Search for 'Huge Missing Piece' of Universe Fails — Unearths New Particle Physics Secrets

Unveiling the Secrets of Dark Matter

A groundbreaking investigation, conducted using a particle detector located a mile underground in South Dakota, may have provided new insights into dark matter, the elusive substance thought to constitute most of the matter in the universe.

Using the largest dataset of its kind, the experiment known as LUX-ZEPLIN (LZ) has placed tighter constraints on the potential properties of one of the leading candidates for dark matter with an unprecedented level of sensitivity. While the research did not discover any direct evidence of dark matter, it will assist future studies in avoiding false detections and focusing more accurately on this enigmatic component of the universe.

Rick Gaitskell, head of the particle astrophysics group at Brown University and a member of the LZ research team, explained, "This quest is to try to solve this huge problem, this huge missing piece that we have in terms of understanding our universe."

The results, released on Monday (December 8), have been submitted to the journal Physical Review Letters and are available as a preprint via arXiv. The findings were also presented at a scientific talk at the Sanford Underground Research Facility, where LZ's detector is housed; the research is led by Daniel Akerib, a professor of particle physics at the SLAC National Accelerator Laboratory in California.

WIMPs vs. Neutrinos

The team had two primary objectives for the new study: to understand the properties of a low-mass "flavor" of proposed dark-matter particles called weakly interacting massive particles (WIMPs), and to determine if the detector could observe solar neutrinos—nearly massless subatomic particles produced by nuclear reactions inside the sun. The researchers suspected that the detection signature of these particles might be similar to that predicted by certain models of dark matter, but they needed to confirm this by detecting solar neutrinos.

Before the experiment, which lasted 417 days from March 2023 to April 2025, the detector's sensitivity was enhanced to search for rare interactions with fundamental particles. A cylindrical chamber filled with liquid xenon served as the site of these interactions. Researchers could observe either WIMPs or neutrinos colliding with the xenon, each producing flashes of photons along with positively charged electrons.

The experiment advanced the science for both the WIMP and neutrino questions. Regarding neutrinos, the researchers increased their confidence that a type of solar neutrino, known as boron-8, is indeed interacting with the xenon. This knowledge will help future studies avoid false detections of dark matter.

Physics discoveries typically require a confidence level called "5 sigma" to be considered valid. The new work achieved 4.5 sigma—a significant improvement over sub-3-sigma results reported in two detectors last year. This achievement was especially notable given that boron-8 detections occur only about once a month in the detector, even when monitoring 10 tons of xenon, according to Gaitskell.

As for the dark matter question, however, the researchers did not find anything definitive for the low-mass types of WIMPs they were seeking. Scientists would have recognized it if they had seen it, the team said; if a WIMP hits the heart of a xenon molecule, the energy of the collision creates a distinctive signature, as best as models predict.

"If you take a nucleus, it is possible for dark matter to come in and actually simultaneously scatter from the entire nucleus and cause it to recoil," Gaitskell explained. "It's known as a coherent scatter. It has a particular signature in the xenon. So it's those coherent, nuclear recoils that we're looking for."

The team did not detect this signature in their experiment.

Doubling the Run

Another, longer run will begin in 2028, when the detector is expected to collect results for a record-breaking 1,000 days. Longer runs give researchers a better chance of catching rare events.

The detector will continue to hunt not only for more solar neutrino or WIMP interactions but also other physics that may fall outside the Standard Model of particle physics, which is used to describe most of the environment around us.

Gaitskell emphasized that the role of science is to keep pushing forward even when "negative" results arise.

"One thing I've learned is, don't ever assume that nature does things in the way that you think it should, exactly," said Gaitskell, who has been studying dark matter for more than four decades.

"There are plenty of elegant [solutions] that you would say, 'That's so beautiful. It has to be true.' And we tested them … and it turned out, nature ignored it and nature did not want to go down that particular route."