Stawell underground lab clears crucial hurdle for dark matter hunt
Australia’s bid to detect elusive dark matter has taken a major step forward, with new research confirming that cosmic radiation levels deep inside the Stawell Underground Physics Laboratory (SUPL) are low enough to support the world-class experiment that will commence later this year.
ARC Centre of Excellence for Dark Matter Particle Physics researchers recorded muon – or cosmic radiation – levels inside and outside the laboratory over a year. They detected 30,000 muons inside the underground laboratory, while 8.4 billion muons would be expected to be detected on the surface of the earth.
Published in Astroparticle Physics, the SABRE Collaboration paper is the first to use data collected in SUPL, marking a major achievement for Australian and international scientists involved in the project.
SUPL was constructed in the Stawell Gold Mine in 2022 to house the SABRE South dark matter experiment and other research requiring deep underground conditions. It’s the first facility of its kind in the Southern Hemisphere, placing Australia firmly on the world stage as a centre for science.
The SABRE South experiment mirrors a similar experiment undertaken in Italy called the DAMA/LIBRA, which detected signals that scientists believe could be dark matter.
By replicating this in the Southern Hemisphere, scientists can account for seasonal variations and verify if the signals found in Italy were dark matter.
A low-radiation environment, paired with ultra-pure crystals placed in 12,000 litres of liquid scintillator and protected by a specially-built vessel and around 120 tonnes of steel shielding, create the near-pristine conditions necessary to detect incredibly elusive dark matter particles.
The muon count detected in SUPL matched world-leading underground laboratories internationally including the Boulby Lab in the UK and Gran Sasso National Laboratory in Italy.
Project lead, University of Melbourne Professor Phillip Urquijo said the research was an important milestone.
“Over the course of the year, we measured the rate of muons produced in the atmosphere from cosmic ray interactions,” Professor Urquijo said.
“We performed highly detailed simulations of the expected rate of muon, using detailed maps of the Stawell Gold Mine tunnel structures and knowledge of the rock structures in Stawell.
“The measurements and simulations are in excellent agreement, showing that we have a thorough understanding of how much cosmic background radiation we expect in our future dark matter search.
“This gives us a deep understanding of the potential background from cosmic ray muons in dark matter searches in SUPL, particularly with SABRE South.”
Swinburne University of Technology postdoctoral researcher Dr Federico Scutti said the results validated both the underground site and the detector technology by demonstrating that the muon veto system – a key component of the experiment designed to tag and reject muon-related events –is working as expected.
Dr Scutti said the next steps would provide further information about the environment in the underground laboratory.
“First, we will perform more detailed studies of the muons, including how their rate changes with direction and time. This will further refine our understanding of the underground environment,” Dr Scutti said.
“Second, we will integrate this knowledge directly into the operation of SABRE South as it moves toward full data-taking. This includes using the muon veto system to actively reject background events in real time.
“Ultimately, this work helps pave the way toward high-sensitivity dark matter searches at the Stawell Underground Physics Laboratory.”
SUPL Chief Operating Officer Kim Mintern-Lane said the research was an exciting milestone for the facility.
“This work demonstrates that SUPL is not just operational, but scientifically ready,” Ms Mintern-Lane said.
“Achieving results consistent with world-leading underground laboratories is a significant milestone for Australia and an important step in establishing SUPL as part of the global effort to understand dark matter and enable other ultra-sensitive research.”
Scientists from Australian National University, the University of Melbourne, Swinburne University of Technology, Adelaide University and the University of Sydney participated in the research, with muon data analysis completed by University of Melbourne PhD candidate Guangyong Fu.
BACKGROUND NOTES
Dark matter is a mysterious substance that holds galaxies together and constitutes 85 per cent of the matter of the Universe. Its existence is inferred from its gravitational interaction and the exploration of the nature of dark matter is considered science’s next frontier.
The ARC Centre of Excellence for Dark Matter Particle Physics was established in 2020 to bring together physicists from across Australia, in partnership with key international researchers and institutions, to pursue the discovery of dark matter.
The Centre is the first of its kind in the Southern Hemisphere and its researchers aim to make discoveries that could change our understanding of the universe.
The SABRE experiment is a dark matter direct-detection experiment that uses a detector comprising a large steel vessel encasing seven ultra-pure crystals made of sodium iodide, submerged in liquid
scintillator. It will search for WIMPs (Weakly Interacting Massive Particles), one type of candidate dark matter particle.
The DAMA/NaI and DAMA/LIBRA experiment is located in Laboratori Nazionali del Gran Sasso (LNGS), in Italy. It has been taking data on what might be dark matter for almost 30 years.
The SABRE South detector, the first of its kind in the Southern Hemisphere and the nation’s first major dark matter experiment, will be located in SUPL.
SABRE South’s location in the Southern Hemisphere allows researchers to discount the impact of seasonal modulation on potential dark matter signals to confirm or refute the results of DAMA/Nalnal and DAMA/LIBRA.
The SABRE South dark matter experiment will begin taking data in the underground laboratory in 2026.
For interviews, contact: Fleur Morrison | 0421 118 233 | fleur.morrison@unimelb.edu.au