RUI: Fundamental Symmetries and Interactions Studies in Weak and Electromagnetic Decays

Wittenberg University, Springfield OH

Investigators

Abstract

The Standard Model of nuclear and particle physics is remarkably successful in describing a wide range of physics topics but there are still many important unanswered questions. One crucial question has to do with differences in how matter and anti-matter behave. And the Standard Model’s description of the weak interaction, one of the four fundamental forces of nature, is believed to be incomplete. This research searches for new physics beyond the Standard Model by making precise measurements in simple systems such as in the annihilation of electrons by their anti-particles (positrons) or by measuring the beta decay of a nucleus. Three projects are proposed: one to measure the decay of positronium (an unstable bound state of an electron and a positron), and two to measure the beta decay of Carbon-14 and Fluorine-20. These projects will enable talented undergraduate students at Wittenberg University, where the proportion of women physics majors is well above the national average, to work closely with faculty on leading-edge research. The students involved in this research will develop instrumentation and computer skills important for many post-college paths in STEM fields. Further, they will learn to think independently, solve practical problems, and communicate clearly. These skills prepare them well for graduate work in physics, engineering, and related fields, and for the STEM workforce, including physics teaching. The major effort will be a high-precision search for CP violation in positronium decay, using an instrument under development with support from a previous MRI award. CP violation is described in the Standard Model (SM) through the CP-violating phase of the CKM quark mixing matrix. CP violation is necessary to explain the observed matter-antimatter asymmetry in the universe, but the level of violation present in the SM is far too small to explain this asymmetry. This has motivated searches for CP violation in a broad range of systems. Searches for new mechanisms of CP violation in observables where the SM CP-violating effects are strongly suppressed are among the most sensitive probes to search for new physics. With Michigan State University collaborators, the group is developing and will employ a highly efficient, symmetric, and high-rate-capable instrument to search for CP violation in the decay of tensor polarized positronium (Ps). The goal is to improve by an order of magnitude the current limits on a CP-violating correlation involving the spin of Ps and the momenta of the two most energetic gammas in the 3-gamma decay of Ps. The group will also complete two high precision measurements of the shapes of nuclear beta spectra as tests of the SM description of the weak interaction, that is, Vector and Axial Vector parts (V-A description) in a left-handed, maximally parity violating combination. Other, non-SM interaction forms (e.g. Tensor) give rise to shapes of nuclear beta spectra that differ from those expected from a pure V-A interaction. The group is measuring the shape of the beta spectrum of 20F to put tighter limits on such non-V-A contributions to the weak interaction. The SM also predicts a contribution (“weak magnetism” (WM)) to the shape of beta spectra due to the theory’s formulation in terms of a conserved vector current (CVC). A precision spectrum shape measurement in 14C, as well as the 20F shape measurement, will quantitatively test CVC in its strong form. The 14C experiment complements a recent high-precision measurement by the group in the 14O system. 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.