World's Most Sensitive Dark Matter Detector Is Closing In On What It Can – And Cannot – Be

Deep in a mine in South Dakota is the LUX-ZEPLIN dark matter detector, a pair of nested titanium tanks filled with 10 tons of transparent, pure liquid xenon nestled far underground to shield it from cosmic particles that may drown out any faint signals. If dark matter is made of Weakly Interacting Massive Particles (WIMPs) as suspected, they should occasionally slam into xenon atoms, producing a flash of light.

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The LZ detector will run for at least 1,000 days through 2028, and scientists have now released the data from the first 220 days. No dark matter particle has been discovered, but scientists can now rule out several possibilities, narrowing down the hunt and getting closer to defining what dark matter can or cannot be.

"While we always hope to discover a new particle, it is important for particle physics that we are able to set bounds on what the dark matter might actually be," explained UC Santa Barbara experimental physicist Hugh Lippincott, who is a member of the LZ collaboration, in a statement.

WIMPS are one of the first and long-standing proposed dark matter candidates. A hypothetical type of particle, it's thought that they are massive and only interact very weakly with normal matter. The idea is that the WIMPs must be passing through Earth all the time as it moves through space, so to detect them, we simply need detectors sensitive enough to capture those weak interactions, like the LUX-ZEPLIN.

“The tricky thing about neutrons is that they also interact with the xenon nuclei, giving off a signal identical to what we expect from WIMPs,” Makayla Trask said. “The OD [Outer Detector] is excellent at detecting neutrons and confirms a WIMP detection by not having any response.” 

Being deep underground is key to protecting the experiment from false events. But the detector itself can release false signals, such as neutrons interacting with the xenon, as well as radioactive decay of the xenon itself. A similar experiment saw the “rarest event ever”, a decay of xenon with an incredibly long lifetime.

"Our experiment is also sensitive to rare events with roots in diverse areas of physics," post-doc researcher Chame Amarasinghe said. "Some examples are solar neutrinos, the fascinating decays of certain xenon isotopes, and even other types of dark matter. With the intensity of this result behind us, I'm very excited to spend more time on these searches."

The team will continue to analyze the data as well as improve the data analysis. At the same time, researchers are considering the next generation of these dark matter detectors, which will be needed whether dark matter is found by LZ or not. One of the possible locations for XLZD, the future upgrade, is the Boulby Dark Matter lab in the UK, which IFLScience visited last year.

The study is published in Physical Review Letters.