Searching for dark matter, deep in the Earth

[MUSIC PLAYING] FLORA LICHTMAN: Hey, it’s Flora. And you are listening to Science Friday. When I think about dark matter, my mind goes to outer space. I imagine– I don’t know– I imagine mysterious cosmic dust bunnies floating around in the distant universe. But a lot of them, because we know dark matter makes up 80-something percent of the total matter of the universe.

But researchers are looking for signs of dark matter right here on Earth. An experiment called SuperCDMS is searching for the signatures of dark matter deep, deep underground. Here to tell us more is Dr. Priscilla Cushman. She’s a physicist at the University of Minnesota and has been working on this dark matter hunting experiment for over 20 years. Priscilla, welcome to Science Friday.

PRISCILLA CUSHMAN: Thank you for inviting me. I love your show, by the way.

FLORA LICHTMAN: Oh, thank you. Appreciate it. Also, welcome back to the surface of Earth. I understand you’ve been deep below for a bit.

PRISCILLA CUSHMAN: I have. I have, indeed.

FLORA LICHTMAN: Should I picture you in a lab coat and a miner’s helmet?

PRISCILLA CUSHMAN: Well, the lab coat, no. You’ll have to think more about a large miner’s coverall and a utility belt and a backpack filled with not only my own stuff I need, but also a self-rescuer that weighs about 20 pounds, marching about a kilometer to get to the lab, actually, at 2 kilometers below the surface.

FLORA LICHTMAN: Wow. Why do you need to be so deep underground to do this research?

PRISCILLA CUSHMAN: Well, basically, we have very, very sensitive detectors, right? And so they’re sensitive to everything. And that includes the cosmic rays, which are intersecting us and the Earth at all times. And they get blocked by the Earth between the surface and where the lab is. But the dark matter particles do not, because they are so weakly interacting. Basically, they pass through the Earth. You would count as many of them at night as at day, because they can just come through the other side of the Earth any way they want.

FLORA LICHTMAN: This is something that, I think, upends one of my preconceived notions. So is dark matter everywhere? Is it in the room with us right now?

PRISCILLA CUSHMAN: At this very moment. But it is moving quite fast because not so much that it is moving fast, but that we are moving fast through it, as we move around the sun and as our solar system moves around our galaxy.

FLORA LICHTMAN: Is it staying still? Is it floating there, and we’re moving through it?

PRISCILLA CUSHMAN: Yeah, it’s more think of it that way. Because the relative motion is all that counts, because we’re having detectors that measure the kinetic energy of the particles. And so from our point of view, it’s like this dark matter wind that’s moving through our detectors, but only one in a trillion actually gets close enough to one of our target atoms to move the nucleus a tiny bit. And that’s why we have a hard time seeing it, although there’s so much of it.

FLORA LICHTMAN: Yeah, let’s talk about that. What are you looking for exactly?

PRISCILLA CUSHMAN: So what we’re looking for is an interaction between a dark matter particle and a nucleus. Our target material, for example, are crystals of germanium or silicon. The target material exists to be interacted with, if you like. So we have all of this collection of nuclei, and the dark matter particles are moving through. And they have to be very close to the nucleus to make an interaction. But that’s what we’re looking for.

Imagine that you are standing in the middle of a large stadium, and then the nucleus would be like putting a little grape down in the middle. And all the electrons are out at the edges of that stadium. So for a dark matter particle that needs to get very, very close to that grape, there is a ton of open space for it to go through. And that’s our main problem, is that there’s a lot of dark matter particles. But as they move through us and through the Earth and also through our detectors, we have a hard time detecting them because they mostly just pass through.

Obviously, the more detectors you have, the more nuclei you have, and the longer you wait to see an interaction, the more likely you are to see one. So that’s what drives the need for a larger and larger detector, or, in our case, very sensitive detectors that can look at the very lowest and tiniest energy depositions.

FLORA LICHTMAN: You also cool it down. And we called you now because I saw that this SuperCDMS experiment had cooled down enough to its operating temperature. Why does it have to be cold?

PRISCILLA CUSHMAN: Well, there are actually two reasons. First of all, we are better able to distinguish that deposited energy from the particle interactions we care about from the generalized thermal energy of the surrounding atomic nuclei. But also the crystals are outfitted with superconducting sensors, and they only work when they’re extremely cold.

FLORA LICHTMAN: Well, how cold does it have to be?

PRISCILLA CUSHMAN: So for SuperCDMS, the temperature at which our sensors can operate is somewhere between 20 and 40 millikelvin. The exact temperature depends on the TC or transition temperature of the Tungsten sensors and where they go superconducting. So let’s take 30 millikelvin. That is about 0.03 degrees above absolute 0.

FLORA LICHTMAN: Wow.

PRISCILLA CUSHMAN: It’s actually not unusually cold. It is unusually cold for humans, I suppose. But the helium dilution fridge, which gets us to that temperature, was actually invented back in the 1960s. And it’s used in a lot of experiments, especially condensed matter physics. So getting down to tens of milliK is not what’s unique about our experiment. It’s that we’ve got a payload of 31 kilograms of detectors, along with hundreds of kilograms of associated tower hardware, even cables that have to snake their way out from millikelvin temperatures to 1K, up to room temperature.

And we also have copper vacuum cans. That’s another 5 tons, all nested inside each other at sequentially colder temperatures, and cooled by conduction through a similarly nested cold stem cylinder to the dilution fridge stages. So you need all this cryogenic infrastructure to hold those 24 detectors stably at their operating temperature, so that we got it all to work together. And we’re now able to reliably see pulses in our detectors. That’s the milestone we are celebrating.

FLORA LICHTMAN: Does that mean– if you’re at operating temperature now, does that mean you’re ready to start the hunt?

PRISCILLA CUSHMAN: We are. We are now in what’s called the commissioning stage. And this is because we now have operating detectors, we are putting them through their paces. We’re calibrating them. So we’re now deciding which ones that we really want to concentrate on and get the best performance out of.

FLORA LICHTMAN: When do you start actually looking and collecting data?

PRISCILLA CUSHMAN: Well, we’re collecting data, of course, now. We have what are called data-taking shifts, where we have three to four people who, for a week, look at the data and try to see how to improve our resolution. We expect what we call science data to happen near the end of the summer, most likely, sometime in the summer.

FLORA LICHTMAN: If you spot dark matter or the signature of dark matter with this experiment, what would it look like? Is it a flash of light, a streak? Is it a number?

PRISCILLA CUSHMAN: What happens is a particle– and it could be a dark matter particle, or it could be a particle from trace radiation. And what happens is that that bump creates a vibration in the crystal. That vibration then spreads out in the crystal and begins bumping off the walls. And every time it hits a sensor– so we have thousands of these little sensors that are called transition edge sensors, but you can think of them as very, very sensitive thermometers. And these thermometers register this vibration hitting them multiple times.

And so you build up basically a pulse out of that, over time. And it’s about microseconds long. But the shape of that pulse is very important. What is its rise time? How high does it get? How wide does it get? All the physics of how that vibration expands and the physics of how the interaction actually took place is then mirrored in this pulse. So the final pulse that we get tells us what type of interaction it was, how much energy was deposited, where the incident particle actually bumped a nucleus. And it also tells us something about whether it’s a background particle or a dark matter particle.

FLORA LICHTMAN: Sounds very exciting. I mean, how much data do you think you’ll need– how long will it take before you know if you’ve seen a pulse of interest or a particle of interest?

PRISCILLA CUSHMAN: So we start the science run in the summer. We would like to have about six months of data under our belt and another six months to analyze it. So we’re hoping that a year or a year and a half for our first run is about what we expect. Of course, we don’t stop. We continue. We artificially say, OK, we’re going to look at that first six months, and we’re going to start the next chunk of data.

What we learn about the first set of data, of course, will influence how we make that choice. Perhaps we’ve actually fixed a few of the detectors. Maybe we have a different physics goal in mind. There are a number of different categories of dark matter. There are electron recoil dark matter. There’s nuclear recoil dark matter. There are solar axions. We’re actually looking for a lot of different candidates. And so we might choose to concentrate on one or the other, depending on what we find in the first chunk of data.

FLORA LICHTMAN: We’re talking about dark matter as one thing. Is there a chance it could be multiple things that we haven’t seen, and we’re just lumping them together in this cosmic junk drawer?

PRISCILLA CUSHMAN: [CHUCKLES] Yeah, in fact, I think that’s the most likely explanation, honestly. When we started this, we had a very specific theory in mind, and that turned out not to be a simple explanation for it. We thought, perhaps, it was the lightest member of a supersymmetric family of new particles at the masses we expected. But then supersymmetry was not found at the LHC, and direct dark matter experiments didn’t find wimps where they were expected.

So that just tells us nature is more exotic and interesting than we thought, but it might also go to what you’re saying, that when we talk about how much there should be, we’re always talking about a mass density. We know that from gravity, from astrophysical measurements, and from cosmology, that there’s 85% of the stuff of the universe is dark matter. There’s a lot of it there. We know a lot about it from gravity. But what we don’t know is how it interacts with standard model particles, the stuff we know and love.

And because we know the mass density, we know that if it was one of these wimps, then we should have seen it already. The fact that it’s taken us so long to find it tells us that there seems to be less of one kind. I don’t know if this makes sense, but to me, it really does open this door, which might be Pandora’s box, actually, of a lot of multiple candidates, perhaps. Maybe it’s a whole new, dark sector with a family of shadow particles. It might even lead us to a new way to understand gravity.

So in all of it, I feel encouraged, in one sense. We’re looking at a whole new parameter space where the lighter dark matter particles exist or would exist, and we might find several. And it could also not be a particle. Indeed, it could be a wave. And there are a number of experiments that are looking for axions or axion-like particles, particles that fulfill all of the limitations from cosmology and the relic densities we see, but come about it a different way.

FLORA LICHTMAN: That’s so interesting. I love this intersection in science. And so it feels to me almost where experiment meets not just theory, but almost philosophy.

PRISCILLA CUSHMAN: Well, I think it does, in a sense. There’s two motivations for me, for example. I think with all the evidence pointing to the fact that dark matter points up 85% of the stuff in the universe, I mean, its gravitational pull is responsible for holding our galaxy together. It’s the main engine driving the whole evolution of the universe, from the Big Bang to how we look out now and see the large-scale structure we do, and yet, we don’t know anything about how it interacts with normal matter. It’s sort of like a known, unknown.

We know there is this treasure trove of new discoveries to be made. And I just know that’s where the next breakthrough will be. But we’re not there yet. We need to explore all the different avenues. But it’s not like we don’t know it exists. And that’s what’s so– well, both frustrating, I suppose, but also intriguing.

FLORA LICHTMAN: Very intriguing. Dr. Priscilla Cushman is spokesperson for the SuperCDMS SNOLAB experiment and a professor in the School of Physics and Astronomy at the University of Minnesota. Thank you for coming on today.

PRISCILLA CUSHMAN: Sure. That was fun.

FLORA LICHTMAN: [LAUGHS] This podcast was produced by Charles Bergquist. And if you have a cosmic question about the nature of the universe or some mysterious shadow particle in your life, please give us a call. 8774-SCIFRI is our number. 8774-SCIFRI. We love hearing from you. Thanks for listening. I’m Flora Lichtman.

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