One of the biggest mysteries in physics is understanding dark matter, an invisible substance that makes up most of the mass in our universe.
Scientists have been searching for it for decades, and a recent experiment has made significant progress in narrowing down the possibilities.
The LUX-ZEPLIN (LZ) detector, the world’s most sensitive dark matter detector, has set a new record in this quest.
LZ is an experiment led by the Department of Energy’s Lawrence Berkeley National Laboratory. It is located almost a mile underground at the Sanford Underground Research Facility in South Dakota.
This deep location helps shield the detector from cosmic rays, making it easier to detect dark matter particles, particularly a leading candidate called weakly interacting massive particles, or WIMPs.
The latest results from LZ are groundbreaking.
They explored weaker dark matter interactions than ever before, allowing scientists to rule out many potential WIMP models.
“These are new world-leading constraints on dark matter and WIMPs,” said Chamkaur Ghag, a professor at University College London (UCL) and spokesperson for LZ. He emphasized that the detector and the analysis techniques are performing even better than expected.
The team did not find evidence of WIMPs with a mass greater than 9 gigaelectronvolts/c² (GeV/c²), which is slightly more than the mass of a proton.
However, the experiment’s extreme sensitivity allowed researchers to eliminate many WIMP models that don’t match the data, significantly narrowing down the possibilities for what dark matter could be.
The new results were presented at two physics conferences on August 26, 2024, and a paper will be published soon.
These findings are based on 280 days of data collection, including a new set of 220 days gathered between March 2023 and April 2024, combined with data from LZ’s first run. The experiment aims to collect 1,000 days of data before it concludes in 2028.
Scott Kravitz, LZ’s deputy physics coordinator, explained the significance of the experiment’s achievements: “If you think of the search for dark matter like looking for buried treasure, we’ve dug almost five times deeper than anyone else has in the past.”
LZ’s incredible sensitivity comes from its ability to minimize background signals—false alarms that could be mistaken for dark matter.
The detector is built from thousands of ultraclean, low-radiation parts, and it is designed like an onion, with layers that block outside radiation or track particle interactions to rule out false signals.
Advanced analysis techniques further help distinguish real dark matter interactions from other noise, particularly from radon, a common contaminant.
One unique method used in the experiment is called “salting,” where fake WIMP signals are added to the data during collection. This helps prevent researchers from developing unconscious biases as they analyze the data, ensuring that any discovery is accurate and not influenced by wishful thinking.
Although LZ has yet to detect dark matter, it remains a powerful tool in the search. The experiment uses 10 tons of liquid xenon, a dense material that could be struck by a WIMP, causing detectable reactions.
The detector is also being used to explore other rare physics processes, like neutrinoless double beta decay and interactions involving neutrinos from the sun.
LZ is a collaboration of about 250 scientists from 38 institutions worldwide. They are already looking forward to analyzing more data and planning for future upgrades, as well as a next-generation detector called XLZD. “Our ability to search for dark matter is improving at a rate faster than Moore’s Law,” said Kravitz.
“Just wait until you see what comes next.”
