University of Michigan: Successful startup of particle detector aims to pin down dark matter

A new particle detector searching for dark matter has closed in on the region where physicists may detect dark matter—an elusive particle that creates 85% of the mass in the universe—all in its first four months of operation.

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 (LZ) Dark Matter Experiment, led by Lawrence Berkeley National Lab and supported by University of Michigan researchers—has passed a check-out phase of startup operations and delivered first results.

The experiment is performing well and in a study posted online on the LZ website, researchers report that with the initial run, LZ is already the world’s most sensitive dark matter detector. The study will appear on the online preprint archive arXiv.org. This first result covers only a small fraction of the total data to be taken with many leading dark matter and other science results to come.

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 shapes the form and movement of galaxies, and it is invoked by researchers to explain what is known about the structure and expansion of the universe.

“We know that dark matter exists. We see its imprint in the universe; we see how it tracks around stars and galaxies. We see it in the primordial radiation of the Big Bang,” said U-M physicist Bjoern Penning, whose lab has contributed significantly to setting up the detector. “We have the world’s most powerful result already, after just four months, and we’re going to run this detector for another five years. It’s very, very powerful and this bodes well for our ability to detect 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.

“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.”

The heart of the LZ dark matter detector is comprised of two nested titanium tanks filled with ten tons of very pure liquid xenon and viewed by two arrays of photomultiplier tubes 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.

“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,” said LZ spokesperson Hugh Lippincott of the University of California Santa Barbara.

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.

An underground detector
Tucked away about a mile underground at Sanford Underground Research Facility, or SURF, in Lead, South Dakota, LZ is designed to capture dark matter in the form of weakly interacting massive particles, called WIMPS. The experiment is underground to protect it from cosmic radiation at the surface that could drown out dark matter signals.

Particle collisions in the xenon produce visible scintillation or flashes of light, which are recorded by the photomultiplier tubes, said Aaron Manalaysay of 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,” he 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.

In order to get a clear picture of these interacting particles, the interior of the detector has to be ultra clean, as free as possible from other particles and radiation, says Penning. Led by U-M physicist Wolfgang Lorenzon and Penning, U-M graduate students and postdoctoral researchers were instrumental in mitigating backgrounds.

Penning built a detector to mitigate neutrons, the main dark matter background, while U-M students and postdocs, led by Lorenzon, developed a system to remove radioactive particles from the detector. In addition, the U-M team was heavily involved in assembling the detector, which took place to a large extent during the COVID-19 pandemic.

“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,” said Mike Headley, executive director of SURF Lab.

Fiorucci said the onsite team in South Dakota took excellent care of LZ and 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.

“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.

With confirmation that LZ and its systems are operating successfully, the researchers say it is time for full-scale observations to begin in hopes that a dark matter particle will collide with a xenon atom in the LZ detector very soon.

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

LZ is supported by the U.S. Department of Energy’s 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. More than 35 institutions of higher education and advanced research provided support to LZ. The LZ collaboration acknowledges the assistance of the Sanford Underground Research Facility.