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的物质结构，即一飞米（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）——是不完整的。介子（π介子）转变为电子和中微子的罕见衰变，在大约一万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。PEN实验是由弗吉尼亚大学（UVa）领导的七个国际合作机构在瑞士Paul Scherrer研究所进行的，它建立在PIBETA实验的成功基础上，PIBETA实验在1999年至2004年的一系列测量中，将几种罕见的介子和介子（电子的巨大相对物）衰变模式的精度提高了大约一个数量级。一个新的项目，包括实验“Nab”和“abBA”，已经在田纳西州橡树岭国家实验室的散裂中子源启动，目的是精确地同时测量中子衰变的几个衰变参数。通过它们的精确性，这些实验将探索SM同样有趣的可能扩展。在过去的五年里，这个项目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，计划很快会有第三名博士生加入这个小组。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 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.