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是已知最小尺度（1飞米或更小）物质结构的原因。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。此外，SM在低能量的强相互作用中变得不可行。介子（介子）罕见的β衰变，大约在1亿个普通介子衰变中发生一次，理论上为SM的某些可能扩展提供了一个清晰的窗口。pibeta实验由弗吉尼亚大学（UVa）领导的7个国际合作机构在瑞士Paul Scherrer研究所进行，在1999年至2001年的一系列测量中，pibeta实验已经将先锋衰变率的准确性提高了5倍，从4%提高到0.8%。未来一两年，通过改进数据分析，精度将进一步提高，降至0.5%左右。所有其他罕见的介子和介子衰变通道的精度也得到了类似的提高。在我们的介子辐射衰变数据中发现了一个重要的异常，促使我们在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，这一过程对于确定自然界是否以完全相同的方式对待我们熟悉的电子和它更大质量的表亲介子具有深远的意义。即使是微小的偏离也会标志着新的物理学。在过去的五年里，这个项目在弗吉尼亚大学获得了两个博士学位，在合作大学获得了三个博士学位。预计未来两年还会有四个弗吉尼亚大学的博士学位。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 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.