Most of the Universe is Hidden in the Dark; These UMass Amherst Physicists Have A Strong Chance of Finding It

One of the universe's most perplexing questions is: why can't we detect more of it? The universe is truly massive, and yet scientists are only able to account for about 15% of that mass, which means that the vast majority of everything that exists is made up of so-called "dark matter." The problem is that no one has ever observed dark matter; no one knows exactly what it is or how it works, but we do know that our models of the universe don't work without it.<_o3a_p>

Scott Hertel with an earlier version of the detector that is searching for dark matter.

"There are lots of problems in astrophysics which are solved by dark matter," says Scott Hertel, professor of physics at UMass Amherst. "There is more dark matter in the universe than normal matter. It tells us how galaxies form, how stars orbit and how things worked in the early universe, which was hot and dense. Dark matter is the main source of the gravity we see in astrophysics. We need dark matter. But no one has seen a dark matter particle. We have no idea what the particle is."<_o3a_p>

Or rather, Hertel, along with his graduate student Mark Murdy, don't know for sure which particle is responsible for dark matter, but they, along with more than 250 scientists from 39 institutions in the United States, UK, Portugal, Switzerland, South Korea and Australia, believe that weakly interacting massive particles - WIMPs - are the most likely candidates.<_o3a_p>

Hertel and Murdy are part of a team led by Lawrence Berkeley National Lab (Berkeley Lab) running LUX-ZEPLIN (LZ), the world's most sensitive dark matter detector, which is located nearly a mile beneath the Black Hills of South Dakota. And though they haven't yet determined whether or not dark matter is indeed a WIMP, LZ did recently complete its first 220-day run, which greatly narrowed down the search for WIMPs.<_o3a_p>

The experiment's new results explore weaker dark matter interactions than anyone else has ever been able to search for, and so they were able to further limit what WIMPs could be. The collaboration found no evidence of WIMPs between the masses of 9 gigaelectronvolts (GeV) and 10 teraelectronvolts (TeV). The new results were presented at two physics conferences on August 26 - TeV Particle Astrophysics 2024 in Chicago, Illinois, and LIDINE 2024 in São Paulo, Brazil - and a science paper will be published in the coming weeks.<_o3a_p>

The results analyze 220 days' worth of data collected over a year and improve the previous world's best published measurement of WIMP dark matter set by LZ in 2022. The experiment plans to collect 1,000 days' worth of data before it ends in 2028.<_o3a_p>

UMass Grad student Mark Murdy carefully prepares radioactive calibration sources for the LZ experiment.

"These are new world-leading constraints by a sizable margin on dark matter and WIMPs," said Chamkaur Ghag, spokesperson for LZ and a professor at University College London. He noted that the detector and analysis techniques are performing even better than the collaboration expected. "If WIMPs had been within the masses we searched, we'd have been able to robustly say something about them. 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."<_o3a_p>

LZ uses 10 tons of liquid xenon 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 and electrons emitted during interactions, LZ captures potential WIMP signals alongside other data.<_o3a_p>

"We've demonstrated how strong we are as a WIMP search machine, and we're going to keep running and getting even better - but there's lots of other things we can do with this detector," said Amy Cottle, lead on the WIMP search effort and an assistant professor at the University College London. "The next stage is using these data to look at other interesting and rare physics processes, like rare decays of xenon atoms, neutrinoless double beta decay, boron-8 neutrinos from the sun, and other beyond-the-Standard-Model physics. And this is in addition to probing some of the most interesting and previously inaccessible dark matter models from the last 20 years."<_o3a_p>

Building the research muscle<_o3a_p>

"I started working on the LZ experiment as an undergrad at Brandeis University," says Murdy. "When I first started doing experimental physics research, I wasn't sure it was for me because particle physics is so complex and I knew so little. But by my senior year, I felt grounded in the research and I knew this is what I wanted to do."<_o3a_p>

So Murdy came to work with Hertel, whose previous graduate student, Chris Nedlick, helped build the detector's calibration unit. The LZ detector is extraordinarily sensitive-Hertel says that "if one innocent object that has normal background radiation, like a banana, is around the detector, it could kill the experiment."<_o3a_p>

"I'm very lucky that I began graduate school right as the experiment was taking off," says Murdy. "The instrument had already been built, and I get to play with the data."

And data there is-many terabytes' worth, all of which will help Hertel and the rest of the LZ team continue to further calibrate the detector.

"It sees just about everything," says Murdy, "but if we're going to find a WIMP, we need to see even more. This work is really rewarding, because we're discovering something fundamental about the universe."<_o3a_p>

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