Grant: Discovery Grants Program - Individual

Ultrahigh-precision measurements in atoms and molecules are key to determining fundamental constants of nature and testing for physics beyond the standard model. E.g., our measurement of the hydrogen Lamb shift (Science 365 1007(2019)) determines the charge radius of the proton, but also tests quantum-electrodynamic (QED) predictions for the Lamb shift, thus putting limits on possible beyond-the-Standard-Model physics (BSMP). Tests of BSMP are of extreme interest given major gaps in our knowledge of fundamental physics (e.g., Dark Matter and the asymmetry between matter and antimatter). This proposal consists of two programs that use ultrahigh-precision measurements to measure fundamental constants, to test QED and to search for BSMP. The first continues a successful decades-long program of measuring the fine structure of the n=2 triplet P states of helium. In a previous experiment (PRL 121 143002 (2018)), we measured the smaller J=1-to-2 interval to a precision of 11 parts per billion (ppb) using the newly-introduced frequency-offset separated fields (FOSOF) technique. More recently, a FOSOF microwave measurement of the larger J=0-to-1 interval with a precision of 60 Hz (2 ppb) was completed (arXiv:2310.17040 (2023)). We propose here to improve both measurements to a precision of 10 Hz (0.3 ppb). The new level of precision will require an improved signal-to-noise ratio and years of careful study of all systematic effects. When QED calculations are completed at a similar precision, these measurements will lead to a test of 2-electron QED at an unprecedented level of accuracy, provide a precision test for BSMP in the form of exotic spin-dependent interactions between electrons, and provide a new 0.15-ppb determination of the fine-structure constant. A comparison between precision determinations of this constant in different systems tests our understanding of the physics of each system and puts limits on BSMP in each. The second program of research involves measurements of the barium monofluoride (BaF) molecule. This molecule is being embedded into a cryogenic argon solid to measure the electron's electric dipole moment (eEDM) by the EDM3 collaboration, which includes physicists and chemists at York University, the University of Toronto, and Michigan State University. That work aims to test for the BSMP T-violation, which is needed to understand the asymmetry between matter and antimatter in the universe. The present Discovery Grant will be used to study BaF molecules to help guide the work of the EDM3 collaboration. In particular, we will measure the spectroscopic properties (electronic excitation wavelengths, vibrational excitation wavelengths, excited-state lifetimes, ground-state hyperfine structure, and Stark shift rates) of this molecule when it is embedded in an argon matrix. Whereas these properties are well known in the gas phase, each property is affected by the solid, thus requiring a new set of measurements.