Research in Intermediate Energy Physics

这个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下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了已知最小尺度的物质的结构，飞米(氢核的大小)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。Pi介子(Pion)罕见地衰变为一个电子和一个中微子，在大约一万次普通的pion衰变中发生一次，这为SM的某些可能的扩展提供了一个理论上非常干净的窗口，在非常大的质量范围内，任何现有或计划中的加速器都无法直接访问。PEN实验在瑞士的Paul Scherrer研究所进行，由弗吉尼亚大学(UVA)小组领导的七个机构的国际合作开展，建立在PIBETA实验的成功基础上，在1999年至2004年的一系列测量中，该实验将pion和muon(电子的大质量亲属)的几个罕见衰变模式的精度提高了约一个数量级。田纳西州橡树岭国家实验室的散裂中子源启动了一个新的项目，包括实验“NAB”和“ABBA”，目的是精确和同时测量中子β衰变的几个衰变参数。通过它们的精确度，这些实验将探索类似有趣的SM的可能扩展。在过去的五年里，这个项目已经在弗吉尼亚大学获得了四个博士学位。两名弗吉尼亚大学的博士生目前正在准备他们的论文，计划很快就会有第三名学生加入这个小组。每年通常有一到两名本科生参与这个项目的研究，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 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 from 1999 through 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 precisely and simultaneously several of the decay parameter of the 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. Two UVa Ph.D. students are currently working towards their theses, plans call for a third to join the group soon. Each year typically one to two undergraduate students are engaged in research on this project, thus gaining practical laboratory experience.