Taking science underground

A kilometre underground in western Victoria’s Stawell gold mine’s dank, dark environment, in a cavity surrounded by basalt, physicists from the University of Melbourne are among researchers taking initial measurements for a study that could determine what holds the universe together.

It may seem bizarre, but to find out why the stars are in their places in the sky, we need to go almost as far from them as we can easily reach on Earth.

Understanding the nature of dark matter is regarded as one of the most important questions of modern particle physics.

“If we nail it, it’s a Nobel Prize-winning experiment,” says the leader of the effort, University of Melbourne Professor of Physics Elisabetta Barberio, a chief investigator of the Australian Research Council Centre of Excellence for Particle Physics at the Terascale (CoEPP).

Alongside collaborators from other Australian universities, Princeton University in the US, the Australian Nuclear Science and Technology Organisation and the Italian Institute for Nuclear Physics, the researchers plan to construct a $3.5 million laboratory to try to detect the elusive cosmic glue.

But it could mean a whole lot more for the people of the Stawell region, most of whom are being confronted by the concept of dark matter for the first time.

They are hoping the lab can provide employment and investment, technology transfer and a stimulus to local industry, a source of education, possibly even a tourist attraction, and most certainly endless fascination.

“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.”

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 it 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 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.

Professor 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.”

Yet dark matter could be just the tip of the research iceberg for the laboratory, according to Professor Barberio. A lot of useful nuclear physics research is conducted in a low-radiation environment, she says. And then there’s biology.

“Already there are researchers from Australian universities at Gran Sasso studying the effect of low radioactivity on cells, particularly cancer cells. Then there are studies on general relativity, on underground micro-organisms and on chemistry and materials science.”

There seems real confidence at the mine and in the local community that the laboratory will go ahead. Things are already gearing up.

www.unimelb.edu.au/3010
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