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 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 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 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 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.
“I’m thrilled to see this complex detector ready to address the long-standing issue of what dark matter is made of,” said Berkeley Lab Physics Division Director Nathalie Palanque-Delabrouille. “The LZ team now has in hand the most ambitious instrument to do so!”
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.
Henrique Araújo, from Imperial College London, leads the UK groups and previously the last phase of the UK-based ZEPLIN-III program. He worked very closely with the Berkeley team and other colleagues to integrate the international contributions. “We started out with two groups with different outlooks and ended up with a highly tuned orchestra working seamlessly together to deliver a great experiment,” Araújo said.
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 weakly interacting massive particles (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 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 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.
“Lots of subsystems started to come together as we started taking data for detector commissioning, calibrations and science running. Turning on a new experiment is challenging, but we have a great LZ team that worked closely together to get us through the early stages of understanding our detector,” said David Woodward from Pennsylvania State University who coordinates the detector run planning.
Maria Elena Monzani of SLAC, the 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, allowing for a flawless commissioning of the detector. The support of NERSC [National Energy Research Scientific Computing Center] was invaluable.”
With confirmation that LZ and its systems are operating successfully, Lesko said, 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 UC Berkeley team designed, built and extensively tested the high voltage feed to the cathode of the LZ experiment. This is the highest voltage applied to the LZ experiment. Prof. Dan McKinsey says, "It was essential that the liquid xenon did not light up like a xenon lamp when exposed to high voltages. We are very pleased by the performance of the high voltage delivery system, which reached a stable operating voltage of -32 kV, without producing light backgrounds. It was extremely important that the LZ experiment reached high-enough drift field that radioactive background events (from beta decay of Rn-222 daughters and Kr-85) could be sufficiently distinguished from nuclear recoil signals from WIMPs. Also, a higher drift field allowed us to keep the electron drift speed high, and reduce spurious backgrounds from event pile-up."
The UC Berkeley team also contributed extensively to the understanding of backgrounds and development of statistical tools used to quantify the final result. Below are some quotes from team members:
Dr. Burkhant Suerfu, a UC Berkeley postdoctoral researcher, says "Ar-37 is a dangerous radioactive background for dark matter searches in liquid xenon detectors. We realized the bombardment of xenon atoms by cosmic rays on the Earth's surface can produce Ar-37. Combining a nuclear reaction model with detailed logistics of xenon surface purification, storage and transport, we predicted a low but significant level of Ar-37 in the LZ detector, which we later confirmed during the data taking. This background will decay away as time goes by, so should be less significant in future LZ searches for dark matter."
Andreas Biekert, graduate student working on LZ, says "We performed the final analysis on 335 candidate events and concluded there are at most 4 or 5 dark matter scatters in there, which is how we set our world-leading limits. For some WIMP masses it's as little as 1 scatter. I think that's quite remarkable and demonstrates the power of our statistical tools."
Postdoctoral researcher, Ibles Olcina, says "I led a team of talented graduate students to develop a model of an important background called accidentals, caused by the random coincidence of light pulses. We built the model that was used in the statistical analysis to search for WIMPs and developed analysis cuts to bring the accidental background rate down to a safe level."