Research in Intermediate Energy Physics

该项目涉及反映在介子和核子（原子核内发现的粒子）的性质及其在低能量下的相互作用中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（一毫微微米）的物质结构（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）-是已知的是不完整的。π介子（π介子）罕见的衰变为电子和中微子，大约一万次普通π衰变中发生一次，为SM的某些可能扩展提供了理论上非常干净的窗口，其质量规模非常大，无法在任何现有或计划的加速器上直接访问。 PEN实验，安装在瑞士的保罗·谢勒研究所，由弗吉尼亚大学（UVa）小组领导的七个机构的国际合作，建立在PIBETA实验的成功基础上，该实验在2004年完成的一系列测量中将π和μ子（电子的大质量相对物）的几种罕见衰变模式的准确性提高了大约一个数量级。 在田纳西州橡树岭国家实验室的中子源启动了一个新项目，包括实验“Nab”和“abBA”，目的是同时精确测量中子β衰变的几个关键参数。 通过他们的精度，这些实验将探索同样有趣的可能扩展到SM。在过去的五年里，这个计划已经导致了四个博士学位在UVa。 三个UVa博士学生们目前正在写论文。 每年通常有一到两名本科生参与该项目的研究，从而获得实际的实验室经验。

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 decay of the pi-meson (pion) into an electron and a neutrino, occurring once in about ten thousand 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, mounted at the Paul Scherrer Institute, Switzerland, by an international collaboration of seven institutions led by the University of Virginia (UVa) group, builds on the success of the PIBETA experiment which improved by about an order of magnitude the accuracy of several rare decay modes of the pion and muon (a massive relative of the electron) in a series of measurements finished in 2004. A new project, encompassing experiments "Nab" and "abBA," has been initiated at the Spallation Neutron Source at Oak Ridge National Lab in Tennessee, with the aim to measure simultaneously and precisely several key parameters of neutron beta decay. Through their precision, these experiments will explore similarly interesting possible extensions to the SM.During the past five years this program has resulted in four doctoral degrees at UVa. Three UVa Ph.D. students are currently working towards their theses. Each year typically one to two undergraduate students are engaged in research on this project, thus gaining practical laboratory experience.