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. South Dakota
Mines physicists played an integral role in LZ
by creating technology that reduced the amount of background
radiation that could skew the experiment’s results. They
are continuing to make important contributions by calibrating and analyzing the
experiment.
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,
LZ researchers report that with the initial run, LZ is already the world’s most
sensitive dark matter detector. 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!”
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 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 (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.
Reducing particles that could mimic a dark
matter signal is critical for the experiment’s success and Mines researchers
Richard Schnee and Juergen Reichenbacher have led a team, including graduate students, to
ensure the highly sensitive
LZ experiment is sufficiently free of
background radiation, such as radon, that could contaminate the
results, as well as controlling dirt and dust during the detector assembly that
could have spoiled the very expensive ultra-pure xenon.
“Radon’s radioactive grand-daughter, lead-214,
can produce a decay that looks like a dark matter signal,” says Richard Schnee,
Ph.D., head of the Department of Physics at Mines. “We worked hard to measure
how much radon emanates from the materials used to build LZ. Sometimes we told
collaborators that they had to find another material because the one they were
planning on had too much radon."
Mines researchers also continue to make
crucial contributions by calibrating the detector so that dark matter inside
the LZ chamber could be reliably identified. Mines researchers also make daily
corrections of the detector performance utilized by every data analyzer in the
LZ collaboration. In this photo, Mines physics
graduate student Madan Timalsina mounts a calibration source on top of the LZ
detector. “Without our calibrations of the LZ detector we would be
operating it nearly blind,” says Juergen Reichenbacher, Ph.D., associate
professor of physics at South Dakota Mines.
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 over 35 institutions from the US, UK, 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.
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 South Dakota Science and Technology
Authority, which manages SURF through a cooperative agreement with the US
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.
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 U.S. National Science Foundation; 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.
Mines researchers are also helping lead
the next generation dark matter detector being planned after LZ. Researchers from
around the world gathered in June for the first joint meeting of all major liquid xenon dark matter experiments at the Karlsruhe Institute
of Technology in Germany to continue the planning effort.