a complementary way, and with sensitivity competitive to what is expected from the LHC. The current Long Range Plan for Nuclear Science devotes a paragraph to this effort (see LRP, page 75).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.