The ACME EDM Experiment

The ACME experiment uses a cryogenic molecular beam of the heavy polar molecule thorium monoxide (ThO) to measure the electron electric dipole moment (EDM). The existence of an electron EDM would manifest itself as very small energy shifts in certain molecular states when the molecules are in an electric field. The electron EDM is a very strong probe of physics beyond the Standard Model.

Principle of the experiment

The molecular beam is created by buffer gas cooling an ablated sample of ThO, which then flows out of an orifice. We use optical pumping to rotationally cool more molecules into the X, J=0 state. The molecules then enter a magnetically shielded vacuum chamber that contains uniform electric and magnetic fields. We transfer the population using STIRAP into the H electronic (EDM sensitive) state, and then prepare a coherent superposition of M=1, M=-1 states by optically pumping undesired polarizations out of the H state. The quantum state of the molecules precesses in the electric and magnetic fields. Finally, we read out the accumulated phase by projecting the precessed state into orthogonal bases using rapidly switched linearly polarized readout lasers. By reversing the electric field, we can reverse the amount of precession accumulated from the electron EDM; taking the difference between these two phases yields the electron EDM.

Advantages of ThO

While there are many experiments looking for permanent EDMs with the same basic principle, the ACME experiment has several features that will enhance our sensitivity:

  • Polar Molecule. Polar molecules have the highest known sensitivity to an electron electric dipole moment. The electric field inside a polar molecule can be as large as tens of gigavolts per centimeter, almost a million times larger than any controlled field that can be created in a laboratory. Because the energy of a dipole moment in an electric field is proportional to the field strength, the electrons orbiting a polar molecule can experience these large fields and give proportionally larger energy shift. The electric field in ThO has been calculated to be 84 GV/cm, one of the largest known.
  • Internal Co-magnetometer. Since we measure tiny energy splittings, a small stray electric or magnetic field can could destroy our signal. Even worse, these stray fields could look like an EDM in certain situations, for example from leakage currents between the electric field plates. Our molecule has a parity doublet: two close-lying (a few hundred kilohertz in zero field) states of opposite parity in the ground rotational level of the H state. In addition to allowing complete polarization of the molecule in very weak fields, the two components of this doublet have equal yet opposite shifts from an electron EDM. In other words, the two different doublet states correspond to spectroscopically reversing the internal electric field experienced by the electron. Therefore, we can perform our field reversal without reversing any external fields, which will give us very powerful rejection of systematic errors. Other limiting systematics, such as geometric phases, can also be eliminated because they do not reverse between populating the different doublets.
  • Cryogenic Beam Source. Our hydrodynamically-enhanced buffer gas cooled beam is high flux, cold, and slow. We produce beams of ThO with time-averaged fluxes of over 10^13 molecules/state/sr/second moving with a forward velocity of 170 m/s, without the need for stark or optical deceleration.
  • Small Magnetic Moment. The magnetic moment of the experiment H-state is small (~0.0044 Bohr magnetons). This makes our experiment relatively impervious to stray magnetic fields causing noise and systematic errors.

Progress of the experiment

In 2014, ACME I placed a limit on the size of the electron EDM which is twelve times smaller than the previous best measurement. This measurement is a sensitive probe of new physics at the TeV scale, and complements accelerator-based searches (such as the LHC) for new particles and interactions.

ACME II featured several upgrades which increase its statistical sensitivity by over an order of magnitude compared to the ACME I result, such as using STIRAP to efficiently transfer molecules from the ground to the EDM experiment state. In October 2018, ACME II further improved the upper limit on the electron EDM by a factor of 8.7. This result placed constraints on new time-reversal symmetry (T) violating physics at the 3-30 TeV level.

The next generation of the experiment, Advanced ACME, is currently under development (as of February 2020). It will feature a hexapole molecular lens to reduce geometric losses in the molecular beam, a ~4-5 times extended precession time to take full advantage of the H-state lifetime, silicon PMT detectors (in collaboration with Okayama University). These and other improvements are projected to boost the statistical sensitivity of the experiment by an order of magnitude. In addition, fully redesigned magnetic shielding and a new transition for performing STIRAP for population transfer will help reduce systematic errors.

Funding

We would like to thank the National Science Foundation (NSF) and the National Institute of Standards and Technology (NIST) for funding our experiment.