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 atom 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 "pi(e2)" decay of the pi-meson (pion) into an electron and a neutrino, occurring once in about 10,000 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, with measurements carried out at the Paul Scherrer Institute (PSI), Switzerland, by an international collaboration of 7 institutions led by the University of Virginia (UVa) group, aims to improve the current pi(e2) branching ratio result by about an order of magnitude. The data analysis is planned to be largely completed during the current 3-year award period. Plans for a possible follow-up measurement will be evaluated at that time.
A new project of precise measurement of neutron beta decay parameters has been initiated at the Spallation Neutron Source (SNS) at the Oak Ridge National Lab, TN. The first phase, Nab, will focus on a precise measurement of the electron-neutrino correlation parameter "a" and Fierz interference term, "b." Nab should contribute (along with improved neutron lifetime measurements) to a final resolution of a longstanding problem in the determination of the ratio of axial and vector nucleon form factors, G_A/G_V, and a nuclear model independent determination of the Cabibbo-Kobayashi-Maskawa u-d quark mixing. Through their precision, these experiments will explore interesting possible extensions to the SM. It is planned that during the current 3-year award period, Nab apparatus will be constructed and initially tested.
As the work on PEN concludes, the group will tranisition to the Muon g-2 experiment (E989) at Fermilab. The UVa group has contributed to the detector design through Monte Carlo simulations, is leading the Calorimeter Bias Voltage (CBV) project, and plans to play a significant role in the data analysis. The CBV system is planned to be completed during the 3-year award period, with instrument installation to follow.
Broader significance and impact of the project
Common to PEN, Nab and Muon g-2, the three main experiments in this research project, is the search for signals of new physics, not included in the Standard Model of particles and fields. Each uses its unique sensitivities to probe a hitherto unexplored region of fundamental physics, and in doing so contributes to the overall advancement of understanding of our universe at the most basic level.
In addition to the basic science impact, there is the impact on education of young scientists: a number of undergraduate and graduate students have gained, and will continue to gain state of the art training in research. Seven PhD and one MS degrees were earned on this project during the past decade at UVa alone; a dozen or more undergraduate students got extensive hands-on research training, and have gone on to engaging careers. Several of these students have been members of underrepresented groups in science (women and minorities).
