We May Have Just Observed Axions for the Very First Time

Caroline Delbert
Photo credit: XENON1T

From Popular Mechanics

  • The dataset from an underground lab could show the existence of solar axions.
  • Axions are a telltale solar or dark matter particle suggested by physicists, but never observed.
  • This measured "excess" could be several things, including axions as well as new background noise.

Scientists say they may have observed axions for the very first time. The data is unconfirmed by outside observers, and the scientists themselves say their data could be from any of several other explanations, but the likelihood points toward axions.

What’s an axion? So far, most study of the idea of axions has focused on what they’re not—but hypothetically, the idea of the axion could help to explain an unresolved difference in the amount of “light matter” and “dark matter” scientists believe the universe contains. Because dark matter has never been observed, scientists rely on telltale signals to help them extrapolate where and how dark matter might impact other particles.

“Axions are hypothetical particles that were proposed to preserve a time-reversal symmetry of the nuclear force, and the Sun may be a strong source of them,” the laboratory reports.

Today’s news is in the form of a press release from a long-term experimental observatory in Gran Sasso, Italy called XENON1T, where a liquid xenon detector monitors some of the lowest energy signals possible. The liquid and gas detection chamber records even the most minute disturbances, and these signals are collected over time as evidence of potential dark matter interactions.

The overall principle is like triangulating lightning during thunderstorms. Instruments measure “scintillation light” generated by interacting particles, and the electrons released as that light are measured a second time. Then the two measurements are compared, using the time delay to extrapolate how deep the interaction was.

The XENON experiment, as the overall facility and studies are known, includes the XENON1T detector and the associated observations. University of Chicago graduate student Evan Shockley presented the results on Wednesday, and XENON1T’s statement explains in more detail.

Over a certain time period, the scientists expected a certain number of these light and electron interactions, and they assign most expected events to known phenomena. Instead of an expected 232 such events, there were 285—an excess of 53, representing about 23 percent more events than scientists estimated.

“This raises the exciting question: where is this excess coming from?” the statement says. “One explanation could be a new, previously unconsidered source of background, caused by the presence of tiny amounts of tritium in the XENON1T detector.”

This might sound like the sad-trombone outcome, but in the search for dark matter, the more scientists can understand about what they’re seeing and hearing, the more precisely they can continue to tune their search efforts.

But the scientists have reason to feel optimistic, too. They explain:

“[T]he excess observed has an energy spectrum similar to that expected from axions produced in the Sun. While these solar axions are not dark matter candidates, their detection would mark the first observation of a well-motivated but never observed class of new particles, with a large impact on our understanding of fundamental physics, but also on astrophysical phenomena.”

They say the solar axion outcome is the most likely given this observation of excess signals. And, they say, their next-generation XENONnT detector could expose the additional information required to decide once and for all.

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