Improved magnetic shielding and coil systems for ACME III electron EDM search *

The measurement of electron electric dipole moment (EDM) is a powerful probe for physics beyond the Standard Model. The ACME II Experiment in 2018 reported the current most stringent upper limit for electron EDM, of d_e<1.1×10^-29 e·cm  (Nature, 562, (2018) 355-360). The next generation of the experiment is currently under development, aiming to improve the sensitivity by at least an order of magnitude. This upgrade features a longer interaction region to increase the spin precession time, and significantly smaller magnetic-field-related noises. To achieve this, a custom-built 3-layer mu-metal magnetic shield covering a larger interaction region is under construction. With optimum geometric design and state-of-the-art degaussing systems, the shield is expected to reduce the ambient magnetic field to <0.1nT. A self-shielding coil and a set of gradient coils were also under construction, enabling the precise control of the magnetic field in the interaction region and the detailed assessment of systematic uncertainties associated with magnetic fields.