Precision Studies in Subatomic Physics

About This Grant

This award supports a program of experimental research by the University of Virginia group, 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. The research program is focused on the intrinsic low-energy properties of each of them, properties that are sensitive to some of the basic tenets of the Standard Model (SM). The SM describes all known (to date) elementary particles, their basic characteristics, and their interactions. Determining these properties precisely enough to establish whether they differ from SM predictions has the potential to uncover new physical processes and particles. 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 tapping the broadest available talent pool. Specifically, in the course of the award period, it is anticipated that two graduate students will complete the requirements for a Ph.D. degree in physics, and another one will graduate with an M.Sc. degree. Finally, the project aims 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) carrying out the measurements of the Nab experiment, a program of precise measurements of the neutron beta decay parameters at the Spallation Neutron Source (SNS), Oak Ridge National Laboratory; (b) finalizing data analysis and publication of results on the muon gyromagnetic ratio determined in the Muon g-2 experiment at Fermilab, (c) analyzing the Phase-I results of the MUonE experiment at CERN, and (d) conclusion of the data analysis and publication of results for the PEN experiment, a precise measurement of the electronic and radiative electronic rare decays of the pion. Nab is a high-rate unpolarized measurement of the electron-antineutrino correlation parameter "a" with accuracy of a few parts in 1000, and a precise 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. This will ultimately 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. The Muon g-2 and MUonE experiments are focused on the hadronic corrections to the extremely precisely predicted electroweak processes that determine the interaction of charged leptons with the vacuum, a research subject with one of the highest potentials for discovery of beyond-SM physics. The Muon g-2 experiment has completed data taking and the main data analysis, and is in the process of publishing details of the analysis. The MUonE experiment at CERN, having established full viability, is wrapping up Phase-1 data acquisition. 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 providess 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 This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.