Country Victoria is set to host one of the coolest experiments going, an attempt to solve the mystery of dark matter. Will they succeed? And what’s the Italian connection? JACINTA BOWLER dived underground
to find out.
This story has been republished from Cosmos Magazine Issue 94, on sale now to celebrate the Stage one completion of Stawell Underground Physics Laboratory. This piece has been selected to be part of the anthology Best Australian Science Writing 2022.
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A small town in regional Victoria called Stawell will soon have a secret deep below its main street. One thousand metres underground in a dusty, dark gold mine will sit a brand-new, dust-free, laboratory with a dark matter detector at its core.
If you’re a physicist who needs to head down there for work, the trip isn’t easy. For openers you’ll need to don 10kg of protective mining equipment and drive in a specialised ute down a tunnel for 45 minutes in the dark on the bumpiest road imaginable. Then you’ll have to strip and shower off all the dirt and grime acquired on the way. Finally, after changing into a fresh, low-dust jumpsuit, you’re ready to enter the Stawell Underground Physics Laboratory (SUPL).
“I was overwhelmed when I went down that tunnel,” says Professor Alan Duffy, Swinburne University of Technology astrophysicist and former lead scientist of the Royal Institution of Australia. “It was a 45-minute journey to see this huge facility. The roof was over 12 metres high. It’s such a larger space than I imagined, but clearly so full of promise.”
At this depth, the smell is completely foreign. Breathable air is pumped throughout the facility, but by the time it reaches a kilometre below the surface it’s hot and humid and rank. To Wayne Chapman, one of the managers of the SUPL building project, it’s the smell of decades of “miners, sweat, kebabs, beer and blast fumes”.
Right now, the lab – a cavernous area on a mostly unused section of the mine – is just a construction site. But if all goes to plan, by the end of 2022 a team of researchers from the University of Melbourne, ANSTO, Swinburne, and more will be collecting data, and hopefully helping to answer one of the most befuddling questions in astrophysics: what is dark matter made from?
STAWELL AND SABRE
For a town of just 6,000 that’s so entwined with its gold mine, it’s not surprising that when plans began rumbling back in 2014 about the first underground lab in the Southern Hemisphere being built beneath Stawell, the locals had to be convinced of the appeal.
Stawell has similarities with many regional towns of its size across Australia. It’s got one high school, two post offices and a nearby national park – the Grampians. Its biggest attraction is the Stawell Gift, the oldest short-distance footrace in the country, and famously the one with the richest prize purse. If you visit outside of the Gift carnival, the locals might suggest you visit the Diamond House; the distinctive, diamond-patterned restaurant is one of the oldest houses in Stawell, having stood there since 1868.
But unlike many gold rush towns, Stawell’s gold has had a resurgence. Mining in the town dates back to the 1850s, and although the gold had seemingly run out by the 1920s, in the 1980s full-scale mining recommenced, and more than 2.3 million ounces of the precious metal has since been extracted. The mine, just a few minutes’ drive from the centre of town, still employs more than 300 people.
Eight years after SUPL was first announced, there’s much better understanding about the experiment – thanks to the then mayor, Murray Emerson, enthusiastic science teachers at the local schools, and outreach programs run by SUPL scientists.
In particular, it’s now known that the reason the detector – called the Sodium Iodide with Active Background Rejection Experiment (SABRE) – had to be so far underground was not to keep the experiment in, but to keep the rest of the world’s radiation out.
“We’re pushing the boundaries for what you can do for a scientific instrument, as well as a new type of scientific lab in Australia,” says Phillip Urquijo, a technical coordinator of SABRE and University of Melbourne associate professor of particle physics. “It turns out that there is sufficient expertise in Australia to pull this together. It’s been impressive.”
But with years-long delays of the project due to the mine closing and then reopening when its ownership changed, plus the months of COVID lockdowns in Victoria during the pandemic, SUPL is years behind schedule and only now coming to fruition.
The tennis-court-sized cavern must still undergo some significant changes. The rock needs to be covered in a flexible coating, and rooms will be constructed and painted. Showers need to be installed, along with airconditioners to keep out the dust and humidity from the rest of the mine. Once construction is completed, the equipment for the experiment will be painstakingly transported down through the bumpy tunnels.
A COOL BIT OF KIT
In place, the multi-million-dollar detector will look from the outside like a four-metre square box of steel. This steel casing – and its placement deep beneath the Earth’s surface – is to repel as much stray radiation as possible. Housed inside is a vat called the veto vessel, which will be covered with reflector foil and filled with a compound called linear alkylbenzene. Studded throughout the inner walls of the vat are photon sensors called photomultiplier tubes that look like vintage yellow light bulbs.
Descending from the vat’s lid are long copper cylinders filled with 50kg of incredibly pure sodium iodide crystal. This is the critical part – although what it does won’t happen often. When a dark matter particle travels through the crystal, it will interact with an atom inside of it; the atom will recoil and the crystal will produce a flash of light, which the detectors will be able to pick up.
The crystals need to be as heavy – and therefore as big – as possible to detect as many potential dark matter particles as they can. Think of a hand swatting a fly – the bigger the hand, the more swatting ability. In the same way, the bigger the crystal, the more likely that a weakly interacting massive particle (WIMP) will pass through and alert the detector. They also need to be as “pure” as possible to prevent radiation from interfering with accurate readings. Extending the hand analogy, impurities act like phantom flies, alerting the detector with false readings. Creating each of these individual parts has taken years of study. For example, the crystals have been developed by an international team and will have the lowest level of radioactivity ever created – a state the researchers call “radiopure”.
By the 1980s, scientists were sure that dark matter existed, but since then they’ve struggled to agree on what it is or how best to detect it
Zuzana Slavkovska, one of the researchers testing these crystals at the Australian National University, has been using a particle accelerator to measure the crystals’ purity.
“We are trying to identify and quantify these radio impurities – meaning isotopes that are radioactive – as they might produce signals that mimic dark matter,” says Slavkovska. “We want to make sure when we detect something, it really is dark matter.”
This is also the reason why the lab needs to be dust-free. Radiation is all around us, all the time. Ionising radiation emits from bananas (because of the potassium) for example, and from modern steel (because the production uses atmospheric air). Each piece of material that makes up SUPL has been checked for minute levels of radioactivity at specialised labs to minimise any chance of the detector picking it up. Even the talcum-powder-like mine dust would introduce small amounts of radiation, so it has to be kept out.
Each piece of the experiment needs to be transported into the mine’s depth’s, taking exceptional care not to damage the fragile equipment. Then the detector will be assembled piece by piece inside the mine. First the veto vessel will need to be installed, with enough reinforcements to make sure it can eventually hold thousands of litres of liquid inside. The photomultiplier tubes will be screwed into the walls. Then the team needs to coat the entire inside of the container in reflector foil.
The foil needs to cover all sides and it can’t be touched. Urquijo suggests that the installation of the final bottom piece might have to be “Mission Impossible” style.
“We’ll try to figure out something that doesn’t involve dangling from the ceiling,” he says. “But maybe we’ll have to.”
THE MATTER AT HAND
Dark matter hasn’t been an easy thing to get our hands (or in this case, detectors) on. We’ve been seeing the evidence of whatever dark matter is since the turn of the 20th century, when scientists discovered that the mass and rotation of galaxies just don’t make sense without an added unseen gravitational force.
By the 1980s, scientists were sure that dark matter existed, but since then they’ve struggled to agree on what it is or how best to detect it. It could be a brand-new type of particle, a boson, or even a primordial black hole. Each potential candidate requires its own expensive experimental set-up in order to test for its existence.
One of physicists’ favourite candidates over the past few years is a hypothetical particle class called weakly interacting massive particles, or WIMPs. Scientists don’t yet have a great definition for what a WIMP would be, but if it exists, it’ll neither absorb nor emit light. Although WIMPs don’t often interact with other particles, researchers are hoping that they’ll occasionally smack into the atoms inside detectors.
Despite WIMPs being one of our best bets for dark matter, detector after detector has been unable to find anything that even remotely resembles a WIMP. Well, all detectors but one.
The DAMA/LIBRA experiment, set up in a large underground lab underneath the mountain Gran Sasso in Italy, has been recording a significant signal for 20 years now. DAMA/LIBRA is a controversial experiment at best – while other large experiments that are far more sensitive and specific have found nothing, DAMA/LIBRA took a simpler route: it charts the data for change over time.
Radiation doesn’t just exist in this dark matter. Even thousands of metres under rock where the detector lives, other kinds of radiation – such as muons from the Sun’s cosmic rays or neutrons from radioactive decay – can travel.
SABRE, and many other detectors, have ways to minimise these unwanted particles. Muons, for example, can travel through the Earth from one side to the other, but they produce ionising energy, which the team can easily detect and remove from its data. Neutrons are likely to be stopped by the 12,000 litres of linear alkylbenzene inside the vat.
Luckily, due to how rarely WIMPs interact with particles, they are very unlikely to hit both the crystal and the outer detector.
“We also have high-density polyethylene shielding around, and that tends to absorb neutrons as well,” says Urquijo. “Plus, the neutrons will actually produce light. If we see some light occurring in the outer detector before it hits the crystal, then that tells us it’s probably a neutron coming from outside.”
Although scientists don’t know what a dark matter signal is going to look like, one thing that we do know is that we live in a galaxy filled with dark matter.
Because the Earth is physically moving through space as it travels around the Sun, researchers think that we might see a change in the amount of dark matter we see as we move, creating a signal that would repeat annually. The signal doesn’t have to show the exact specifications of WIMPs or any other potential dark matter characteristics; any repeating signal that can’t be explained by error or other sources could be dark matter. This repeating signal is known as an “annual modulation”.
This is what the DAMA/LIBRA team has found: an annual modulation signal. Even more impressively, they’ve managed to confirm it to a high degree of confidence for almost two decades. But so far, no other detectors have been able to find this same signal. There could be a number of reasons for this.
One suggestion is that instead of the annual modulation signal of the Earth moving around the Sun, it is instead a seasonal change caused by something else entirely. This makes having a dark matter detector on the other side of the world incredibly handy. If the SABRE team finds modulation opposite to DAMA/LIBRA, it means that what the Italian team is seeing is seasonal variation – and it isn’t dark matter. However, if the modulations match, it would confirm that we’re able to detect dark matter.
But the SABRE team might also find no modulation at all. In the last few years, some physicists have suggested that the way that DAMA/LIBRA collects its results introduces a bias that could cause this modulation. An experiment called ANAIS in an old railway tunnel in Spain recently concluded that they could not find evidence of this annual modulation, despite adopting the same target and technique of DAMA/LIBRA. This result was described by some as the nail in the coffin of DAMA/LIBRA’s modulation.
Urquijo strongly disagrees with that assessment, suggesting it was a form of scientific “clickbait”.
“The ANAIS collaboration didn’t jump to a conclusion themselves. They didn’t say ‘we’ve ruled out DAMA/LIBRA’,” he says. “If you look at the difference between the two, it wasn’t that much… It was not to a level that you would say it ruled out [the DAMA/LIBRA] measurement.”
THE ITALIAN JOB
Clearly, SABRE is closely connected to the DAMA/LIBRA experiment. In fact, the SABRE being built in Stawell is only one of two related experiments: a second one is slated to be built in the same Gran Sasso laboratory as DAMA/LIBRA. The two projects are called SABRE North and SABRE South. They’ll be identical, each one looking for an annual modulation on their own side of the world to confirm DAMA/LIBRA’s result. Without the Italian project wanting to probe the hypothesis of seasonal variation, the Stawell laboratory would probably not exist.
“At the beginning, it was very important to push to have this counterpart in the Southern Hemisphere, but at this point we have progressed so much that now we are actually at the same level,” says Claudia Tomei, a particle physicist at Italy’s National Institute for Nuclear Physics (INFN) and one of the lead researchers on the SABRE North project. “Both projects will be able to obtain an important result – but not as important as the combination of the two.”
Tomei is one of the many scientists from around the world who visited Stawell to inspect the mine and its location long before the experiments begin.
“The excitement and the potential value to the world would be akin to the Higgs boson discovery. Perhaps even greater”
“I went there with colleagues who were interested in the project,” she says. “I also got the chance to meet the community of physicists there. We are used to collaborating with scientists from abroad, of course, but Australia is so far away I have never been involved there before.”
Australians, in turn, headed north to learn from DAMA/LIBRA. Even Emerson, the enthusiastic Stawell mayor when the project was first announced, travelled to Gran Sasso to see the DAMA/LIBRA detector. The collaboration is so wide-reaching that the universities of Rome, Milan and Princeton, as well as INFN, are all involved.
“This collaboration was at the beginning very strong,” says Tomei. Of course, it isn’t easy to collaborate on three different continents. “But we will be more connected as more of the projects enter the active stage,” she says.
Although it’s taken a while to get the Stawell lab off (well, under) the ground, and international trips are no longer viable to foster those connections, many people around the world – including those working at DAMA/LIBRA – are holding their breath to see what the SABRE team discovers.
Even when all the parts are in, assembled and the detector is turned on, it’ll be at least three years until we can know what they’re going to find. SABRE will just have to sit there waiting for the occasional mystery particles to come whizzing by. If we’re lucky, some of them could be dark matter.
“What I think we’ll see is a result that rules out DAMA/LIBRA,” says Duffy.
“What I hope we find is a result that rules in DAMA/LIBRA as a discoverer of dark matter, because the excitement and the potential value to the world would be akin to the Higgs boson discovery. Perhaps even greater.”
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