Dark matter makes up more than 85 percent of all matter in the universe. Their presence has had a decisive influence on cosmic development and continues to influence the behavior of galaxies, gases and other objects to this day. Nevertheless, dark matter is a mystery: what it is made of is still unknown. Despite intensive searches using various methods and detectors, researchers have not yet been able to detect any of the postulated candidate particles.
This also applies to the WIMP (Weakly Interacting Massive Particle), a heavy particle that hardly interacts with other matter. Long considered the most promising candidate for dark matter particles, all attempts to detect WIMPs have so far failed. Many detectors found nothing at all, some supposed signals subsequently turned out to be measurement errors or background effects, as was the case with the XENON1T detector in 2020.
WIMP search underground
Now there is a new – potential – discovery: using the LUX-ZEPLIN detector (LZ) in the USA, physicists have discovered a signal whose characteristics match those of a WIMP. “It is very exciting to find this event in the data – exactly in the area where we expect signs of dark matter and where background signals are very weak,” says LZ spokesman Rick Gaitskell of Brown University.
The physicists detected the signal using the LZ detector at the Sanford Underground Research Facility (SURF) in South Dakota. The detector consists of an underground tank containing ten tons of liquid xenon, the walls of which are equipped with extremely sensitive photosensors. Should a dark matter particle fly through this tank and collide with one of the xenon atoms, this should cause characteristic light signals that are captured by the sensors.
The LUX-ZEPLIN detector consists of a cylindrical tank with liquid xenon in the center, surrounded by special gas layers and photosensors. — © Greg Stewart/SLAC National Accelerator Laboratory
Telltale flashes of light in the xenon tank
In such an event, the first flash of light occurs directly at the site of particle interaction. A second glow is generated by electrons that are knocked out of the xenon atom by particle interaction and accelerated in a shell layer of the tank. The ratio of these two light signals reveals whether they come from normal radioactive decay or from the collision of a dark matter particle with a xenon atom nucleus, as Gaitskell and his colleagues explain.
For their current analysis, the physicists evaluated data that the LZ detector recorded between March 2023 and April 2024. Using complex procedures, they filtered out disruptive effects and signals of “normal” interactions from the raw data. They specifically looked for signals that were in a higher energy range than previously considered. It included collision events that indicate an elastic recoil of the xenon nuclei with energies up to 270 kiloelectron volts.
Recoil signal with 248 kiloelectron volts
The physicists actually found what they were looking for: “One event stood out that indicated an atomic nucleus collision with a recoil of 248 kiloelectron volts,” they report. The LZ detector captured this signal on June 16, 2023. “We know our detector and the background effects so well that even a single standout event – like the one we found – is significant,” says lead author Sam Eriksen from the University of Bristol.
But does this signal really come from a WIMP – and therefore one of the long-sought dark matter particles? The statistical reliability is not yet sufficient for this, as Eriksen and his colleagues admit: They determined a significance of 2.6 standard deviations (sigma) for their signal – which corresponds to approximately a probability of 0.5 percent that this signal was generated by a background process. For an event at the exact origin location in the detector, the local significance was a maximum of 3.4 sigma.
“Plausible outlier”
“We do not claim that we have detected dark matter here,” emphasizes LZ spokesman Gaitskell. “We don’t want to jump to conclusions based on just one single event. But we have found something interesting here that we want to share with the scientific community – also to get their opinion.” However, if the signal were to be confirmed, then the triggering WIMP would have had a mass of at least 200 gigaelectron volts – around 200 times that of a proton.
The photomultiplier detectors of the LZ detector are extremely sensitive and can therefore capture the weak flashes of light that arise when potential dark matter particles interact with the xenon atomic nuclei. — © Matthew Kapust/Sanford Underground Research Facility
To determine whether there really is anything to this potential WIMP signal, the physicists will now examine other data packets from the LZ detector in a similar manner. If the signal is real, its statistical significance should continue to increase. If it is a mere measurement error or disturbing effect, it will disappear. “Outlier events in the data are not uncommon and, upon closer inspection, they usually turn out to be background signals,” explains Aaron Manalaysay from the Lawrence Berkeley Laboratory in California.
In this case, the physicist sees a good chance that the signal will persist: “Of all the experiments I have been involved in so far, this is the first case of an outlier that seems plausible in every respect,” says Manalaysay. “Of course, we continue to rack our brains as to whether we might have missed a rare background mechanism. But it could also be that we are on the trail of evidence of dark matter here.”
Source: Sam Eriksen (University of Bristol, UK) et al (LZ Collaboration; 2026 TeV Particle Astrophysics Conferencepreprint