ACME III Electron Electric Dipole Moment Search

The electric dipole moment of electron (eEDM, d_e) with a value around the current limit is theorized to originate from time-reversal- (T-) violating physics beyond Standard Model. The ACME experiment uses a cold beam of ThO molecules to probe for the eEDM. The third generation experiment (ACME III) aims to improve on the most recently published (ACME II) result of |d_e|<1.1×10^-29 e·cm. ^{1 2}

Improved statistical sensitivity is sought in ACME III by employing several new techniques. Listed along with their projected gain in eEDM sensitivity over ACME II in parentheses, these improvements are: A new cryogenic beam source with in-situ target changing (×1.2); An electrostatic lens to focus the ThO molecular beam into the detection region³ (×4.4); A longer interaction chamber that permits longer spin-procession times⁴ (×2.0) and silicon photomultipliers (SiPMs) as photon detectors⁵ (×2.5); Elimination of excess noise from timing jitter in the data acquisition system⁶ (×1.7), corresponding to a total projected improvement by a factor of ∼40.

Aiming also to improve on systematic errors, the transparent electric field plates in ACME (through which state preparation and detection lasers pass) will be mounted in a stress-free manner, and low birefringence vacuum window material will be used, to reduce dangerous birefringence gradients. A new magnetic shielding system is designed to provide <0.1 nT field control. Using ThO in the Q state as a magnetometer will further suppress systematic errors through better magnetic field control.

We will review the performance of each of these improvements.