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Record-Breaking New Measurement of Electron “Roundness”

Illustration showing the electrons in a cloud of hafnium fluoride ions.
Illustration showing the electrons in a cloud of hafnium fluoride ions. Credit: Steven Burrows / JILA.
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Scientists have made a new measurement of the electric dipole of electrons, improving upon previous best measurements by a factor of 2.4. They found no significant asymmetries in the charge distribution across electrons, which may have important consequences as physicists attempt to unravel what occurred shortly after the Big Bang.

The research was published in the journal Science and led by scientists at JILA, a joint astrophysics research institute from the University of Colorado Boulder and the National Institute of Standards and Technology.

Beyond the Big Bang

Why are we here? It’s something that philosophers have argued over for centuries, but it is also a question that still mystifies physicists to this day.

The Big Bang – the moment when all matter and antimatter were suddenly created and spat out into the universe – should have created equal amounts of matter and antimatter. These protons, neutrons and electrons should have collided with their antimatter counterparts as the universe expanded, leaving behind only flashes of light in the form of photons.

What is antimatter?

Antimatter is a special substance composed of so-called antiparticles. In theory, every particle of matter we see around us should have an antimatter companion that is virtually identical to itself, but with the opposite charge. For example, the antimatter companion to our negatively-charged electron is a positively-charged particle known as a positron. When a particle and an antiparticle come into contact, they annihilate each other, producing high-energy photons and neutrinos.

The existence of antimatter particles was first predicted by Paul Dirac, a British physicist, in the early 20th century. Since then, researchers have successfully used large particle accelerators to produce and study antimatter particles.

Yet, we are still here. When we observe the universe around us, there is little antimatter to be found. 

“If the universe had been perfectly symmetrical, then there would be nothing left but light. This is a hugely important moment in history. Suddenly there is stuff in the universe, and the question is, why?” said Eric Cornell, a NIST/JILA fellow and the senior author of the new study. “Why do we have this asymmetry?”

One theory for why any matter exists at all is that there might be some asymmetry hiding from us at the electron level. As a charged particle, electrons have a north and south pole. If the electron is completely round, with the charge spread exactly evenly between these two poles, then scientists would say it has an electric dipole moment (EDM) of zero. But any slight deviation, an EDM value above zero, and that means that the electron could be more egg-shaped. Such an asymmetry in the shape of electrons might be the answer that physicists have been looking for.

“We need to fix our math to be closer to reality,” added Tanya Roussy, a graduate student in Cornell’s research group at JILA. “We’re looking for places where that asymmetry might be, so we can understand where it came from. Electrons are fundamental particles, and their symmetry tells us about the symmetry of the universe.”

Ultraviolet laser and magnetic fields help scientists “see” the shape of electrons

Practically, measuring the symmetry or “roundness” of an electron is incredibly difficult. But with their new methodology, the JILA researchers were able to improve on previous measurements by a factor of 2.4. If an electron were the size of the Earth, their new method would be able to find any asymmetry down to the size of an atom, Roussy explained.

The principle behind the new method is rather ingenious. If a very strong electric field is applied across a molecule and its electrons are asymmetric, then the electrons would want to align with the direction of that field and shift around inside the molecule. On the other hand, if they were completely round, no changes would be seen.

For these new measurements, the researchers bombarded molecules of hafnium fluoride with an ultraviolet laser to ionize them. These ions were trapped within a strong alternating electromagnetic field and were forced to either align or not align with the field. Using more lasers, the researchers were able to accurately measure the energy levels of the aligned and unaligned ions. If the levels were any different, this would indicate some asymmetry in the molecule’s electrons.

“We found up to our measurement the electron is symmetric. If we would have found nonzero, it would be a big deal,” Roussy said. Speaking on what this means for the asymmetry present in our early universe, Roussy continued, “The best bet is to have teams of scientists around the world looking at different options. As long as we all keep measuring the truth, eventually someone will find it.”

Experiment demonstrates usefulness of alternative techniques

While the experiment might not have led to any revelations in our understanding of electrons, the new measurement technique is an important advancement for particle physicists and quantum chemists.

Firstly, the technique allowed the researchers to have longer measurement times than their previous attempts, which focused more on sensitivity.

It is also significant that a tabletop experiment was able to achieve this level of precision – it shows that expensive particle accelerators are not the only means of exploring questions in fundamental physics. Indeed, the uncertainty of the measurement compared favorably to similar measurements that have been carried out in accelerators.


Reference: Roussy TS, Caldwell L, Wright T, et al. An improved bound on the electron’s electric dipole moment. Science. 2023;381(6653):46-50. doi: 10.1126/science.adg4084

This article is a rework of a press release issued by NIST. Material has been edited for length and content.