Members of the LZ team in the LZ water tank after the installation of the outer detector. (Credit: Matthew Kapust/Sanford Underground Research Facility)
– By William Schulz
Deep beneath 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) – passed a check-out phase of startup operations and delivered the first results.
The 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 work well within expectations,” he said.
In a newspaper posted online today about the experiments of website, LZ researchers report that LZ is already the world’s most sensitive dark matter detector on its first run. The paper will appear in the online preprint archive arXiv.org 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 next few years, so we’re just getting started. There’s a lot of science to do and it’s very exciting! ”
Dark matter particles have never really been detected — but maybe not for much longer. The countdown may have started with the results of LZ’s first 60 “live days” testing. This data was collected over a three-and-a-half-month period of initial operations that began in late December. This was a period long enough to confirm that all aspects of the detector were functioning properly.
Looking up into the LZ Outer Detector, used to veto radioactivity that can mimic a dark matter signal. (Credit: Matthew Kapust/Sanford Underground Research Facility)
Unseen, because it does not emit, absorb or scatter light, the presence and attraction of dark matter are nevertheless fundamental to our understanding of the universe. For example, the presence of dark matter, estimated to be about 85 percent of the universe’s total mass, determines the shape and motion of galaxies, and is invoked by researchers to explain what is known about the universe’s large-scale structure and expansion. .
The heart of the LZ dark matter detector consists of two nested titanium tanks filled with ten tons of high-purity liquid xenon and viewed through two arrays of photomultiplier tubes (PMTs) that can detect weak light sources. The titanium tanks are contained within a larger detector system to capture particles that can mimic a dark matter signal.
“I’m thrilled that this complex detector is poised to tackle the long-standing problem that makes up dark matter,” said Nathalie Palanque-Delabrouille, director of the Berkeley Lab’s Department of Physics. “The LZ team now has the most ambitious tool for this!”
The design, fabrication and installation phases of the LZ detector were led by Gil Gilchriese, Berkeley Lab Project Director in collaboration with an international team of 250 scientists and engineers from more than 35 institutions from the US, UK, Portugal and South America. Korea. LZ’s operations manager is Simon Fiorucci of Berkeley Lab. Together, the collaboration hopes to use the instrument to capture the first direct evidence of dark matter, the so-called missing mass of the cosmos.
Henrique Araújo, from Imperial College London, leads the UK groups and was previously the final phase of the UK-based ZEPLIN-III programme. He worked very closely with the Berkeley team and other colleagues to integrate the international contributions. “We started with two groups with different visions and ended up with a well-tuned orchestra that worked together seamlessly to deliver a great experiment,” said Araújo.
An underground detector
Tucked away about a mile underground at SURF in Lead, SD, LZ is designed to capture dark matter in the form of weakly interacting massive particles (WIMPs). The experiment is underground to protect it from surface cosmic rays that could drown out signals from dark matter.
(Left) A schematic of the LZ detector. (Right) Illustration of LZ action – particles interact in liquid xenon, releasing a flash of light and charge that are collected by top and bottom photomultiplier tube arrays. (Credit: Schematic left: LZ collaboration. Image right: LZ/SLAC)
Particle collisions in the xenon produce visible scintillation, or flashes of light, which are registered by the PMTs, explained Aaron Manalaysay of Berkeley Lab, who as a physics coordinator led the collaboration’s efforts to produce these first physics results. “The collaboration worked well together to calibrate and understand the detector’s response,” Manalaysay said. “Since we just 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 repel electrons from xenon atoms, sending them to the top of the chamber under an applied electric field where they produce a new flash that allows reconstruction of spatial events. The characteristics of the scintillation help determine the types of particles that interact in the xenon.
The South Dakota Science and Technology Authority, which manages SURF through a partnership agreement with the US Department of Energy, has 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, director of SURF Lab: “The entire SURF team congratulates the LZ Collaboration on achieving this important milestone. The LZ team has been a great partner and we are proud to welcome them to SURF.”
Fiorucci said the on-site team deserves special credit on this startup milestone, as the detector was shipped underground in late 2019, just before the start of the COVID-19 pandemic. He said that with travel severely limited, only a few LZ scientists could make the trip to help on the spot. The team in South Dakota took excellent care of LZ.
The central LZ detector in the cleanroom of the Sanford Underground Research Facility after assembly, before going underground. (Credit: Matthew Kapust, Sanford Underground Research Facility)
“I want to share the credit for 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 LZ, many of whom worked full-time from home institutions that ensured that the experiment would be a success and will continue to do so,” says Tomasz Biesiadzinski of SLAC, the operations manager of the LZ detector.
“Many subsystems came together as we started collecting data for detector commissioning, calibrations and scientific execution. Setting up a new experiment is challenging, but we have a great LZ team that has worked closely to take us through the early stages of understanding our detector,” said David Woodward of Pennsylvania State University, who plans the of the detector run.
Maria Elena Monzani of SLAC, the Deputy Operations Manager for Computing and Software, said: “We had great scientists and software developers throughout the collaboration, tirelessly supporting data movement, data processing and simulations, enabling flawless detector commissioning. The support from NERSC [National Energy Research Scientific Computing Center] was invaluable.”
With confirmation that LZ and its systems work successfully, Lesko said, it’s time for full-scale observations in the hopes that very soon a dark matter particle will collide with a xenon atom in the LZ detector.
LZ is supported by the US 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 of Science and Technology; and the Institute for Basic Science, Korea. More than 35 higher education and advanced research institutions have supported LZ. The LZ collaboration recognizes the assistance of the Sanford Underground Research Facility.
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