For many years, physicists have been chasing a ghost. Darkish matter, an invisible substance thought to make up a lot of the universe’s mass, has revealed itself solely via its gravitational pull on galaxies and cosmic constructions. However what it truly is stays one in every of science’s best mysteries.
What has now been noticed by the LUX-ZEPLIN (LZ) experiment, positioned deep inside the Black Hills of South Dakota, in a vault of rock practically a mile underground: a single particle interplay that doesn’t match neatly into the catalog of identified backgrounds. It’s not proof of darkish matter, however it’s probably the most compelling trace but from this state-of-the-art detector.
LZ is meant for detecting the very faint alerts of WIMPs (Weakly Interacting Huge Particles), that are one of many predominant candidates for darkish matter. The detector holds 10 tonnes of ultrapure liquid xenon, ready for the uncommon second when a WIMP would possibly collide with a xenon nucleus and produce a tiny flash of sunshine.
To defend in opposition to cosmic rays and different noise, the experiment sits beneath a mile of rock on the Sanford Underground Analysis Facility (SURF). Layers of water tanks, outer detectors, and complicated computational filters additional strip away false alerts. Outlier occasions are anticipated, however often, they are often defined. This one is completely different.
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In a brand new evaluation of 220 reside days of information (March 2023–April 2024), researchers discovered a single nuclear recoil occasion depositing about 248 keV of power. That’s proper within the area the place sure darkish matter fashions predict alerts, and the place background expectations are very low.
The statistical evaluation reveals a worldwide significance of two.6 sigma (roughly a 0.5% likelihood it’s simply background), with an area peak of three.4 sigma; in physics, the criterion for a discovery is 5 sigma, so the result’s not at all conclusive. However for a subject the place alerts are anticipated to be vanishingly uncommon, even one unexplained occasion is sufficient to spark pleasure.
“If this was attributable to darkish matter, the WIMP that generated it might doubtless have a mass of at the least 200 GeV/c2 (gigaelectronvolts), or greater than 200 instances the mass of a proton,” defined the staff. That will level to a heavier WIMP and a extra advanced interplay than the only fashions predict.
Rick Gaitskell of Brown College, LZ’s spokesperson, struck a cautious steadiness: “We’re very intrigued to see this occasion within the knowledge, within the area the place we anticipate darkish matter to indicate up and the competing backgrounds are very low. With just one occasion, we don’t wish to get forward of ourselves. We’re not claiming to have seen dark matter. However now we have seen one thing fascinating that we wish to share with the scientific group.”
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Aaron Manalaysay of Berkeley Lab added: “That is the primary instance in any experiment I’ve labored on of an outlier that seems legitimate in each means. After all, we’re nonetheless twisting our brains attempting to assume if there’s a uncommon background mechanism we may’ve missed, however it’s thrilling to marvel if this might be the primary trace of a dark-matter commentary.”
The LZ collaboration, which incorporates 250 scientists from 39 establishments worldwide, has already constructed the biggest darkish matter dataset ever collected. As extra knowledge accumulates, researchers will see whether or not this anomaly grows in statistical weight or fades into the noise.
In any case, the occasion is efficacious because it confirms LZ’s sensitivity within the new power ranges, improves the background fashions, and maintains the intriguing risk that the very first direct detection of darkish matter may end result from uncommon, high-energy recoils similar to this one.
Darkish matter has formed galaxies, prompted gentle to bend throughout the cosmos, and quietly fashioned the universe as we see it; but on Earth, its presence continues to be elusive. Though a single flash in a tank of xenon doesn’t show now we have gained the search, it is sufficient to remind us that the search continues to be removed from over.
Within the phrases of 1 researcher: “We anticipate darkish matter occasions to be extraordinarily uncommon, so solely a handful may mark the primary detection.”
Maybe, simply maybe, this lone sign is the primary whisper of a solution to a cosmic riddle.
The outcomes have been introduced in a scientific speak on the 2026 TeV Particle Astrophysics conference in Japan, and can be posted on the arXiv on-line repository along with a submission to the journal Bodily Assessment Letters.
