Dark Matter and Stawell Underground Physics Laboratory : Short Wave An underground lab is opening early next year in Australia. Its quest: to help detect dark matter and thereby also help answer some of physics' biggest questions about this mysterious force. It is the only detector of its kind in the Southern Hemisphere. Swinburne University astronomer Alan Duffy takes us on a journey to the bottom of this active gold mine, where researchers will try to detect a ghost-like particle.

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Seeking Answers To The Universe Deep In A Gold Mine

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EMILY KWONG, BYLINE: You're listening to SHORT WAVE...

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KWONG: ...From NPR.

GEOFF BRUMFIEL, HOST:

In a hole in the ground, there works an astronomer named Alan Duffy. This hole is in southeastern Australia, about 150 miles from Melbourne in a small town called Stawell.

ALAN DUFFY: So this is a active gold mine. So you get your hard hat, you know, hi-viz (ph) jackets, you get in the truck. There's lots of huge machinery going on around. And you drive. And there's just this impossibly ominous-looking ramp and cave structure. There's a tunnel that goes down deep into the Earth and is completely black. And you drive into this thing, and then you basically go on this incredibly long spiral.

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DUFFY: It takes nearly 40 minutes in this vehicle to go all the way down to that operational depth at 1 kilometer. As you go, you're passing the old excavations of, you know, decades, centuries, in fact, past - collapsed tunnels, old wooden structures. And as you go deeper, you're seeing the more recent digs. All the while, the tunnel is dark. There's water dripping. You have the spinning flashlight hazard lights of the truck illuminating it. It's incredibly atmospheric.

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BRUMFIEL: Alan works here because at the bottom of this mine sits a detector, a device that may pick up faint traces of dark matter, the mysterious particles that many scientists think make up the vast majority of matter in our universe but don't really interact with other regular particles that you and I see every day. The detector has to be at the bottom of the mine because right now, everything on the Earth's surface is being hit with particles from the deep cosmos.

DUFFY: Constant bombardment, a rain of high-energy particles, naturally occurring particles from space known as muons or cosmic rays. And these are exploding stars. Feeding black holes generate these incredible particles. And they rain down constantly and would completely blind you to any of those sensitive searches.

BRUMFIEL: Meaning with so much coming at Earth all the time, it's hard to detect dark matter.

DUFFY: So we go a kilometer underground, and that rock acts to stop the cosmic rays - or most of them. The dark matter, a ghostlike particle able to travel through this rock, travels unimpeded. And really, you've just hopefully made your detector a far easier job of seeing what was the dark matter collision versus this otherwise completely blinding number of events that you get on the surface.

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BRUMFIEL: So today on the show, a journey to a kilometer inside the Earth to see a lab scientists are building to push the boundaries of our known universe. I'm Geoff Brumfiel. You're listening to SHORT WAVE, the daily science podcast from NPR.

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BRUMFIEL: Scientists have been trying to detect dark matter for decades, and they haven't found it for decades. But Alan and many others say it is one of the fundamental questions in physics right now.

DUFFY: You can literally see in the motion of the stars they're being pulled around by the gravity of an unseen partner. It's in exactly the same way as when you look out at a view of trees, and you can see that they're being moved around, but you can't see the air, but you can see the wind's effect. So in the same way, we're able to see the movement of the visible component - the stars, the gas in the galaxies and other indicators - and it is the gravity of this unseen partner that is revealing itself by their otherwise, you know, inexplicable motions. And it's not small. I mean, there is a lot of dark matter out there, and it's been essentially seen for decades. We literally create maps of the universe outlining where this dark matter is, and yet we still don't know what it is. And that's - that makes this, rightly, one of the big physics questions of this century.

BRUMFIEL: Well, I did want to talk about one detection that has raised some controversy. This was, of course, in Italy. There's a detector that claims to have seen something. Tell me a little bit about what they have and whether or not it could be dark matter.

DUFFY: The Italian experiment, known as DAMA/LIBRA, is based in the Italian nuclear agency INFN's Gran Sasso Lab. This is literally inside a mountain. And they are looking for a very special kind of signal from dark matter, and that is one that changes through the course of the year. Indeed, they think they've seen this for several years now. And it is a beautiful experiment. It's a huge amount of sodium iodide crystals that will flash a little bit of light when struck by dark matter.

What they're looking for is the fact that the Earth goes around the sun. Basically, we have a cloud of dark matter that our galaxy lies within, and the sun is orbiting around the galaxy, so it's rushing through this dark matter. Now, if you've ever driven, it's a still day, but if you put your hand out the car window, you're going to feel that force of the air on your hand. That's you going through the air at that speed. In the same way, our sun is going around the Milky Way, experiencing that rush of dark matter towards it, just as your car going through the air gives you a rush of this air towards your hand.

BRUMFIEL: As Earth moves around the sun, it's moving in the galactic headwinds, with June being the month the Earth is moving the fastest. So more headwinds, more dark matter. The Earth's movement relative to the headwinds slows towards December, so that means less dark matter hitting us.

DUFFY: That's just the way the Earth goes around the sun, and that's what's been reported by DAMA/LIBRA. The problem is the other thing that changes through the year are the seasons. And the best way to test is it the season or something outside of the Earth, so to speak, is to build and experiment at the other side of the Earth, where if it's dark matter, we're going to see the same signal just rush - this headwind of dark matter rushes through the Earth. Both detectors see more in June and less in December. But if it's something instead to do with the seasons, well, June is summer in the Northern Hemisphere and for Italy. We're in winter, so we'll see less particle collisions when you see more, and vice versa. We switch over for the wintertime. So it's as simple as literally going the other side of the Earth to build a detector to rule out the fact of seasons and perhaps the dark matter.

BRUMFIEL: And some physicists have been skeptical of the Italian result, right? They think it could be seasonal.

DUFFY: Yeah. And, look; this is because so many other experiments have been searching for dark matter have not been able to find this collision. And it's - these experiments have used slightly different detectors, different kinds of atoms. There's ways you can build a model that, you know, DAMA/LIBRA might see something, but another detector won't.

BRUMFIEL: Enter the subterranean experiments of Alan and his teammates in Australia. When their lab officially opens soon, they'll be using the same experimental setup as the one in Italy to make sure as many variables as possible are consistent. Together, this is called the SABRE Project.

DUFFY: So the SABRE Project is led by Elisabetta Barberio. And really, this is based around the sodium iodide crystal. So I wanted you to imagine something not too dissimilar to table salt, but a single, beautiful, clear, transparent crystal cylinder, in fact, about the size of a water bottle that will flash when the dark matter collides or, indeed, any particle collides with it. We will place two very sensitive cameras either end of that cylinder of the crystal looking for the flash of light, wrap the whole thing in copper so it's not blinded by background light. It's completely dark. And then we place this in a very large vessel, and it will have about 10 tons worth of liquid called benzene, which is another thing that flashes when struck by particles. If you see a flash of the crystal but you don't see a flash in the liquid around it, then it's possible you might have dark matter. The dark matter just slipped through the liquid, no collision, hit the crystal and revealed itself. Anything else on the way to your crystal is going to cause a flash in the liquid.

BRUMFIEL: Because ordinary particles will create a tiny burst of light when they interact with benzene, it's another layer designed to keep out unwanted visitors, but it's not the only one. The detective work scientists are doing in the quest to find dark matter these days, it's so precise. They've got to worry about the experiment they just built. The very atoms it's made from could give off radiation that messes with it.

DUFFY: All of these experiments are now so sensitive that the material you make it from matters. There's nuclear fallout from the testing over decades that has sprinkled radioactive materials on everything. So sometimes you have to go to extraordinary lengths to secure material that's never been exposed to nuclear fallout.

There were experiments in the Northern Hemisphere, for example, in the Gran Sasso labs, where they've actually sourced the building material, the lead shielding, in fact, from ancient Roman galleys that were trading across the Mediterranean that were sunk. And lead ingots have been recovered from the bottom of the Med. The top was sliced off and preserved for a museum, and the rest was melted down and used to create new shielding because it's - had all this time to become radioactively quieter and been preserved from, you know, contamination with fallout.

I mean, these are the levels of effort that some of the experiments go to to ensure they're not blinded by the radiation that we might bring in with us. For example, if you and I ate a banana, there's a lot of - well, there's not a lot. There's a little bit of potassium-40. It's totally fine. It's very healthy to eat bananas. But got a no banana policy down there.

BRUMFIEL: (Laughter) Radioactive banana potassium aside, Alan says one of two things is going to happen.

DUFFY: Either we can see the same signal, and that will be a huge boon to dark matter research - everyone will immediately begin to, I think, redirect their attention towards that kind of a particle - or - which would be wonderful for DAMA/LIBRA, a wonderful vindication of all their hard work over years - or we can't see it, and perhaps we're seeing the seasonal effect instead, which would also help, I think, everyone because it would free up us to explore different directions and different kinds of dark matter. So it's - either way, we really can't lose in that sense. I mean, that's the beauty of science. You either - you - either way, you gain knowledge about what it is or what it isn't. And at this point, this is an incredibly valuable contribution.

BRUMFIEL: Got it. And the simplest thing to do is build a nearly identical experiment on the opposite side of the planet a kilometer underground in a gold mine in a completely sterile vault using scientists from all over the world.

DUFFY: Look; when you say that's the easiest way, yeah, it is.

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DUFFY: It certainly is, but it doesn't sound easy. And it's taken us many years to get there, so it certainly isn't. But yes. In essence, when you're hunting for a ghost, I guess, this is the easiest way to do it.

BRUMFIEL: Do you have a hunch which one you're going to see? (Unintelligible).

DUFFY: Well, I know which one I want to see.

BRUMFIEL: Yeah, I...

DUFFY: I - yeah. I think I'm - I think my hunch is that the sheer number of other detectors that have searched this similar kind of space makes me think that if we do see a signal, it will track the seasons. That's not to say I'd be unhappy with that. I'd also certainly not be unhappy if we saw the same signal that indicated it was dark matter. But I think the key is that we are all planning and looking forward to the next - both the upgrade of SABRE to make it even bigger and more sensitive, but maybe we have to build ones that are directional, an entire new, exciting branch of detectors.

BRUMFIEL: To find something that may or may not be there.

DUFFY: It's an extraordinary undertaking, but you spend - and you can spend decades - in fact, you can build an experiment that you may never be still actively researching when it operates and when it begins to potentially find that dark matter. You're doing it for the next generation of researchers. These are generational efforts, and everyone's in it for the long haul. We know we could find it in the next year or the next decade, or maybe nature really is unkind, and it's a century hence. But you never know if you don't look.

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BRUMFIEL: Alan, thank you so much. I've really enjoyed this.

DUFFY: Oh, it's been a real thrill. Thanks for the chance to share this extraordinary experiment.

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BRUMFIEL: This story was edited by Gisele Grayson, produced by Rebecca Ramirez and Margaret Cirino. And Marge (ph) also checked the facts.

DUFFY: SABRE is led by University of Melbourne's Elisabetta Barberio. It sees contributions from University of Adelaide, ANU, the Australian nuclear agency and Stawell and, in particular, efforts from Frank Calaprice and his team at Princeton and the Italian nuclear agency INFN. It's a huge global effort to try to search for dark matter.

BRUMFIEL: I'm Geoff Brumfiel. You're listening to SHORT WAVE, the daily science podcast from NPR.

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