A Thermochemical Cryogenic Buffer Gas Beam Source of ThO for Measuring the Electric Dipole Moment of the Electron

Abstract

The discovery of an electric dipole moment of the electron (eEDM) within a few orders of magnitude of the current best limit would reveal the existence of time-reversal symmetry (T) violating physics beyond the Standard Model. Certain polar molecules with unpaired electron spins possess highly advantageous qualities for eEDM searches, and recent experiments using such species [10, 103] have pushed the frontier of eEDM searches into regimes of unprecedented sensitivity. By performing a spin-precession measurement on a beam of thorium monoxide (ThO), the ACME collaboration has shown that the eEDM is less than 10^−28 e cm, the most stringent upper bound to date [10, 11]. This null result severely constrains many theoretically proposed T-violating mechanisms, and many of the theories that remain viable predict eEDMs within one or two orders of magnitude below this bound. In order to probe this tantalizing regime, we have developed a new cryogenic buffer gas beam (CBGB) source that exploits a high-temperature chemical reaction [56] between thorium and thorium dioxide to produce gas-phase ThO. For a single target over a single day, this source produces ThO fluxes nearly an order of magnitude higher on average than those produced by the laser-ablation-based CBGB used in our previous measurement [106]. Other beam properties, such as forward velocity, rotational temperature, and divergence have been measured and shown to be comparable to or only marginally less favorable than those of the ablation source. By enhancing the experiment’s achievable count rate, this new thermochemical beam source could improve the statistical sensitivity of a future iteration of the ACME eEDM measurement by up to a factor of 2.5. In this thesis, I discuss the background of and motivation for the new beam source, its design, and its characterization and optimization for use in a future eEDM measurement.