LZ water tank

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

LZ water tank

LZ team members in the LZ water tank after installing the outdoor detector. Credit: Matthew Kapust, Sanford Underground Research Facility

Berkeley Laboratory Researchers Record Successful Startup 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 passed a verification phase of start-up operations and delivered the first results. LZ is located deep beneath the Black Hills of South Dakota in the Sanford Underground Research Facility (SURF) and is led by DOE’s Lawrence Berkeley National Laboratory (Berkeley Lab).

The take-home message from this successful startup: “We’re ready and all is well,” said Kevin Lesko, Berkeley Lab senior physicist and former LZ spokesperson. “It’s a complex detector with many parts and they all work well within expectations,” he said.

In a July 7 article on the experiment’s website, LZ scientists report that with the first run, LZ is already the most sensitive dark matter detector in the world. The paper will appear on the arXiv.org online preprint archive later. LZ spokesman Hugh Lippincott of the University of California, Santa Barbara said: “We expect 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 Outdoor Detector

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

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

Although it is invisible because it does not emit, absorb or scatter light, the presence and gravitational pull of dark matter is nevertheless fundamental to our understanding of the universe. For example, the presence of dark matter, which is estimated at around 85% of the total mass of the universe, shapes the shape and motion of galaxies, and it is invoked by researchers to explain what is known about the large-scale structure. and the expansion of the universe.

Two nested titanium tanks filled with ten tons of high-purity liquid xenon and visualized by two photomultiplier tube (PMT) arrays capable of detecting faint light sources form the heart of the LZ dark matter detector. The titanium tanks reside in a larger detection system to catch particles that might mimic a dark matter signal.


A schematic of the LZ detector. Credit: LZ collaboration

“I’m excited to see this complex detector ready to solve the long-standing problem of the composition of dark matter,” said Nathalie Palanque-Delabrouille, director of the Berkeley Lab’s physics division. “The LZ team now has the most ambitious instrument in hand to achieve this!”

The design, fabrication and installation phases of the LUX-ZEPLIN detector were led by Berkeley Lab project director Gil Gilchriese in collaboration with an international team of 250 scientists and engineers from more than 35 institutions in the United States. , the United Kingdom, Portugal and South Korea. LZ’s COO 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, of[{” 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 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.

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 large-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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