Dark matter hunters may have finally spotted a hint of the particles
The invisible substance pervades the cosmos but has eluded direct detection so far
The LZ detector (shown under construction) spotted a single event that could be a dark matter particle, but scientists need more data to check.
Matthew Kapust/Sanford Underground Research Laboratory
One day in June 2023, the numerous, unblinking electronic eyes of the LUX-ZEPLIN detector may have glimpsed something remarkable — possibly the first direct detection of the shadowy dark matter that pervades the cosmos. Or it may have been just another Friday.
A new analysis of data from LUX-ZEPLIN, or LZ, found a single particle interaction that is consistent with dark matter, researchers report September 1 at the TeV Particle Astrophysics meeting in Tendo, Japan, and in a paper posted on LZ’s website. But one event is not conclusive. Without more data, scientists can’t be sure what it is.
The lone unexpected event has physicists buzzing. “It’s the most interesting thing that’s come up in recent times,” says theoretical physicist Wick Haxton of the University of California, Berkeley, who was not involved in the research. “So I definitely think that there’ll be a flood of people looking at this event.”
But physicists are a cautious breed. “It’s only one event. So who knows what’s really going on here,” says theoretical physicist Dan Hooper of the University of Wisconsin–Madison. “That said, it’s intriguing.”
Dark matter is an invisible substance that’s never been directly observed, despite decades of searching. Its existence is gleaned based on its gravitational influence on the universe.
LZ, located at the Sanford Underground Research Facility in Lead, S.D., searches for a class of dark matter called weakly interacting massive particles, or WIMPs, crashing into atomic nuclei in a tank of liquid xenon. Like an atomic game of billiard balls, such a collision would send a xenon nucleus zinging away, producing a small flash of light and releasing electrons that LZ’s sensors can spot. The new analysis targeted a hypothesized version of dark matter that flings the nucleus at particularly high energy.
Dual signals isolate potential dark matter
The LZ detector is designed to detect two signals when a dark matter particle interacts with an atomic nucleus in the detector. As the nucleus recoils, it creates a flash of light, which is detected immediately by arrays of light sensors (circles at top and bottom). The event also releases electrons (red) which drift through the detector due to an electric field. When the electrons reach the top, they make another flash of light.
After combing through 220 days’ worth of data, “we got left with one event, which is fascinating — absolutely fascinating,” says LZ spokesperson Rick Gaitskell, a physicist at Brown University. Based on physicists’ best assessments, there’s only about a 1 in 100 chance for known particles to produce such an event, a statistical significance of 2.6 sigma. Three sigma is usually needed to claim evidence in physics, and five sigma is the bar for a detection.
A previous analysis had combed through the same data looking for atomic nuclei recoiling at low energies, which is the typical expectation for WIMP interactions. That effort came up empty. But dark matter particles might interact in a way that makes atomic nuclei recoil with particular oomph.
For example, dark matter particles could have different energy states, much like an atom does. The dark matter might interact only if there’s enough energy to bump it up to the next level, putting a threshold on the energy of the atomic nucleus it kicks away. For that type of interaction, dark matter events are expected to occur preferentially in the summer, when Earth is meeting the galaxy’s stream of dark matter head-on, producing higher-energy collisions. That makes an event in June particularly notable, says theoretical physicist Neal Weiner of NYU.
A dark matter hint stands alone?
LZ searches for two back-to-back light signals to hunt for dark matter. The relationship between the two signals helps scientists isolate dark matter particles from other interactions. Data points that fall in the arcing band are consistent with known sources such as radioactive decays. The search window for this analysis is indicated by the dashed box. The lone event in the top right is the potential dark matter particle. The bottom left point is consistent with a signal 1 accidentally matching up with an unrelated signal 2.
If the event is dark matter, it wouldn’t be the simplest possible version of it. But that doesn’t necessarily make it less tenable. “It’s not a crazy signal,” says theoretical physicist Matthew Reece of Harvard University. Theoretical physicists don’t have to invent brand-new ideas to explain it. “From that point of view, it’s very plausible … which makes me relatively optimistic.”
Dark matter experiments have a history of getting scientists’ hopes up and then dashing them. For example, in 2020 the XENON1T experiment saw a possible sign of unidentified new particles that was quashed by its successor.
Dark matter or no, the analysis proves the potential of LZ and other dark matter experiments. “We have these incredible machines that these teams have put together,” Weiner says. “And the question is, can they use them to look for these broader categories of scenarios? And one really exciting top-line result is, yes, they can.”
Meanwhile, LZ has already collected additional data — at least three times as much as what the researchers have already analyzed. If dark matter really is behind this event, there are likely others waiting to be found.
“People have looked for [dark matter] for such a long time, and with growing frustration that it hasn’t been found,” Haxton says. “It would be very nice if this event is followed by a few more.”
