ACME electron EDM search

The electron Electric Dipole Moment (eEDM) is a background-free probe of general new CP-violating physics. New physics with CP-violation generally predicts finite eEDM at an observable level in the Advanced Cole Molecule Electron (ACME) experiment. We are aiming to measure the eEDM at 3*10^-31 e･cm level, which corresponds to 50 TeV scale CP-violating physics.

Several features are important for high sensitivity, and we use the ThO molecule to achieve them.

High Electric Field
ThO has an internal electric field of 80 GV/cm, which is ~1,000,000 times larger than lab electric fields.
Long Coherence Time
ThO's science state 3Δ1 state has low excited energy, and its decay to the ground state is forbidden to the lowest order.
Low B-field Sensitivity
ThO's science state 3Δ1 has nearly zero g-factor, which makes magnetic field control much easier
Internal Comagnetometer
In the ThO's science state 3Δ1, we can selectively align or anti-align the electron's spin to the internuclear axis. Since the internal electric field is aligned with the internuclear axis, this gives a spectroscopic reversal of the applied electric field without physical, electrical, or magnetic change of experimental parameters.
High Flux
ThO is known to be the molecule that can achieve the highest flux using the buffer gas beam source.
Simple Nuclear Structure
Both 232Th and 16O do not have nuclear spins, which makes all controls (cooling, state preparation, readout, etc) efficient.
Convenient Transition Wavelength
All transitions required for cooling, state preparation, and readout are in red to NIR range, which is easily accessible with commercial diode lasers.

The ACME experiment is ongoing at Northwestern University. I am one of the PIs and work as the central worker.

Schematic of the ACME III experiment apparatus. Three major statistical improvements from ACME II: x5 longer spin precession region, an electrostatic molecular lens, and new photon detectors, SiPM.

The anticipated statistical gain is summarized on the right. Toward this goal, we are working on the construction of the experiment. We aim to start taking data in the middle of 2024.

A rebuilt vacuum chamber with large rectangular magnetic shields (B< 10 uG) allows 5 times longer spin precession.
A hexapole electrostatic molecular lens is installed to collimate the ThO beam.
Silicon Photo-Multiplier (SiPM) will be used to improve the collection efficiency.
Improved data-acquisition hardware reduces timing jitter noise and improves systematic rejection
The ablation target can be changed regularly with a load lock system to increase the duty cycle.
A compact and more efficient rotational cooling is implemented.

With all improvements, we expect more than order of magnitude improved eEDM limit. Below are some pictures of the apparatus we constructed.

These developments are joint effort in the ACME collaboration! Please visit the ACME webpage for details.

Supported by
- National Science Foundation
- Gordon and Betty Moore Foundation
- Alfred P. Sloan Foundation.

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