Dr Theresa Fruth, from the School of Physics, prepares to descend a mile underground at the LZ experiment facility in South Dakota, USA.
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. New results from the world's most sensitive dark matter detector, LUX-ZEPLIN (LZ), have narrowed down possibilities for one of the leading dark matter candidates: weakly interacting massive particles, or WIMPs.
LZ, led by the United States Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab), hunts for dark matter from a cavern nearly one mile underground at the Sanford Underground Research Facility in South Dakota. The experiment's new results have set further limits on what WIMPs could be.
Dr Theresa Fruth from the School of Physics at the University of Sydney was instrumental in commissioning the LZ detector in South Dakota and is an active participant in the hunt for dark matter at the LZ experiment. She has worked on the project for nine years, including during her time at the University of Oxford and University College London.
"This detector is the best asset we have anywhere in the world in our hunt for WIMP dark matter over coming years. This result shows how sensitive the detector is and how useful it will be in helping us to solve this most intriguing of scientific puzzles," she said.
LZ's central detector in a surface lab clean room before delivery underground. Photo: Matthew Kapust/Sanford Underground Research Facility
Dark matter, so named because it does not emit, reflect, or absorb light, is estimated to make up 85 percent of the mass in the universe but has never been directly detected, though it has left its fingerprints on multiple astronomical observations.
Dr Fruth said: "We wouldn't exist without this mysterious yet fundamental piece of the universe; dark matter's mass contributes to the gravitational attraction that helps galaxies form."
In the new result, the team found no evidence of WIMPs above 9 giga-electronvolts/c2 (GeV/c2), which is 1.6 x 10-26 kilograms, about ten times the mass of a proton.
"While finding 'nothing' doesn't sound like much of a result, this is hugely important in narrowing down where we could find direct evidence of dark matter," Dr Fruth said.
"Will dark matter fit snugly into the Standard Model of particle physics, or will its discovery need us to rewrite our theoretical models? We simply don't know yet."
The important new data has been presented today at physics conferences in Chicago, USA, and São Paulo, Brazil. A paper will be prepared for peer-review in coming weeks.
"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," said Professor Scott Kravitz, LZ's deputy physics coordinator and a professor at the University of Texas at Austin. "That's something you don't do with a million shovels, you do it by inventing a new tool."
Researchers sit between two outer layers of LZ during construction. Photo: Matthew Kapust/Sanford Underground Research Facility
Professor Chamkaur Ghag, LZ spokesperson and professor University College London said: "These are new world-leading constraints by a sizable margin on dark matter and WIMPs. 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 experiment's sensitivity to faint interactions helps researchers reject potential WIMP dark matter models that don't fit the data, leaving fewer places for WIMPs to hide.
This result is also the first time that LZ has applied "salting"- a technique that adds fake WIMP signals during data collection. By camouflaging the real data until "unsalting" at the very end, researchers can avoid unconscious bias and keep from overly interpreting or changing their analysis.
"We're pushing the boundary into a regime where people have not looked for dark matter before," said Scott Haselschwardt, the LZ physics coordinator and a recent Chamberlain Fellow at Berkeley Lab who is now an assistant professor at the University of Michigan. "There's a human tendency to want to see patterns in data, so it's really important when you enter this new regime that no bias wanders in. If you make a discovery, you want to get it right."
LZ uses 10 tonnes of liquid xenon at 175 Kelvin (minus 98.15 degrees) to provide a dense, transparent material for dark matter particles to potentially bump into. The hope is for a WIMP to knock into a xenon nucleus, causing it to move, much like a hit from a cue ball in a game of pool. By collecting the light emitted during such interactions by the detector's 494 light sensors, LZ could capture WIMP signals with other rare events.
LZ is a collaboration of about 250 scientists from 38 institutions in the United States, United Kingdom, Portugal, Switzerland, South Korea, and Australia.
Dr Fruth leads the only Australian-based research group working on LZ. She is also a collaborator at the Australian dark matter detector (SABRE South) being built in an active gold mine in Stawell, Victoria.
LZ Completes TPC Assembly from Sanford Lab on Vimeo.
Declaration
LZ is supported by the US Department of Energy, Office of Science, Office of High Energy Physics and the National Energy Research Scientific Computing Center, a DOE Office of Science user facility. LZ is also supported by the Science & Technology Facilities Council of the United Kingdom; the Portuguese Foundation for Science and Technology; the Swiss National Science Foundation, and the Institute for Basic Science, Korea. More than 38 institutions of higher education and advanced research provided support to LZ. The LZ collaboration acknowledges the assistance of the Sanford Underground Research Facility.
