Research in Intermediate Energy Physics

点击翻译按钮获取中文摘要

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.