A particle detector buried almost a mile beneath the Black Hills of South Dakota has picked up a single interplay that physicists can’t clarify with any identified supply of background noise, elevating the chance that scientists have caught their first direct glimpse of darkish matter.
The occasion, recorded on June 16, 2023 by the LUX-ZEPLIN (LZ) experiment on the Sanford Underground Analysis Facility (SURF) in Lead, South Dakota, appeared in an power area the place researchers had predicted darkish matter indicators would present up, and the place competing background indicators are extraordinarily low. The findings had been introduced on September 1 on the 2026 TeV Particle Astrophysics convention in Japan and detailed in a preprint submitted to Bodily Assessment Letters.
“We’re very intrigued to see this occasion within the information, within the area the place we anticipate darkish matter to point out up, and the competing backgrounds are very low,” mentioned Rick Gaitskell, a professor of physics at Brown College and the spokesperson for LZ. “With just one occasion, we do not wish to get forward of ourselves. We’re not claiming to have seen darkish matter. However now we have seen one thing fascinating that we wish to share with the scientific group for his or her enter.”
A needle in a cosmic haystack
Darkish matter is believed to account for roughly 85% of all matter within the universe. It emits no gentle, absorbs nothing, and displays nothing, but its gravitational affect is regarded as the scaffolding that holds galaxies collectively and explains why they rotate sooner than seen matter alone would enable. For many years, physicists have tried to detect it instantly, and the main theoretical candidate has been the Weakly Interacting Huge Particle, or WIMP.
The LZ detector is purpose-built for this hunt. It consists of a 10-ton tank of ultra-pure liquid xenon, instrumented with photomultiplier tubes able to sensing gentle on the single-photon degree. When a particle collides with a xenon atom, it produces a faint flash and a small electrical sign that the detector can report. Sitting 1.6 kilometers underground, the equipment is shielded from cosmic rays by a thick layer of rock, whereas a surrounding water tank and outer detectors assist filter out different particles that might mimic a darkish matter signature.
The newly reported sign was recognized throughout an evaluation of 220 days of knowledge collected between March 2023 and April 2024. Not like earlier searches that targeted on the only attainable WIMP interactions, this examine examined a broader vary of interplay sorts that might deposit extra power into the detector.
“This was an in depth examine in a area we hadn’t explored inside this dataset, and we spent months of extra effort to know all of the attainable causes of background occasions,” mentioned Sam Eriksen, a senior analysis affiliate on the College of Bristol and the paper’s lead writer.
If the sign did come from a WIMP colliding with a xenon nucleus, the particle would wish to have a mass of at the least 200 GeV/c2, equal to greater than 200 instances the mass of a proton. The interplay additionally seems to have been unusually energetic, depositing about 248 keV into the detector. That mixture of excessive mass and excessive power switch doesn’t match neatly into the only WIMP fashions.
The statistical hurdle
Regardless of the intrigue, the consequence falls properly wanting the brink required to assert a discovery in particle physics. The sign carries a worldwide significance of two.6 sigma after accounting for the look-elsewhere impact, which means there may be roughly a 0.5% likelihood that identified background processes might have produced it. By conference, physicists require a 5-sigma significance, comparable to a few one-in-3.5-million chance of a fluke, earlier than declaring a brand new particle discovered.
“It is genuinely thrilling to contemplate whether or not this may be an early glimpse of darkish matter lastly revealing itself,” mentioned Luiz de Viveiros, an affiliate professor of physics at Penn State and a member of the LZ collaboration. “We’re nonetheless racking our brains over whether or not some uncommon background course of might account for it, however to date nothing has turned up.”
| Metric | Worth |
|---|---|
| Detection date | June 16, 2023 |
| Knowledge analyzed | 220 days (Mar 2023 – Apr 2024) |
| Sign significance | 2.6 sigma (world) |
| Likelihood of background origin | ~0.5% |
| Estimated WIMP mass (if actual) | Not less than 200 GeV/c2 |
| Detector depth | ~1.6 km underground |
Observe: Figures are as reported by the LZ collaboration of their preprint and convention presentation.
An unexplained puzzle
If the occasion really represents a high-energy WIMP interplay, the usual theoretical framework suggests LZ ought to have already noticed quite a few lower-energy occasions as properly. It has not. This discrepancy might point out that WIMPs work together with bizarre matter in methods scientists don’t but anticipate, or that the particle acquired an uncommon increase in velocity. Alternatively, the sign may stem from a wholly totally different sort of unidentified particle or bodily course of.
The LZ crew has solely analyzed about one-third of the information it has already collected, leaving a considerable reservoir for follow-up research. The collaboration now holds the world’s largest darkish matter dataset, and future analyses might reveal extra occasions in the identical power vary. Two competing experiments, PandaX in China and XENONnT in Italy, might additionally assist affirm or refute the discovering in the event that they observe related indicators.
Worldwide collaboration contains Korean researchers
LZ is a collaboration of roughly 250 scientists and engineers from 39 establishments, led by the U.S. Division of Power’s Lawrence Berkeley Nationwide Laboratory. Among the many individuals is a crew from South Korea’s Institute for Primary Science (IBS), led by Kim Younger-deok, director of the IBS Heart for Underground Physics. IBS is the one Asian analysis establishment concerned within the undertaking.
“IBS is the one Asian analysis establishment to have participated, and we’re additionally becoming a member of the next-generation XLZD experiment,” Kim mentioned. “In liquid xenon detector experiments which are displaying robust efficiency in WIMP observations, Korea can take a extra main function.”
Douglas Leonard, an IBS analysis fellow, added: “One thing we don’t but perceive has appeared, and whichever method it’s decided, we’re studying and discovering one thing new.”
The outcomes had been first posted to the preprint server arXiv on September 1 and have been submitted to Bodily Assessment Letters for peer evaluation. The collaboration introduced the findings on the 2026 TeV Particle Astrophysics convention in Japan the identical day.
For now, the darkish matter thriller stays unsolved. However the June 2023 occasion provides a concrete lead, a selected power signature that future information can both corroborate or rule out. If extra indicators emerge, June 16, 2023 could at some point be remembered because the second physicists first caught darkish matter within the act, or maybe stumbled upon one thing much more surprising.
