Intriguing results in the search for dark matter

The LUX-ZEPLIN (LZ) experiment reports a signal that cannot be explained by any known background.

A single particle interaction is the most intriguing signal reported so far by the LUX-ZEPLIN (LZ) dark matter experiment — though researchers say more data is needed before any claim can be made.

Dark matter is thought to make up around 85% of all the matter in the universe, yet it has never been directly detected. Now, a new analysis from the LZ experiment has identified a single particle interaction that researchers have struggled to explain using any known background process. The result falls short of the statistical threshold required to claim a discovery but is the most compelling hint of dark matter the experiment has reported to date.

LZ is an international collaboration of around 250 scientists and engineers. The detector, which uses 10 tonnes of ultrapure liquid xenon to search for weakly interacting massive particles (WIMPs), is managed by the U.S. Department of Energy's Lawrence Berkeley National Laboratory and operates nearly a mile underground at the Sanford Underground Research Facility (SURF) in South Dakota, USA.

Funded in the UK by the Science and Technology Facilities Council (STFC) for both construction and operations, ten UK teams play a central role in LZ, including researchers from the University of Edinburgh. The analysis behind this result was led by the LZ group at the University of Bristol and was presented at the 2026 TeV Particle Astrophysics conference in Japan. The paper is available on the LZ Dark Matter Experiment website and has been submitted to the journal Physical Review Letters.

Dr Sam Eriksen, a senior research associate at the University of Bristol and lead author of the study said:

This was a detailed study in a region we hadn’t explored within this dataset, and we spent months of additional effort to understand all the possible causes of background events. We understand our detector and the backgrounds so well that even a single outstanding event, like the one we found, is important. We expect dark matter events to be extremely rare, so only a handful could mark the first detection of WIMP dark matter.

If the event was caused by dark matter, it would point to a WIMP with a mass of at least 200 GeV/c² (gigaelectron volts), or more than 200 times the mass of a proton, interacting with ordinary matter in a way not covered by the simplest theoretical models. The result stands at 2.6 sigma significance, meaning there is roughly a 0.5% chance it could be explained by known backgrounds alone. This is well short of the 5-sigma threshold that physics requires before a result can be called a discovery.

LZ's results so far include the most sensitive dark-matter searches to date, the most significant observation of solar boron-8 neutrinos, and today's result, which could be an early hint of a dark-matter detection. Together, they reflect years of collaborative work across the international team and point to the value of continuing the search with a larger successor experiment.

The UK team is already working with international partners on the next phase: XLZD (XENON-LUX-ZEPLIN-DARWIN), a next-generation rare-event observatory for dark matter detection and neutrino physics, drawing on the double-phase liquid xenon technology pioneered in the UK. The project is currently supported by a UKRI Infrastructure Fund preliminary activity, and the UK is exploring the possibility of hosting the experiment at the Boulby Underground Laboratory.

Professor Alex Murphy, Personal Chair in Nuclear & Particle Astrophysics, at the University of Edinburgh, said:

One of the first to consider the possibility – and perform calculations – of unseen matter in the galaxy was Lord Kelvin, of Glasgow University, way back in 1904. So there’s a lot of history here in the UK. Rather than perhaps being an end to this journey, the interesting event we’ve seen might be a whole new start. It’s especially exciting to think that the Boulby Underground Laboratory could feature highly in whatever comes next.
 

University of Edinburgh researchers in this field also include academic Dr Sally Shaw, postdoctoral researchers Dr Alberto Uson and Dr Sam Woodford, and PhD students Ellie Bishop and Huan Zhang.  They specialise in areas key to the dark matter search – neutron backgrounds and simulations, and more exotic searches for signals from theorised particles such as axions and axion-like particles.

Dr Sally Shaw said:

This is definitely the most exciting thing LZ has seen so far but we’ll need to work hard now to analyse more data to determine if this is a real dark matter signal. If we see more interactions that fit the bill, we’re onto something. We’re intrigued because that ‘something’ would be our first steps in understanding a huge missing piece of the puzzle of our universe!