A crack at solving the asymmetry mystery

November 10, 2023

David DeMille

Ramsey Prize winner David DeMille explains the importance of egg-shaped electrons.

Congratulations to David DeMille, Professor of Physics, who has been named a Norman F. Ramsey Prize winner by the American Physical Society. The prize recognizes outstanding accomplishments in the two fields of Norman Ramsey: atomic, molecular, and optical physics; and precision tests of fundamental laws and symmetries.

DeMille, along with Gerald Gabrielse (Northwestern), and John M. Doyle (Harvard), earned the prize “for pioneering work in molecular physics, cooling, and spectroscopy that has profoundly advanced the search for the electric dipole moment of the electron, and for placing stringent constraints on modifications to the Standard Model in a tabletop experiment.”

In an interview with the Physical Sciences Division, DeMille describes his groundbreaking ACME experiment and how it puts the Standard Model to the test.

How would you describe your research to a general audience?

The goal of our research is to detect new forces and particles beyond those in the Standard Model of particle physics theory—the theory that explains all laboratory observations so far. Such new particles can—due to the quantum uncertainty principle—constantly wink in and out of existence in a cloud that surrounds ordinary particles like the electron.

Certain types of particles in this cloud would cause the electron to appear slightly egg-shaped rather than perfectly spherical. And the types of particles capable of this distortion could be responsible for the fact that the universe is asymmetrical—made almost entirely out of matter rather than equal parts matter and antimatter. We use ultra-precise quantum measurement techniques to detect those new particles by looking for egg-shaped electrons.

ACME II team
Students around the ACME II vacuum chamber ca. 2017, with two on a platform known as the "Laser Lounge"—used to direct laser beams vertically through the experiment. Photo by then-postdoc Adam West.

What is the ACME experiment?

ACME (Advanced Cold Molecule Electric Dipole Moment Search) looks at electrons that are bound inside a polar molecule: thorium monoxide. If the electron has an egg shape, the axis of the egg must lie along the same axis the electron constantly spins around. To detect the egg shape, we use a series of lasers to align the spin axis along a known direction in the lab. Simultaneously, we apply a small electric field to orient the molecule in a known direction perpendicular to the spin. The much larger electric field inside this oriented molecule exerts a torque on the electron and causes its axis to rotate (much like Earth’s gravitational field causes a real egg to fall over when placed upright).

A beam of these molecules flies through a vacuum chamber where this field is applied, for roughly a thousandth of a second, after which we use more lasers to detect the final direction of the electron spin. Then, we repeat the experiment with the molecule oriented in the opposite direction. If the electron is egg-shaped, the final angle of the spin will change between the two configurations. All of this is operated by a team of roughly ten researchers, in a lab not much larger than a two-car garage.

Can you explain the experiment’s results thus far and their impact on the field?

ACME has (so far) achieved a sensitivity to the electron’s deformed shape that is roughly one hundred times better than experiments prior to its inception. While we have not (so far) detected any deformation, the limits we place on its size are sufficient to rule out certain types of hypothesized particles that could have explained the matter-antimatter imbalance of the universe. More generally, ACME is sensitive enough to detect various hypothetical particles with masses roughly 30, or even 300, times greater than could be produced and detected at the world’s largest particle accelerator, the Large Hadron Collider. That is: ACME has shown that small-scale, precision experiments are very much at the cutting edge of particle physics today.

To achieve this, ACME developed several new methods to control and measure the quantum states of molecules and the electrons inside them, and to reduce the possibility that spurious effects could fool us into thinking we detected a deformed shape even if it were not there. These advances are now inspiring many new experiments that will take advantage of methods devised for ACME, with the goal to reach much higher sensitivity over the next decade.

How does your research affect our understanding of the Standard Model?

According to the Standard Model, the electron should in fact have an egg shape—but the size of the deformation would be roughly one million times smaller than ACME could detect. So, ACME’s results tell us only about new physics beyond the Standard Model—or, so far, the lack thereof. However, the near-total absence of this deformation in the Standard Model means that should a deformation be detected in the future, it must come from new physics rather than the Standard Model. This is motivating many experiments aiming for much higher sensitivity to the electron’s shape.

What’s next for you and your lab?

Our ACME team is near to completing construction of a third generation of the experiment. This incorporates several additional or upgraded parts of the experiment to deliver more molecules, observe them for a longer time, and detect them more efficiently. Altogether, we anticipate our sensitivity will improve by a factor of about 30 relative to our last result, within the next few years.

In the meantime, we are also building new experiments to detect a similar deformation in the shape of an atomic nucleus rather than that of the electron. These experiments are sensitive to different classes of new particles and forces; one has projected sensitivity that would be sufficient to detect even more massive particles than the third generation of ACME.

What does it mean to you to win the Norman F. Ramsey Prize?

It is an extraordinary honor to be awarded the Ramsey Prize. Norman himself initiated the first searches for deformed particle shapes and pioneered both the production of molecular beams and the measurement concept that is at the heart of ACME (and nearly all of quantum information science and quantum metrology). In addition, he set a tone of openness, collegiality, and respectful interactions between researchers at every level in the field of Atomic, Molecular, and Optical Physics—a tone that persists to this day. He was an inspiring role model for me and many others in the field, and I am proud to help keep his spirit alive in the community.

The ACME experiment has been driven by a remarkable set of PhD students and postdocs. Without their ingenuity and hard work, none of our goals could have been reached. They all should be celebrated with this prize as well.

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