LZ Sees Surprising Result in Search for Dark Matter
An interaction observed in the LZ experiment could be interpreted as a WIMP signal, but more data will be needed to confirm this.
An event that is difficult to explain
A single event lies at the heart of the latest results from the LUX-ZEPLIN (LZ) experiment. Identified in an analysis that extended the search for dark matter to higher energies, it cannot readily be accounted for by known background processes arising from ordinary matter.
The result, released on the LZ website on 1 September and presented at the international 2026 TeV Particle Astrophysics conference in Japan, does not yet reach the level of statistical significance required to claim a discovery. It is, however, the most intriguing hint of dark matter reported by LZ to date.
LIP's role
“We know our detector extremely well. After months of careful review, this event appears to be genuine and incompatible with the interactions we would expect,” says Paulo Brás, a researcher at LIP and currently LZ's lead physics coordinator. Paulo Brás played a central role in coordinating this analysis and in reviewing and approving the results.
LIP and the University of Coimbra have been members of the LZ collaboration since it began in 2012. The Portuguese team currently comprises six researchers and two PhD students.
A search at higher energies
This dataset had previously been analysed by the collaboration at lower recoil energies, focusing on the simplest types of interaction between dark matter and ordinary matter. This time, LZ extended the search to a broader range of possible interactions involving weakly interacting massive particles (WIMPs), including those capable of producing higher-energy nuclear recoils in the detector.
Paulo Brás nevertheless stresses the need for caution when interpreting the result: “This analysis has a global significance of 2.6σ (two point six sigma), after accounting for the look-elsewhere effect. Under the background-only hypothesis, this corresponds to a probability of approximately 0.5% of obtaining a result at least this signal-like. Only with more data and further observations will we be able to determine whether this signal is genuinely significant.”
If the event was indeed caused by dark matter, the particle responsible would probably have a mass of at least 200 GeV/c² — more than 200 times the mass of a proton. The result would also point to a type of interaction between WIMPs and ordinary matter that goes beyond the simplest models typically considered in these searches.
The need for caution is also emphasised by Rick Gaitskell, LZ spokesperson and a professor at Brown University in the United States: “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. 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.”
Searching for dark matter
Dark matter is one of the fundamental constituents of the Universe, and its presence is needed to explain a wide range of observations of the dynamics and evolution of the cosmos. However, its existence has so far been inferred only through its gravitational effects, and it has never been directly detected. Within the standard cosmological model, precision measurements by the Planck satellite indicate that the Universe contains roughly five times as much dark matter as ordinary matter.
The LZ collaboration brings together around 250 scientists and engineers from 39 institutions, including LIP and the University of Coimbra. The search for dark matter uses a large detector with an active target of seven tonnes of ultrapure liquid xenon. A hypothetical dark matter particle colliding with a xenon nucleus would cause it to recoil, producing scintillation light and ionisation electrons that generate signals the detector can record.
These signals must, however, be distinguished from the much more frequent background events arising, among other sources, from radioactivity in the detector materials and the surrounding environment. To minimise these backgrounds and, in particular, the contribution from cosmic rays, LZ is located almost 1,500 metres underground at the Sanford Underground Research Facility (SURF) in South Dakota, United States.
More data until 2028
LZ has already accumulated the world's largest dataset dedicated to the direct search for dark matter. The experiment will continue to operate at SURF until 2028, collecting more data and steadily increasing its sensitivity. The new data will help determine whether this intriguing result fades in statistical significance or, on the contrary, becomes stronger. LZ researchers will continue to investigate this latest result.