Research in Intermediate Energy Physics

本项目研究介子和核子（在原子核内发现的粒子）的性质及其在低能下的相互作用所反映的电弱和强相互作用的某些基本方面。强相互作用和弱相互作用决定了已知最小尺度（一飞米（氢原子原子核的大小）或更小）的物质结构。这项研究的动机是，目前的综合理论——标准模型（SM）——是不完整的。π介子（π介子）转变为电子和中微子的罕见“π (e2)”衰变，在大约10,000次普通介子衰变中发生一次，在理论上为SM的某些可能扩展提供了一个非常干净的窗口，在任何现有或计划中的加速器都无法直接达到的非常大的质量尺度上。由弗吉尼亚大学（UVa）领导的7个机构组成的国际合作小组在瑞士保罗·谢勒研究所（PSI）进行了PEN实验，旨在将目前的pi（e2）分支比结果提高约一个数量级。数据分析计划在目前的3年奖励期内基本完成。届时将对可能的后续测量计划进行评估。位于田纳西州橡树岭国家实验室的散裂中子源（SNS）已经启动了一个精确测量中子衰变参数的新项目。第一阶段，Nab，将专注于精确测量电子-中微子相关参数“A”和Fierz干涉项“b”。Nab应该有助于（以及改进的中子寿命测量）最终解决一个长期存在的问题，即确定轴向和矢量核子形状因子的比率G_A/G_V，以及独立于核模型的Cabibbo-Kobayashi-Maskawa u-d夸克混合的确定。通过它们的精确性，这些实验将探索SM有趣的可能扩展。计划在目前的3年合同期内，将建造Nab设备并进行初步测试。随着PEN工作的结束，该小组将过渡到费米实验室的μ子g-2实验（E989）。UVa小组通过蒙特卡罗模拟为探测器设计做出了贡献，正在领导量热计偏置电压（CBV）项目，并计划在数据分析中发挥重要作用。CBV系统计划在3年的合同期内完成，随后进行仪器安装。PEN、Nab和μ介子g-2（该研究项目中的三个主要实验）共同的项目更广泛的意义和影响是寻找新物理学的信号，这些信号不包括在粒子和场的标准模型中。每一个都使用其独特的灵敏度来探索迄今为止未被探索的基础物理领域，这样做有助于在最基本的层面上全面推进对宇宙的理解。除了对基础科学的影响外，还有对年轻科学家教育的影响：一些本科生和研究生已经并将继续获得最先进的研究训练。在过去的十年里，仅在弗吉尼亚大学就有七个博士和一个硕士学位是通过这个项目获得的；十几名或更多的本科生获得了广泛的实践研究培训，并继续从事有吸引力的职业。这些学生中有几个是科学界代表性不足的群体（妇女和少数民族）的成员。

Technical description of the project This project addresses certain basic aspects of the electroweak and strong interactions reflected in the properties of mesons and nucleons (particles found inside atomic nuclei) and their interactions at low energies. The strong and weak interactions are responsible for the structure of matter at the smallest known scale, a femtometer (size of the hydrogen atom nucleus) or less. The research is motivated by the fact that the present comprehensive theory -- the Standard Model (SM) -- is known to be incomplete. The rare "pi(e2)" decay of the pi-meson (pion) into an electron and a neutrino, occurring once in about 10,000 ordinary pion decays, provides a theoretically extraordinarily clean window to certain possible extensions of the SM, at a very large mass scale not directly accessible at any existing or planned accelerators. The PEN experiment, with measurements carried out at the Paul Scherrer Institute (PSI), Switzerland, by an international collaboration of 7 institutions led by the University of Virginia (UVa) group, aims to improve the current pi(e2) branching ratio result by about an order of magnitude. The data analysis is planned to be largely completed during the current 3-year award period. Plans for a possible follow-up measurement will be evaluated at that time. A new project of precise measurement of neutron beta decay parameters has been initiated at the Spallation Neutron Source (SNS) at the Oak Ridge National Lab, TN. The first phase, Nab, will focus on a precise measurement of the electron-neutrino correlation parameter "a" and Fierz interference term, "b." Nab should contribute (along with improved neutron lifetime measurements) to a final resolution of a longstanding problem in the determination of the ratio of axial and vector nucleon form factors, G_A/G_V, and a nuclear model independent determination of the Cabibbo-Kobayashi-Maskawa u-d quark mixing. Through their precision, these experiments will explore interesting possible extensions to the SM. It is planned that during the current 3-year award period, Nab apparatus will be constructed and initially tested. As the work on PEN concludes, the group will tranisition to the Muon g-2 experiment (E989) at Fermilab. The UVa group has contributed to the detector design through Monte Carlo simulations, is leading the Calorimeter Bias Voltage (CBV) project, and plans to play a significant role in the data analysis. The CBV system is planned to be completed during the 3-year award period, with instrument installation to follow. Broader significance and impact of the project Common to PEN, Nab and Muon g-2, the three main experiments in this research project, is the search for signals of new physics, not included in the Standard Model of particles and fields. Each uses its unique sensitivities to probe a hitherto unexplored region of fundamental physics, and in doing so contributes to the overall advancement of understanding of our universe at the most basic level. In addition to the basic science impact, there is the impact on education of young scientists: a number of undergraduate and graduate students have gained, and will continue to gain state of the art training in research. Seven PhD and one MS degrees were earned on this project during the past decade at UVa alone; a dozen or more undergraduate students got extensive hands-on research training, and have gone on to engaging careers. Several of these students have been members of underrepresented groups in science (women and minorities).