Research in Intermediate Energy Physics

本项目涉及介子和核子(在原子核内发现的粒子)的性质以及它们在低能时的相互作用所反映的电弱相互作用和强相互作用的某些基本方面。强相互作用和弱相互作用导致物质的结构在已知的最小尺度上，一飞米(氢原子核的大小)或更小。这项研究的动机是，目前的综合理论--标准模型(SM)--是已知的不完整的。在大约10,000次普通的π介子衰变中，发生一次罕见的“pi(E2)”衰变为电子和中微子，这在理论上为SM的某些可能的扩展提供了一个非常干净的窗口，在一个非常大的质量范围内，任何现有的或计划中的加速器都无法直接访问。PEN实验由弗吉尼亚大学(UVA)领导的7个机构的国际合作在瑞士保罗·谢勒研究所(PSI)进行，旨在将目前的pi(E2)分支率结果提高约一个数量级。数据分析计划在本3年授权期内基本完成。届时将对可能的后续测量计划进行评估。在田纳西州橡树岭国家实验室的散裂中子源(SNS)启动了一个精确测量中子β衰变参数的新项目。第一阶段，NAB，将集中于精确测量电子-中微子关联参数“a”和菲尔兹干扰项“b”。NAB应该有助于(连同改进的中子寿命测量)最终解决在确定轴向和矢量核子形状因子的比率G_A/G_V中的一个长期存在的问题，并有助于独立于核模型确定Cabibbo-Kobayashi-Maskawa u-d夸克混合。通过它们的精确度，这些实验将探索SM有趣的可能扩展。计划在目前的3年授权期内，建造和初步测试NAB设备。随着PEN的工作结束，该小组将过渡到费米实验室的Muon g-2实验(E989)。UVA团队通过蒙特卡罗模拟为探测器设计做出了贡献，领导了量热仪偏置电压(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年授权期内完成，随后将安装仪器。这个研究项目中的三个主要实验--PEN、NAB和Muon G-2的共同项目的更广泛的意义和影响是寻找新物理的信号，而不是包括在粒子和场的标准模型中。每一个都使用其独特的敏感性来探索到目前为止尚未探索的基础物理领域，并在这样做的过程中有助于在最基本的层面上全面提高对我们宇宙的理解。除了基础科学的影响，还有对青年科学家教育的影响：一些本科生和研究生已经获得，并将继续获得最先进的研究培训。在过去的十年里，仅在弗吉尼亚大学，就有七个博士学位和一个硕士学位在这个项目上获得；十几名或更多的本科生接受了广泛的实践研究培训，并进入了令人着迷的职业生涯。这些学生中有几个人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).