Physicists using the LUX-ZEPLIN (LZ) dark matter detector deep underground in a former South Dakota gold mine say they’ve recorded a single particle interaction that they can’t easily explain, but they’re a long way from claiming a discovery.
To search for dark matter, LZ uses photomultiplier tubes (shown here before installation in the detector) to capture light from particle interactions. Image credit: Matthew Kapust / Sanford Underground Research Laboratory.
The LZ experiment is an international collaboration of 250 scientists and engineers from 39 institutions.
The detector uses 10 tons of ultrapure liquid xenon 1.6 km (one mile) below ground at the Sanford Underground Research Facility in South Dakota.
Managed by Lawrence Berkeley National Laboratory, it is optimized to look for WIMPs (weakly interacting massive particles).
“We expect dark matter interactions to be extremely rare, so it wouldn’t take many to represent the first real detection of dark matter,” said Dr. Carmen Carmona, leader of the LZ experiment group at Penn State.
“That’s why even a single event stands out to us. In fact, this particular signal came from a region of the dataset we hadn’t previously examined, and it took our team months of additional work on understanding possible causes of backgrounds to rule out conventional explanations for it.”
The LZ team analyzed 220 live days of data collected between March 2023 and April 2024, this time searching for a broader range of possible WIMP interactions that could deposit more energy in the detector than previous searches covered.
If caused by dark matter, the particle responsible would likely have a mass of at least 200 GeV/c2 (gigaelectronvolts), or more than 200 times the mass of a proton, suggesting a more exotic interaction than the simplest theoretical models predict
The signal remains well short of the 5-sigma threshold physicists require to claim a discovery. The new analysis stands at 2.6 sigma, meaning there’s roughly a 0.5% chance it’s simply a statistical fluke from known backgrounds.
“We’re very intrigued to see this event in the data, in the region where we expect dark matter to show up and the competing backgrounds are very low,” said Brown University’s Professor Rick Gaitskell, spokesperson for LZ.
“With only one event, we don’t want to get ahead of ourselves. We are not claiming to have seen dark matter.”
“But we have seen something interesting that we want to share with the scientific community for their input.”
The team’s results will be submitted to the journal Physical Review Letters.
