Research in Intermediate Energy Physics

University Of Virginia Main Campus, Charlottesville VA

Investigators

Abstract

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 or less. The research is motivated by the fact that the present comprehensive theory--the Standard Model (SM)--is known to be incomplete. Furthermore, the SM becomes unworkable for strong interactions at low energies. The rare beta decay of the pi-meson (pion), occurring once in about 100 million ordinary pion decays, provides a theoretically clean window to certain possible extensions of the SM. The PIBETA experiment, mounted at the Paul Scherrer Institute, Switzerland, by an international collaboration of seven institutions led by the University of Virginia (UVa) group, has already improved the accuracy of the pion beta decay rate five-fold, from 4% to 0.8% in a series of measurements from 1999 through 2001. Further improvement in precision, down to about 0.5%, is forthcoming in the next year or two through improved data analysis. All other rare decay channels of the pion and the muon have been similarly improved in precision. An important anomaly was discovered in our pion radiative decay data, prompting a new, dedicated measurement of this decay in 2004. After that, we plan to design a new precise measurement of the Pi -> e nu decay, a process of profound significance in determining whether or not nature treats the familiar electron and its more massive cousin, the muon, in precisely the same way. Even a small departure would signal new physics. During the past five years this program has resulted in two doctoral degrees at UVa plus three more at collaborating universities stemming from the PIBETA project alone. Four more UVa Ph.D. degrees are expected in the next two years. Each year typically one to two undergraduate students are engaged in research on this project, thus gaining practical laboratory experience.