Experimental Upgrades For Improving Statistical And Systematic Uncertainties In ACME III

The measurement of the electron electric dipole moment (eEDM), d_e, is a powerful probe of physics beyond the Standard Model. The current most stringent limit of |d_e|<1.1 × 10^-29 e·cm was reported by the ACME II experiment (Nature, 562(2018), 355). ACME III aims to improve this experimental limit by an order of magnitude. Progress has been made in improving the statistical sensitivity. An electrostatic lens that can increase the flux of a ThO molecular beam has been built and tested. We are also upgrading our fluorescence detectors to silicon photomultipliers for higher quantum efficiency. An upgraded cryogenic buffer gas beam (CBGB) source featuring shorter cool-down time and in-situ target changing function is built for increasing the duty cycle of the experiment. Guided by our recent H ³Δ₁ lifetime measurement, a longer interaction region is constructed. We also made progress toward reducing systematic errors and their uncertainty. One significant source of systematic error comes from birefringence gradients in the laser optics. We are implementing a modified measurement scheme to reduce this systematic error and taking several new measures to reduce birefringence gradients. Last, we are building and testing an upgraded magnetic shielding and field control system.