ACME III: moving closer to a more precise electron electric dipole moment measurement

The existence of a nonzero electron electric dipole moment (eEDM) violates CP symmetry, and a measured eEDM that is greater than the standard model prediction of 10-35 e⋅cm implies new physics beyond the standard model. The ACME experiment implements a spin precession experiment by preparing Thorium Monoxide (ThO) molecules in the H3Δ1 state, allowing them to acquire a phase as they spin down our beamline, and optically reading out the final state. The ACME III experiment is well underway and all of the upgrades have been completed including: a 5x longer beamline [1], an electrostatic lens that focuses the molecular beam [2], a load lock mechanism that more easily changes out ThO target [3], and improved Silicon photomultipliers and collection optics [4]. All of these improvements will lead towards statistical sensitivity on the 10-31 e⋅cm level. Checks that compare the spin precession frequency correlated to the eEDM against systematic variables that could contribute to a false nonzero eEDM are close to completion. The progress towards the final ACME III measurement will be reported.

This work was supported by the National Science Foundation, the Gordon and Betty Moore Foundation, and Alfred P. Sloan Foundation, JSPS Kakenhi, and Okayama University RECTOR program.

[1] D. G. Ang et al, Phys. Rev. A 106, 022808 (2022).

[2]  X. Wu et al, New J. Phys. 24 073043 (2022)

[3]  Z. Han et al, arXiv:2505.11647 (2025)

[4] A. Hiramoto et al, Nucl. Instrum. Methods Phys. Res., Sect. A, 1045 (2023)