With major contributions from a Brown team, massive dark matter detector is up and running

The long search for dark matter, estimated to comprise 85% of all mass in the universe, took a major step forward with the underground LUX-ZEPLIN experiment in South Dakota now delivering initial results.

PROVIDENCE, R.I., AND LEAD, S.D. — Deep below the Black Hills of South Dakota in the Sanford Underground Research Facility, an innovative and uniquely sensitive dark matter detector — the LUX-ZEPLIN experiment, led by Lawrence Berkeley National Lab — has passed a check-out phase of startup operations and delivered its first results.

In a study published on Thursday, July 7, the researchers behind the experiment report that with its initial run, LZ is already the world’s most sensitive detector of dark matter — the elusive particles thought to account for a majority of matter in the universe.

The take-home message from this successful start: “We’re ready and everything’s looking good,” said Kevin Lesko, a senior physicist at the Berkeley Lab. “It’s a complex detector with many parts to it, and they are all functioning well within expectations.”

The design, manufacturing and installation phases for the massive project are the product of an international collaboration of more than 250 scientists and engineers from 40 institutions — and a far-reaching group of faculty members, postdoctoral scholars, technicians, and graduate and undergraduate students from Brown University has played a major role in making the project happen. Together, the team is hoping to use the instrument to record the first direct evidence of dark matter, the so-called missing mass of the cosmos.

“This has been a monumental effort and many Brown doctoral, masters and undergraduate students, as well as postdocs and technicians, contributed,” said LZ experiment cofounder Richard Gaitskell, a Brown professor of physics and director of the Center for the Fundamental Physics of the Universe. “It’s amazing how much work we did in the labs and cleanrooms in Barus and Holley. This included building, testing and integrating more than 14,000 components for the two massive photodetector arrays for this unprecedented experiment.”

Brown’s role in making the massive detector a reality

Dark matter particles have never actually been detected. The leading theoretical candidate for a dark matter particle is the WIMP, or weakly interacting massive particle. WIMPs can’t be seen because they don’t absorb, emit, reflect or scatter light, but the particles are nonetheless fundamental to understanding the universe. For example, the presence of dark matter shapes the form and movement of galaxies, and is invoked by researchers to explain what is known about the large-scale structure and expansion of the universe.

The search for the elusive particles has a long history, with humans having speculated about invisible forms of matter as far back as Greek atomists in the fifth century BCE. To date, however, all attempts to observe it have failed, and the LZ collaboration is the most ambitious attempt to observe dark matter yet.

“So far, we have not seen a signal consistent with dark matter,” Gaitskell said. “But we have only run the experiment for 100 days. Already the results are more sensitive than the world’s best results by a significant factor. With the LZ experiment going on to run for 1,000 days, the next results will explore many new models for particle dark matter. We believe we are in a strong position to discover the universe’s missing mass.”

This 2018 video describes the role of the Brown team in building two arrays of photomultiplier tubes installed in the LZ detector.

If the LZ experiment detects dark matter, it will come in the form of a brief flash of light. The heart of the detector is comprised of two nested titanium tanks filled with nearly 10 tons of pure liquid xenon and viewed by two arrays of photomultiplier tubes (PMTs) that are able to detect faint sources of light.

The Brown research group’s work to build the detector’s PMT arrays was central to the experiment’s successful launch. The arrays serve as the “eyes” of the experiment and will do the actual detection of dark matter by looking for the faint flashes of light that would be produced if a dark matter particle collides with a xenon atom inside one of LZ’s tanks.

The Brown team worked with Berkeley Lab and Imperial College London researchers to design, test and assemble all of the array’s components. The PMTs, which the Hamamatsu Corporation manufactures, underwent two years of testing at Brown. Then the team assembled them into the final arrays used in LZ.

Will Taylor, who earned his Ph.D. in physics from Brown in May, worked on building and testing the PMT arrays.

“LZ has the potential to be the most sensitive WIMP search experiment in the world,” he said. “The fact that the PMT arrays, some of the most critical systems in the detector, were built at Brown by our group makes it all the more exciting.”

Because they are looking for extremely weak interactions, the “eyes” of the experiment needed to be sensitive enough to detect single photons and have to be calibrated precisely. The Brown team worked extensively with Adelphi Technology, Inc., to ensure the neutron generator would deliver the performance needed in LZ.

“Because the dark matter search is a top priority, it was important to perform calibrations quickly,” Taylor said. “The neutron calibrations need to acquire many thousands of neutron events to achieve statistical significance. To do that within the timespan allotted for calibrations, we worked with Adelphi to develop impressive neutron production intensities and pulse characteristics.”

Brown Ph.D. student Jihyeun Bang said the team “shot a bunch of neutrons into the detectors to observe how they respond, because neutrons have a weak interaction with xenon similar to what we expect with dark matter. We observed that tens of thousands of neutron events are distributed within a narrow band in the signal space, the nuclear recoil band.”

Ultimately, those calibrations measured the signal that dark matter is predicted to generate when it interacts with xenon.

An underground detector

Tucked away about a mile underground at SURF in Lead, South Dakota, LZ is housed underground to protect it from cosmic radiation at the surface that could drown out dark matter signals. As you approach it, SURF — site of the former Homestake gold mine and home to the precursor LUX experiment until 2016 — looks unimposing, Taylor said. But once inside, there is something awe-inspiring about traveling over a mile underground.

“Going underground can feel a bit surreal,” Taylor said. “The elevator — or cage, as it’s called — takes about 10 minutes to reach the 4,850-foot [underground] level.”

Brown physics Ph.D. student Austin Vaitkus described the trip down the shaft as an incredible ordeal:

“You have to garb up in dirty coveralls and boots,” Vaitkus  said. “You put on safety glasses, a helmet and a headlamp in case you end up trapped somewhere without light. You have to wrap all of your stuff in plastic bags so that it doesn’t get dirty on your way into the lab.”

Members of the team said that once inside, the lab feels like a normal workspace. There’s even wi-fi and a well-stocked espresso machine.

“Once you’ve gone through the changing rooms and gotten out of your coveralls, it’s easy to forget you’re underground,” Taylor said. “One could imagine the lab was in a large building if it weren’t for the exposed rock and lack of windows serving as reminders.”

But it’s difficult to entirely forget where you are: “Frankly, most of the time, you don’t think too much about the fact that you have a mile of rock over your head, but sometimes you do ponder the incredible scale of it all,” Gaitskell said. “And you trust the expertise of the engineers that have been able to put us there.”

In addition to Gaitskell, Bang, Taylor and Vaitkus, Eamon Hartigan-O’Connor, Dongqing Huang, Casey Rhyne and Xin Xiang from Brown worked many shifts on the experiment site. The group was aware of the potential historic significance of their work, Taylor said.

“It’s incredibly exciting to finally see LZ’s first WIMP-search results,” he said. “It’s quite thrilling to be able to say that I helped build part of this massive project with my own hands and to see the results of those many years of labor finally come to fruition.”

Gaitskell said he is deeply grateful to the many Brown community members who participated in the experiment.

“Their work assembling and testing PMTs, constructing the huge arrays, providing nuclear recoil calibrations and analyzing a huge range of data from the detector have been critical to the experiment’s successful launch,” he said. “Now that we have confirmed LZ is operational and full-scale observations have begun, we hope to start observing dark matter particles colliding with xenon atoms in the detector.

“It’s an exciting time,” he added. “We’re ready to make history.”

LZ is supported by the U.S. Department of Energy, Office of Science, Office of High Energy Physics and the National Energy Research Scientific Computing Center, an Office of Science user facility. It is also supported by the Science and Technology Facilities Council of the United Kingdom; the Portuguese Foundation for Science and Technology; and the Institute for Basic Science, Korea. More than 40 institutions of higher education and advanced research provided support to the LZ collaboration, which also acknowledges the assistance of the Sanford Underground Research Facility.

This feature story was adapted from a news release published by Brown’s Department of Physics.