LZ experiment advances the search for dark matter

Figuring out the nature of dark matter — the invisible substance that makes up most of the mass in our universe — is one of the greatest unsolved puzzles in modern physics. Now the world’s most sensitive dark matter detector, LUX-ZEPLIN (LZ), has narrowed down the possibilities for one of the leading theoretical dark matter candidates: weakly interacting massive particles, or WIMPs.

So named because it does not emit, reflect or absorb light, dark matter has never been directly detected, though it has left its fingerprints on multiple astronomical observations; indeed, scientists suspect that it holds galaxies together and makes up about 85% of all mass in the universe. WIMPs also do not absorb, emit or reflect light, and they interact with normal matter only on rare occasions. This makes them very difficult to detect, even when millions may be travelling through the Earth and everything on it each second.

The LZ team — a collaboration of about 250 scientists from 38 institutions in the United States, United Kingdom, Portugal, Switzerland, South Korea and Australia — keep their dark matter detector in a cavern nearly one mile underground at the Sanford Underground Research Facility in South Dakota. The heart of the detector consists of two nested titanium tanks, filled with about 10 tonnes of pure liquid xenon at 175 Kelvin (-98.15°C) and monitored by photomultiplier tubes (PMTs) that will detect the dark matter particles if they’re there. The theory works like this: if WIMPs are present, they may occasionally collide with the nucleus of a xenon atom, causing a tiny flash of light and some movement in the atoms, which the PMTs will catch.

In 2022, the detector delivered its first results, proving itself to be world’s most sensitive detector of dark matter and placing what were until now the strictest limits on how strongly WIMPs should interact with ordinary matter. To get the latest result, the researchers combined 220 days of new data taken in 2023 and 2024 with 60 days from the experiment’s first run.

Having analysed all 280 days of data, the team has found no evidence for WIMP signals above a mass of 9 gigaelectronvolts/c² (GeV/c²), which is 1.6 x 10^-26 kilograms — a measurement roughly equivalent to nine times the weight of a proton. This suggests WIMPs are interacting with matter at weaker levels than previously thought.

These findings represent a significant advance, according to LZ experiment co-founder Professor Richard Gaitskell from Brown University, as they help the researchers to narrow their search. For instance, the experiment’s ability to detect extremely faint interactions allows researchers to rule out many possible dark matter models that predict interactions stronger than what the data shows, ultimately dwindling down the options for where WIMPs could be hiding.

“These are new world-leading constraints by a sizable margin on dark matter and WIMPs,” said LZ spokesperson Professor Chamkaur Ghag, from University College London. “We know we have the sensitivity and tools to see whether they’re there as we search lower energies and accrue the bulk of this experiment’s lifetime.”

The new data was presented this week at physics conferences in Chicago and São Paulo, with a paper set to be prepared for peer review in the coming weeks. LZ is still in its early phases, too; by 2028, the team plans to gather over 1000 days of data.

“This is a huge question we are trying to answer: ‘What is most of the matter in the universe made of?’” Gaitskell said. “It won’t get answered in a matter of weeks, months or even years necessarily. This is one that will take decades to answer, and that is actually pretty typical of most major scientific questions. If you go back and look at the history books, you’ll realise that major discoveries are separated by long periods of time and happened step by step.”

Image caption: The LZ central detector in the clean room at Sanford Lab before beginning its journey underground. Photo by Matthew Kapust, Sanford Underground Research Facility.