– By William Schultz
Deep beneath the Black Hills of South Dakota at the Sanford Underground Research Facility (SURF), an innovative and uniquely sensitive dark matter detector – the LUX-ZEPLIN (LZ) experiment led by Lawrence Berkeley National Laboratory (Berkeley Lab) – has undergone a verification phase of start-up operations and delivered first results.
The take-home message from this successful startup: “We’re ready and everything looks good,” said Berkeley Lab senior physicist and former LZ spokesperson Kevin Lesko. “It’s a complex detector with many parts, and they all function well within expectations,” he said.
In a paper published online today on the experiment’s website, LZ researchers report that with the initial run, LZ is now the world’s most sensitive dark matter detector. The paper will appear in the online preprint archive arXiv.org later today. LZ spokesman 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 just getting started. There’s a lot of science to be done and it’s very exciting!”
Dark matter particles have never actually been detected – but maybe not for long. The countdown may have begun with the results of the first 60 days of LZ testing. This data was collected over a period of three and a half months of initial operations beginning in late December. This was a long enough period to confirm that all aspects of the detector were functioning well.
Invisible because it does not emit, absorb or scatter light, dark matter’s presence and gravitational attraction are fundamental to our understanding of the universe. For example, the presence of dark matter, which is estimated to make up about 85 percent of the total mass of the universe, shapes the shape and motion of galaxies and is used 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 consists of two nested titanium tanks filled with ten tons of very pure liquid xenon and monitored by two arrays of photomultiplier tubes (PMTs) capable of detecting faint light sources. The titanium tanks sit inside a larger detector system to catch particles that can mimic a dark matter signal.
“I’m excited to see this sophisticated detector ready to tackle the long-standing problem of what dark matter is made of,” said Berkeley Lab Physics Department Director Nathalie Palanque-Delabrue. “The LZ team now has the most ambitious tool for this!”
The design, manufacturing and installation phases of the LZ detector were led by Berkeley Lab project director Gil Gilchriese along with an international team of 250 scientists and engineers from over 35 institutions from the US, UK, Portugal and South Korea. LZ’s operations manager is Simon Fiorucci of Berkeley Lab. Together, the collaboration hopes to use the instrument to record the first direct evidence of dark matter, the so-called missing mass of the cosmos.
Enrique Araujo of Imperial College London led the UK groups and previously the final phase of the UK-based ZEPLIN-III programme. He works very closely with the Berkeley team and other colleagues to integrate international input. “We started with two groups with different views and ended up with a well-tuned orchestra working seamlessly together to deliver a great experiment,” Araujo said.
Underground detector
Hidden about a mile underground at SURF in Lead, SD, the LZ is designed to capture dark matter in the form of Weakly Interacting Massive Particles (WIMPs). The experiment is underground to protect it from cosmic radiation at the surface, which can drown out signals from dark matter.
Collisions of particles in the xenon cause visible scintillations, or flashes of light, that are recorded by the PMT, explained Berkeley Lab’s Aaron Manalaisey, who, as physics coordinator, led the collaboration’s efforts to produce these first physics results. “The collaboration works well together to calibrate and understand the response of the detector,” Manalaisey said. “Given that we only turned it on a few months ago and during the COVID restrictions, it’s impressive that we already have such significant results.”
The collisions will also knock electrons off the xenon atoms, sending them flying toward the top of the chamber under an applied electric field, where they produce another flash, allowing the reconstruction of a spatial event. The scintillation characteristics help determine the types of particles interacting in the xenon.
The South Dakota Science and Technology Authority, which operates SURF through a cooperative agreement with the US Department of Energy, provided 80 percent of the xenon in the 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 Hadley, CEO of SURF Lab, said: “The entire SURF team congratulates the LZ Collaboration on reaching this important milestone. The LZ team has been a wonderful partner and we are proud to welcome them to SURF.”
Fiorucci said the team on the ground deserves special praise for this launch milestone, given that the detector was transported underground in late 2019, just before the start of the COVID-19 pandemic. He said that because travel is severely restricted, only a few LZ scientists can make the trip to help on the ground. The team in South Dakota took excellent care of the LZ.
“I would like to commend the SURF team and also express my gratitude to the large number of people who provided remote support during the construction, commissioning and operation of the LZ, many of whom worked full-time from home institutions that ensured that the experiment would be successful, and they continue to do so now,” said SLAC’s Tomasz Bisiadzinski, head of operations for the LZ detector.
“Many subsystems began to come together as we began collecting data for detector commissioning, calibration and science work. Getting a new experiment on board is challenging, but we have a great team at LZ working closely together to guide us through the early stages of understanding our detector,” said Penn State University’s David Woodward, who is coordinating detector launch planning.
SLAC’s Maria Elena Monzani, deputy operations manager for computing and software, said: “We had amazing scientists and software developers throughout the collaboration who tirelessly supported data movement, data processing and simulations, enabling a flawless commissioning of the detector. The support of NERSC [National Energy Research Scientific Computing Center] was invaluable.”
With confirmation that the LZ and its systems are working successfully, Lesko said it’s time to begin full-scale observations with the hope that a dark matter particle will collide with a xenon atom in the LZ detector very soon.
LZ is supported by the US Department of Energy, Office of Science, Office of High Energy Physics, and the National Science Computing Center for Energy Research, a user facility of the DOE Office of Science. LZ is also supported by the UK Science and Technology Facilities Council; the Portuguese Foundation for Science and Technology; and the Institute of Basic Sciences, Korea. Over 35 institutions of higher education and advanced research have provided support to LZ. The LZ collaboration acknowledges the assistance of the Sanford Underground Research Facility.
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