Date March 1, 2020
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Brown physicists at forefront of efforts to identify dark matter

Brown professor, students help construct 10-ton dark matter detector — the most sensitive ever built.

PROVIDENCE, R.I. [Brown University] — In the mid-1960s, an astronomer named Vera Rubin found something very strange about the way galaxies rotate. 

Stars near the outer edges of galaxies orbited the center much faster than expected — so fast, in fact, that they should just fly off into space. The finding suggested that gravity from some enormous yet unseen mass is holding these galaxies together. 

Rubin’s results were eventually confirmed, and the quest to understand the missing mass of the universe — dark matter — began in earnest. The quest goes on today, with Brown physicists playing key roles in solving what many see as the greatest mystery in modern physics.

Scientists think dark matter is the dominant form of mass in the universe, though no one is quite sure what the stuff actually is. The current consensus is that it’s some kind of particle, the leading candidate being the WIMP, or weakly interacting massive particle. WIMPs, according to theory, should have a mass of somewhere between 10 and 1,000 times the mass of a proton, as well as a snobbish tendency to avoid any interaction with normal matter (hence “weakly interacting”). And so they waft ghostlike through space, ubiquitous yet unseen.

“You and I and all the stuff we can touch and see, that’s just flotsam and jetsam in a vast sea of dark matter,” said Rick Gaitskell, a professor at Brown and a leading dark matter hunter. “Yet, because the particles are so weakly interacting, they pass right through us all the time and we never know they’re there.”

“ There is every prospect for the direct discovery of the nature of the dominant mass in the universe. ”

Rick Gaitskell Professor of Physics and Director of the Center for Fundamental Physics

Currently, the only way to study dark matter is through indirect observation deep in the cosmos. Though dark matter doesn’t emit or reflect light, its gravity can warp the fabric of space, causing the path taken by light to bend. A technique called gravitational lensing measures light bending as it travels, and Brown physicist Ian Dell’Antonio is part of a much-anticipated lensing experiment, the Large Synoptic Survey Telescope, which will measure the light of billions of galaxies. 

In the process, the telescope is expected to turn up lots of new dwarf galaxies, which are thought to be rich in dark matter. Telescope data can also be used to measure how “clumpy” dark matter is and  the extent to which dark matter may interact with itself, a key characteristic for understanding the nature of particles. 

JiJi Fan, an assistant professor of physics, is working with data collected by the European Space Agency’s Gaia Satellite, which marks the position of more than a billion stars in our galaxy with unprecedented precision. 

“Although the Gaia telescope doesn’t measure dark matter directly, the motions of visible stars are mostly determined by the invisible dark matter through gravity,” Fan said. “Thus, Gaia data does provide a highly powerful indirect probe to infer the distribution of dark matter.” She and her students used the data to put new constraints on the possibility of a “dark disk” — dark matter aligned with the visible galactic disk of the Milky Way.

While the hunt for WIMPs goes on, Savvas Koushiappas, associate professor of physics, has been studying a different dark matter candidate: primordial black holes. Collapsing stars can form black holes, but renowned physicist Stephen Hawking predicted another type of black hole formed before stars existed, during the first moments after the Big Bang. These primordial black holes might contribute to the matter density of the universe, yet so far there’s no experimental evidence that they exist. Koushiappas has calculated the earliest time stellar black holes could have formed — about 65 million years after the Big Bang. If gravitational wave experiments, which detect ripples in space-time resulting from black hole mergers, detect merger events before that cut-off time, it would be strong evidence that primordial black holes do exist and may constitute part of the dark matter.

While indirect observations are important for understanding the nature of dark matter, Gaitskell and other physicists are hard at work trying to directly detect it on Earth. Gaitskell built his first dark matter detector more than 30 years ago; it weighed about 10 grams and was about the size of a fingertip. “It was an entirely credible dark matter detector at the time,” Gaitskell said.

Since then, things have scaled up considerably. Today, Gaitskell and scientists from around the world are building a 10-ton detector at the Sanford Underground Research Facility in South Dakota. That massive detector, called LUX-ZEPLIN or LZ, consists of a tub of liquid xenon festooned with powerful light sensors designed to capture the tiny flashes of light produced on the rare occasions when a WIMP smacks into the nucleus of a xenon atom. 

To protect the detector from cosmic rays and other radiation that could drown out a WIMP signal, it’s being built a mile below ground in a goldmine turned science lab. When complete, it will be the most sensitive dark matter detector ever built. 

A close-up of dark matter detector parts
Students and faculty did assembly work on detector parts in a cleanroom to keep equipment as dust-free as possible.

LZ is the successor to LUX detector, an experiment Gaitskell co-led that previously held the “most sensitive” distinction, and lessons from LUX informed the new detector. Gaitskell and his students designed and built key parts of the new LZ detector — two large arrays of photomultiplier tubes — in cleanrooms at Brown. The arrays are light sensors powerful enough to detect just a handful of photons coming from the xenon tank. The devices will be first to see the tiny flashes of light associated with a dark matter interaction. 

The equipment was trucked from Providence to South Dakota in late 2018. Detector construction is now nearly complete, and soon it will switch on. 

“Our team is very excited to see the first results of the operation of this detector in 2020,” Gaitskell said. “It will be probing entirely new models of dark matter, and so there is every prospect for the direct discovery of the nature of the dominant mass in the universe.”

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