Dig deep for dark matter

Dig deep for dark matter

Nearly ten kilometers of tunnels take you one kilometer underground to the Stawell Underground Physics Laboratory. Credit: Imogen Crump

It takes about half an hour to get to Stawell’s underground physics lab. Thirty minutes might not seem like a long time, but it’s half an hour spent plunging into a truck as it twists and sinks deeper into the pitch-black tunnels of the Stawell Gold Mine. .

Professor Elisabetta Barberio is unfazed. The University of Melbourne physicist and director of the Center of Excellence for Dark Matter Particle Physics has made the trip several times under a kilometer of rock.

“It’s warmer and more humid,” she says, as the truck’s headlights illuminate another sheer rock wall curving in the dark, “but the lab is air-conditioned.”

The Stawell Underground Physics Laboratory (SUPL) is the only underground physics laboratory in the southern hemisphere and its goal is to answer one of the fundamental questions about our universe: does dark matter exist?

At the moment, despite decades of research, the existence of dark matter particles is theoretical, but the way our universe behaves tells us that something must be there.

In fact, without it, the universe as we know it might not exist at all.

According to Professor Barberio, we can only truly observe about five percent of the entire universe; the rest is partly made up of dark matter – invisible fundamental particles that make up the majority of matter, have no electrical charge, produce no light, and don’t interact much with anything we can see.

The SUPL is the only underground physics laboratory in the southern hemisphere. Credit: hyperbeamtv/University of Melbourne

“No matter where we are on Earth, underground or above ground, we have thousands, if not millions, of dark matter particles passing through us and they do nothing to us. For these particles, we are transparent “, says Professor Barbério.

But why is the lab at the bottom of a gold mine?

“Dark matter research has to go this deep underground to eliminate ‘noise’ and cosmic radiation. Cosmic rays are absorbed by rock, so if you go far enough you can reduce them to almost zero”, explains Professor Barberio.

Italian scientists working on the DAMA/LIBRA project claim to have detected dark matter at the Gran Sasso underground laboratory which sits inside a mountain, but the signal they detected fluctuates throughout the year, depending on the Earth’s seasons.

“As the Earth revolves around the Sun, dark matter particles are blown towards us by a headwind or tailwind. If it’s a headwind, there’s more dark matter – if it is a tailwind, there is less,” explains Professor Barberio.

And that’s why there is the SUPL laboratory in the southern hemisphere, where the Italian tests can be reproduced and any seasonal variation can be ruled out.

The lab itself looks a bit like the underground lair of a Bond villain. Professor Barberio agrees.

“It’s all part of my secret plan to take over the world.” She doesn’t quite pull off the evil laugh.

The major experiment entering the Underground Laboratory is known as the Sodium Iodide South Experiment with Active Background Rejection (or SABER South for short).

The device used to detect dark matter will take up almost a third of the completely sterile lab which is 33 meters long and 10 meters wide with a ceiling 14 meters high.

It will use seven ultra-pure sodium iodide crystals housed in cylinders and wrapped in copper, with two highly sensitive instruments, called photomultipliers, at each end.

These seven crystals, which are grown in the United States and China, are then housed in a radiation-protected tank filled with approximately 12 metric tons of a liquid called benzene.

“If the dark matter particles interact with the crystal, it produces a flash of light that will be picked up by the photomultipliers,” explains Professor Barberio.

“Many experiments have tried with many different elements, but it was the DAMA/LIBRA experiment in Italy using a sodium iodide crystal that produced this light from what we think were interactions with matter. black.”

  • Dig deep for dark matter

    Dark matter research must take place this deep underground to eliminate “noise” and cosmic radiation. Credit: Imogen Crump

  • Dig deep for dark matter

    A model of the crystal housing cylinder that scientists use to get used to handling before the experiment. Credit: Imogen Crump

And it’s the properties of sodium iodide that make it so sensitive.

“Dark matter interacts with the nucleus of the crystal, so the mass of the nucleus is important. Depending on the mass of dark matter, different materials will have different sensitivity.

“So if dark matter is a large mass, a nucleus with a large mass will be more sensitive.”

At this point, Professor Barberio notices my blank face.

“Think of a billiard ball. If you have a big billiard ball and the dark matter is a much smaller billiard ball, you won’t be able to move the big ball – so you won’t produce a signal. But if your dark matter the billiard ball is huge, it will just crush everything.

Dig deep for dark matter

Professor Elisabetta Barberio and the SABER team will collect data from the underground laboratory for the next three years. Credit: Imogen Crump

“You need to have two billiard balls – or cores – of the same size, then you get a clear signal.”

SABER will collect data for approximately the next three years. For context, the Italian DAMA/LIBRA project has been collecting data for over twenty years.

“It’s a difficult experience to reproduce, so sensitive.

“We just need to be able to say ‘yes’ or ‘no’ to know if we’ve seen the same signal as Italy, so it won’t take that long.

“But if it is, oh my god.”

There are now five other experiments trying to verify the results of Italian research – in Spain, Korea, Japan, Austria and the United States. Which feels like a bit of a race to prove the existence of dark matter.

But with the only dark matter detector in the southern hemisphere, Stawell’s research team, made up of scientists from Swinburne University of Technology, the University of Adelaide, the Australian National University, the University of Sydney and the Australian Nuclear Science and Technology Organization (ANSTO), as well as the University of Melbourne – is in the box to make the breakthrough discovery of the century.

Professor Barberio looks outraged when I say that, then laughs.

“Not just this century – it will be one of the greatest discoveries of all time – to find out what the universe is made of.”

Find dark matter in the dark

Provided by the University of Melbourne

Quote: Digging deep to find dark matter (2022, August 22) Retrieved August 22, 2022 from https://phys.org/news/2022-08-deep-dark.html

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