Precision Studies in Subatomic Systems

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This award supports a program of experimental research to explore the limits of our knowledge of certain subatomic particles, and of their interactions at the smallest length scale. The project investigates three systems: the pi meson, the muon (a more massive "cousin" of the electron), and the neutron, focusing on the intrinsic low-energy properties of each of them. Those properties are sensitive to some of the basic components of the Standard Model, which describes the number and basic characteristics of the known elementary particles. Determining these properties thus has the potential to uncover new physics beyond the Standard Model. The project aims to advance basic science in nuclear and elementary particle physics, contribute to building the nation's STEM talent by educating members of the new generation of the STEM workforce, while keeping in focus its diversity and tapping the broadest available talent pool, and, finally, to develop new tools in fundamental research with possible broader applications.The proposed research continues an ongoing program of study of the electroweak and strong interactions at low energies. The immediate goals of the project are: (a) conclusion of the data analysis for the PEN experiment, a precise measurement of the electronic and radiative electronic rare decays of the pion (pi-e2 and pi-e2g), completed at the Paul Scherrer Institute (PSI), and (b) assembly and commissioning of the apparatus for Nab, a program of precise measurements of the neutron beta decay parameters at the Spallation Neutron Source (SNS). The UVa group plays a leading role in both projects. As the work on PEN nears completion, focus will move to (c) the new precise measurement of the muon gyromagnetic ratio, the muon g-2 experiment at Fermilab. The goal of PEN is to reach the precision of 5 parts in 10,000 for the branching ratio of the electronic decay of the pion, which is the most accurate experimental test of lepton universality (LU) available. At present, precision of the pi-e2 decay measurements lags behind the SM theoretical predictions by more than an order of magnitude. A number of physics scenarios outside the SM would lead to LU violation. LU, and lepton properties in general, carry added significance due to developments in neutrino physics. Nab is a highly rate unpolarized measurement of the electron-antineutrino correlation parameter "a" with accuracy of a few parts in 1000, and the first measurement of the neutron decay parameter "b", the Fierz interference term. Nab aims to resolve the persistent inconsistencies in the determination of the nucleon axial vector form factor and, consequently, of the Vud term in the Cabibbo-Kobayashi-Maskawa quark mixing matrix, and to provide new limits on possible extensions of the Standard Model, such as left-right symmetric models, leptoquarks, etc., by exploiting the unique advantages of neutron decay, one of the most basic and theoretically best understood processes in nuclear physics.