Success! First results from the world’s most sensitive dark matter detector

LZ water tank

LZ team members in the LZ water tank after the outer detector installation. Photo credit: Matthew Kapust, Sanford Underground Research Facility

Berkeley Lab researchers mark successful commissioning of LUX-ZEPLIN dark matter detector at Sanford Underground Research Facility

An innovative and uniquely sensitive dark matter detector – the LUX-ZEPLIN (LZ) experiment – has gone through a check-out phase of commissioning and has delivered first results. LZ is located deep beneath the Black Hills of South Dakota at the Sanford Underground Research Facility (SURF) and is managed by the DOE’s Lawrence Berkeley National Laboratory (Berkeley Lab).

The take-home message from this successful startup: “We’re ready and looking good,” said Kevin Lesko, senior physicist and former LZ spokesman for Berkeley Lab. “It’s a complex detector with many parts, all of which work well and perform as expected,” he said.

In an article published on the experiment’s website on July 7, LZ scientists report that LZ is already the world’s most sensitive dark matter detector with its first run. The paper will later appear in the online preprint archive arXiv.org. 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 is a lot of science to do and it is very exciting!”

LZ Outer Detector

Looking up into the outer LZ detector used to prevent radioactivity that can mimic a dark matter signal. Credit: Matthew Kapust/Sanford Underground Research Facility

Although dark matter particles have never actually been discovered, they may not be true for much longer. The countdown may have already started with the results of LZ’s first 60 “live test days”. This data was collected over a three and a half month period of commissioning beginning in late December. This duration was long enough to confirm that all aspects of the detector were functioning properly.

Although invisible 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 make up about 85 percent of the total mass of the Universe, shapes the shape and motion of galaxies and is cited by researchers to explain what is known about the large-scale structure and extent of the Universe.

Two nested titanium tanks filled with ten tons of very pure liquid xenon and viewed by two banks of photomultiplier tubes (PMTs) capable of detecting faint light sources form the heart of the LZ dark matter detector . The Titan tanks are housed in a larger detector system to capture particles that could mimic a dark matter signal.

LUX ZEPLIN scheme

A schematic of the LZ detector. Photo credit: LZ collaboration

“I am very pleased that this complex detector is ready to tackle the long-standing problem of what dark matter is made of,” said Nathalie Palanque-Delabrouille, director of the Physics Department at Berkeley Lab. “The LZ team now has the most ambitious instrument for this in hand!”

The design, manufacturing and installation phase of the LUX-ZEPLIN detector was led by Gil Gilchriese, project manager at Berkeley Lab, in collaboration with an international team of 250 scientists and engineers from over 35 institutions in the US, UK, Portugal and South Korea. The LZ 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.

Henrique Araujo, by[{” attribute=””>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, South Dakota, LUX-ZEPLIN 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.”

LZ Detector Event Diagram

When a WIMP – a hypothetical dark matter particle – collides with a xenon atom, the xenon atom emits a flash of light (gold) and electrons. The flash of light is detected at the top and bottom of the liquid xenon chamber. An electric field pushes the electrons to the top of the chamber, where they generate a second flash of light (red). LZ will be searching for a particular sequence of flashes that cannot be due to anything other than WIMPs. Credit: LZ/SLAC

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.”

Vacuum Distillation System for LZ Dark Matter Experiment

Chemists at Brookhaven Lab used this custom-made vacuum distillation system to purify linear alkyl benzene needed to produce liquid scintillator for the LZ dark matter experiment. Credit: Brookhaven Lab

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 LZ Central Detector in Clean Room

The LZ central detector in the clean room at Sanford Underground Research Facility after assembly, before beginning its journey underground. Credit: Matthew Kapust, Sanford Underground Research Facility

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 working successfully, Lesko said it’s time to begin full-scale observations in hopes that a dark matter particle will collide with a xenon[{” attribute=””>atom in the LZ detector very soon.

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 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.

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