Heart of darkness
“This is a pretty big punt for us, but it’s a good one,” says Murray Emerson, mayor of Northern Grampians Shire, which has applied for regional development funding to develop the lab. “In the long term, it can really be beneficial for our community. We’re right at the start of something pretty exciting.”
“We’re right at the start of something pretty exciting.”
The Victorian government thinks so too. In mid-February, Premier Daniel Andrews toured the gold mine and pledged $1.75 million to kick-start construction of the laboratory, a project he says could generate up to 215 local jobs. He called on the federal government to provide matching funding, which they did in May.
The Crocodile Gold Corporation, which operates the mine, sees the project as a way of putting something back into the community and providing continuing employment for its staff and their hard-won expertise, according to general manager Troy Cole. As long as the mine is operating, Crocodile Gold is prepared to provide the lab with in-kind support in the form of access, technical advice and services such as ventilation, water and power.
Dark matter is so-called because it does not interact with light – or any other radiation for that matter. More than dark, it is invisible. And, because of its lack of interaction, it will penetrate almost anything, including Earth itself.
But dark matter is responsible for 85 per cent of the gravity that holds the universe together, so it must have mass. If a particle of dark matter directly bumps into an atomic nucleus “the nucleus gets excited”, says Professor Barberio. “It’s pushed away and the recoil is seen as light.”
And that is exactly how dark matter particles are detected; by setting up a nuclear target – in this case, a very pure crystal of the salt sodium iodide provided by researchers from Princeton – and checking to see what light is emitted.
But the sodium iodide can also react in a similar way if hit by other particles or radiation. So the detector needs to be located as far as possible from any sources of these, such as sunlight or cosmic rays or radioactivity.
And that is where the muted environment of the Stawell Gold Mine shines. Not only does it provide suitable sites deep underground surrounded by low-radiation basalt, it has another huge advantage – access. Because it is a modern “decline” mine, the laboratory can easily be serviced by trucks, ventilation, electricity and even the internet.
What’s more, while there are at least 15 such underground laboratories in the northern hemisphere, this would be the first south of the equator. That’s important, because its initial job would be to duplicate a northern hemisphere experiment that has provided some of the only credible direct evidence of dark matter. It was undertaken at the world’s largest underground particle physics laboratory, 1,400 metres below Gran Sasso near L’Aquila, about 120 kilometres north‑east of Rome.
The Italian physicists reasoned that, as the sun moves around the centre of our galaxy, it passes through a soup of dark matter particles at about 200 kilometres a second. Earth, orbiting the sun, swims with this current of particles for half a year and against it for the other half. So, you would expect that in one half of the year a dark matter detector would encounter more particles than in the other. And, over several annual cycles, that’s exactly what was found at Gran Sasso.
But critics of the study suggest that it might simply be a seasonal thing. Perhaps more particles are detected in warmer weather than cold, they say, or when the sun is nearer. So the Gran Sasso researchers were keen to help establish an underground laboratory in the southern hemisphere that could run the same experiment simultaneously to eliminate
those seasonal possibilities.
Barberio and her colleagues at CoEPP heard their call. She is a highly respected experimental particle physicist who was a key player in the discovery of the Higgs boson, the so-called “God particle”.
“There is a lot of excitement internationally about this particular dark matter experiment because we are in the southern hemisphere,” she says. “The University of Melbourne has the strongest experimental particle physics group in Australia. We can compete at the international level. So the Americans and Italians are willing to work with us while we are learning about new techniques.”