Research in Intermediate Energy Physics

这个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（pion）罕见的β衰变（约1亿次普通π介子衰变中才发生一次）为SM的某些可能的扩展提供了一个理论上干净的窗口。 PIBETA实验由弗吉尼亚大学（UVa）领导的七个机构的国际合作在瑞士保罗谢勒研究所进行，在1999年至2001年的一系列测量中，已经将π介子β衰变率的准确性提高了五倍，从4%提高到0.8%。 通过改进数据分析，在未来一两年内，精确度将进一步提高，下降到0.5%左右。 π介子和μ介子的所有其他稀有衰变通道在精度上都得到了类似的提高。 在我们的π介子辐射衰变数据中发现了一个重要的异常，促使2004年对这种衰变进行了新的专门测量。 在那之后，我们计划设计一种新的精确测量π- e nu衰变的方法，这一过程在确定自然界是否以完全相同的方式对待我们熟悉的电子及其更大质量的表亲μ子方面具有深远的意义。 即使是一个小的偏离也会标志着新的物理学。在过去的五年里，这个项目已经在弗吉尼亚大学带来了两个博士学位，另外还有三个来自合作大学的博士学位。 又有四个弗吉尼亚大学的博士在未来的两年内， 每年通常有一到两名本科生从事这个项目的研究，从而获得实际的实验室经验。

This project addresses certain basic aspects of the electroweak andstrong interactions reflected in the properties of mesons and nucleons(particles found inside atomic nuclei) and their interactions at lowenergies. The strong and weak interactions are responsible for thestructure of matter at the smallest known scale, a femtometer or less. The research is motivated by the fact that the present comprehensivetheory--the Standard Model (SM)--is known to be incomplete. Furthermore, the SM becomes unworkable for strong interactions at lowenergies.The rare beta decay of the pi-meson (pion), occurring once in about 100million ordinary pion decays, provides a theoretically clean window tocertain possible extensions of the SM. The PIBETAexperiment, mounted at the Paul Scherrer Institute, Switzerland, by aninternational collaboration of seven institutions led by the Universityof Virginia (UVa) group, has already improved the accuracy of the pionbeta decay rate five-fold, from 4% to 0.8% in a series of measurementsfrom 1999 through 2001. Further improvement in precision, down to about0.5%, is forthcoming in the next year or two through improved dataanalysis. All other rare decay channels of the pion and the muon havebeen similarly improved in precision. An important anomaly wasdiscovered in our pion radiative decay data, prompting a new, dedicatedmeasurement of this decay in 2004. After that, we plan to design a newprecise measurement of the Pi - e nu decay, a process of profoundsignificance in determining whether or not nature treats the familiarelectron and its more massive cousin, the muon, in precisely the sameway. Even a small departure would signal new physics.During the past five years this program has resulted in two doctoraldegrees at UVa plus three more at collaborating universities stemmingfrom the PIBETA project alone. Four more UVa Ph.D. degrees are expectedin the next two years. Each year typically one to two undergraduatestudents are engaged in research on this project, thus gaining practicallaboratory experience.