The long search for dark matter, estimated to comprise 85 percent of all mass in the universe, took a major step forward now that the LUX-ZEPLIN experiment, an ambitious international collaboration, is fully operational.

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

The Brown Research Group

Brown University’s LZ research group made many major contributions to the project. Brown University Hazard Professor of Physics and Director of the Center for the Fundamental Physics of the Universe Richard Gaitskell, who leads the Brown group, said, “This has been a monumental effort and many Brown doctoral, master’s and undergraduate students, as well as post-docs and technicians, contributed. It’s amazing how much work we did in the labs and cleanrooms in the basement of Barus and Holley. This included the building, testing and integration of over 14,000 components for the two massive photodetector arrays for this unprecedented experiment.” 

Brown doctoral and master’s students on the team included Amjad Alqahtani, Jihyeun (Jeanne) Bang, Samuel Chan, Chen Ding, Dongqing Huang, Renée Kirk, Runxuan Liu, Jan Makkinje, David Malling, Casey Rhyne, Nat Swanson, William Taylor, Austin Vaitkus and James Verbus. 

Undergraduates who contributed included Sissi Chen, Eamon Hartigan-O’Connor, David Heffren, Yizhong (Richard) Hu, Charles Kocher, Jacob Migneault, Napali Raymundo, Grant Rutherford, Angela White and Anna Zuckerman.

Three Brown post-doctoral research associates–Junhui Liao, Monica Pangilinan, and Xin Xiang–worked on the experiment. Dario Garcia, Jake Lyle and Devon Seymour made huge contributions as technicians.

The heart of the LZ dark matter detector is comprised of two nested titanium tanks filled with nearly ten tonnes of very pure liquid xenon and viewed by two arrays of photomultiplier tubes (PMTs) able to detect faint sources of light. The titanium tanks reside in a larger detector system to catch particles that might mimic a dark matter signal.

Looking up into the LZ Outer Detector, used to veto radioactivity that can mimic a dark matter signal. The conduit on the right, showing a hint of blue, is a DD neutron conduit, which the group uses to send neutrons into the detector to calibrate the expected dark matter signal. The neutrons are produced using an Adelphi Technologies, Inc. deuterium-deuterium (DD) fusion neutron generator. (Credit: Matthew Kapust, Sanford Underground Research Facility.)

The Brown research group’s work building 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.

(Left) A schematic of the LZ detector. (Right) Illustration of LZ operation - particles interact in liquid xenon, releasing a flash of light and charge that are collected by photomultiplier tube arrays at top and bottom. (Credit: The schematic is the LZ collaboration, and the image on the right is LZ/SLAC)

The Brown team worked with Berkeley Lab and Imperial College London researchers to design, test, and assemble all of the array’s components. Testing of the PMTs, which the Hamamatsu Corporation manufactures in Japan, was performed at Brown.

After testing the PMTs for more than two years, the team assembled them into the final arrays used in LZ. Taylor, who worked on building and testing the PMT arrays, said, “LZ has the potential to be the most sensitive WIMP [weakly interacting massive particles] search experiment in the world. 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 it is looking for extremely weak interactions, the “eyes” of the experiment need to be sensitive enough to detect single photons and have to be calibrated precisely. The Brown group also contributed nuclear recoil calibrations using an Adelphi Technology, Inc. deuterium-deuterium (DD) fusion neutron generator. Bang said, “The Brown group is in charge of every aspect of the DD neutron generator from operation to analysis.”

To prepare for the nuclear recoil calibrations, the Brown group worked extensively with Adelphi Technology, Inc. to ensure the neutron generator would deliver the performance needed in LZ. Taylor said, “Because the dark matter search is a top priority, it was important to perform calibrations quickly.” He continued, “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. This custom R&D was possible thanks to our longstanding relationship with Adelphi.”

“We 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,” Bang said. “We observed that tens of thousands of neutron events are distributed within a narrow band in the signal space, the Nuclear Recoil Band.” These calibrations measured the signal that dark matter is predicted to generate when it interacts with xenon.

In addition, the Brown team provided a portion of the data acquisition (DAQ) system and computational power needed to analyze the data using Brown University’s computing cluster run by Brown’s Center for Computation and Visualization (CCV). Gaitskell noted that the Brown team’s work followed on from the precursor Large Underground Xenon (LUX) experiment. He added, “Brown also contributed an enormous amount of horsepower for the data analysis and simulations of that precursor experiment.”

Successful Startup

The take-home message from this successful startup: “We’re ready and everything’s looking good,” said Berkeley Lab Senior Physicist and past LZ Spokesperson Kevin Lesko. “It’s a complex detector with many parts to it and they are all functioning well within expectations,” he said. 

In a paper posted online today on the experiment’s website, LZ researchers report that with the initial run, LZ is already the world’s most sensitive dark matter detector. The paper will appear on the online preprint archive later today. LZ Spokesperson Hugh Lippincott of the University of California Santa Barbara said, “We plan to collect about 20 times more data in the coming years, so we’re only getting started. There’s a lot of science to do and it’s very exciting!”

The design, manufacturing, and installation phases of the LZ detector were led by Berkeley Lab project director Gil Gilchriese in conjunction with an international team of 250 scientists and engineers from more than 35 institutions from the U.S., U.K., Portugal, and South Korea. The LZ Operations Manager is Berkeley Lab’s Simon Fiorucci. Together, the collaboration is hoping to use the instrument to record the first direct evidence of dark matter, the so-called missing mass of the cosmos. 

Members of the LZ team in the LZ water tank after the outer detector installation. (Credit: Matthew Kapust, Sanford Underground Research Facility.)

Dark Matter

Dark Matter particles have never actually been detected—but perhaps not for much longer. The countdown may have started with results from LZ’s first 60 “live days” of testing. These data were collected over a three-and-a-half-month span of initial operations beginning at the end of December. This was a period long enough to confirm that all aspects of the detector were functioning well. 

Unseen, because it does not emit, absorb, or scatter light, dark matter’s presence and gravitational pull are nonetheless fundamental to our understanding of the universe. For example, the presence of dark matter, estimated to be about 85 percent of the total mass of the universe, shapes the form and movement of galaxies, and it is invoked by researchers to explain what is known about the large-scale structure and expansion of the universe. 

The search for dark matter has a long history.

Humans have speculated about the possible existence of invisible forms of matter as far back as the Greek atomists in the fifth century BCE. In 1884 Lord Kelvin estimated that the mass of the Milky Way had to be greater than the total mass of visible stars based on the observed velocity dispersion of stars in the galaxy. But it is Swiss astrophysicist Fritz Zwicky who is generally credited with calculating that dark matter constituted the bulk of the universe’s mass in 1933. 

But to date, all attempts to observe this elusive hypothetical form of matter have failed. 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,” said Gaitskell. “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.” He added, “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.”

If the LZ experiment detects dark matter, it will come in the form of a brief flash of light. Particle collisions in the xenon produce visible scintillation or flashes of light, which are recorded by the PMTs, explained Aaron Manalaysay from Berkeley Lab who, as Physics Coordinator, led the collaboration’s efforts to produce these first physics results. “The collaboration worked well together to calibrate and to understand the detector response,” Manalaysay said. “Considering we just turned it on a few months ago and during COVID restrictions, it is impressive we have such significant results already.” 

The collisions will also knock electrons off xenon atoms, sending them to drift to the top of the chamber under an applied electric field where they produce another flash permitting spatial event reconstruction. The characteristics of the scintillation help determine the types of particles interacting in the xenon.

The LZ central detector in the clean room at Sanford Lab before beginning its journey underground. (Credit: Matthew Kapust, Sanford Underground Research Facility)

An underground detector

Tucked away about a mile underground at SURF in Lead, S.D., LZ is designed to capture dark matter in the form of WIMPs. The experiment is underground to protect it from cosmic radiation at the surface that could drown out dark matter signals.  

As you approach it, SURF, the site of the former Homestake gold mine and home to the LUX experiment until 2016, looks unimposing. But once inside, there is something awe-inspiring about traveling over a mile underground. “Going underground can feel a bit surreal,” said Taylor, “The elevator–or cage, as it’s called–takes about 10 minutes to reach the 4850 level.” 

Vaitkus described the trip down the shaft as an incredible ordeal: “You have to garb up in dirty coveralls and boots. 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. You carry a brass tag with your name on it so they can identify your body in case something horrible happens.”

But 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.” Bang added, “It’s not that different from the basement of Barus and Holley because we don’t have any windows there either.”

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,” said Gaitskell. “And you trust the expertise of the engineers that have been able to put us there.”

In addition to Gaitskell, Bang, Taylor and Vaitkus, Hartigan-O’Connor, Huang, Rhyne and Xiang worked many long shifts on the experiment site. 
The group found the stays incredibly gratifying and were aware of the potential historical significance of their work. Taylor said, “It’s incredibly exciting to finally see LZ’s first WIMP-search results. 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.”

The South Dakota Science and Technology Authority, which manages SURF through a cooperative agreement with the U.S. Department of Energy, secured 80 percent of the xenon in LZ. Funding came from the South Dakota Governor’s office, the South Dakota Community Foundation, the South Dakota State University Foundation, and the University of South Dakota Foundation. 

Mike Headley, executive director of SURF Lab, said, “The entire SURF team congratulates the LZ Collaboration in reaching this major milestone. The LZ team has been a wonderful partner and we’re proud to host them at SURF.” 

Fiorucci said the onsite team deserves special praise at this startup milestone, given that the detector was transported underground late in 2019, just before the onset of the COVID-19 pandemic. He said with travel severely restricted, only a few LZ scientists could make the trip to help on-site. The team in South Dakota took excellent care of LZ. 

“I’d like to second the praise for the team at SURF and would also like to express gratitude to the large number of people who provided remote support throughout the construction, commissioning and operations of LZ, many of whom worked full time from their home institutions making sure the experiment would be a success and continue to do so now,” said Tomasz Biesiadzinski of SLAC, the LZ Detector Operations Manager. 
“I’m deeply grateful to the many doctoral, master’s and undergraduate students and other Brown personnel who have participated in this experiment so far,” said Gaitskell. “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 added, “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. 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, 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; and the Institute for Basic Science, Korea. Over 35 institutions of higher education and advanced research provided support to LZ. The LZ collaboration acknowledges the assistance of the Sanford Underground Research Facility.