Cecilia Levy, an astroparticle physicist at UAlbany, is part of a massive international research team searching for direct evidence of dark matter in a defunct South Dakota gold mine. Levy explains how the painstaking search for invisible particles known as WIMPs could usher in a new era of physics — and offers a glimpse at how humans have stubbornly accumulated knowledge about a universe we still scarcely understand.
Lately, Cecilia Levy’s contributions to LUX-ZEPLIN’s hunt for direct evidence of dark matter come in the form of computational physics — that is, the extraordinarily complex analysis of the data coming out of the dark matter detector. (As a postdoc, Levy also contributed to its assembly and commissioning.)
Cecilia's data analysis work begs a more fundamental question: How would we know dark matter if we found it? What would the data say about a thing we cannot see?
We got deep into these cosmic weeds in a portion of our conversation that did not make the final edit but is nonetheless fascinating.
One programming note: You’re going to read a lot about xenon below. If you’ve heard of it before, you probably know that xenon is considered a noble gas — an element that does not chemically interact with much. It’s used inside LUX-ZEPLIN as a liquid, in part because it’s very dense and helps shield the inside of the detector from cosmic interference.
Here’s how Cecilia explained what happens next:
When you're looking at the data that comes out of the detector, what are you seeing that makes you say, “Aha! A WIMP just crashed into our xenon!”? What does that look like?
CL: The data analysis on something like this is absurdly difficult. Please understand that it's not just like, “Oh, boom, there was a little spike on an oscilloscope,” and we know we just detected the WIMP. It doesn't work that way at all.
You cannot actually detect the WIMP itself. What you detect is its interaction with the xenon nucleus. Imagine a billiard ball collision. One ball is dark matter; the other is a xenon nucleus. So you're playing pool, and the dark matter interacts with the xenon and then goes on its merry way. It's gone. But because it's deposited some energy into the xenon, the xenon has recoiled. And with this recoil, you get light, and then you get a little bit of electrical charge.
We look at this light, and then this charge, and we move it up into our xenon and make it interact again with other xenon atoms to get secondary light. What we're looking at is really the light that is being produced in our detectors. The xenon is what we call our target material. It's not the actual detector. The detector is all the light sensors around it. So the target material is what's going to say, “Hey, boom, there was an interaction.” And then all the light detectors say, “Okay, was there a flash of light in here? That's how we know.
Annoying question.
CL: Go ahead.
Why is that direct evidence and not indirect evidence? Because you're detecting the light, right? Light is an indicator of an interaction in a place so quiet that it almost certainly had to be dark matter. But you didn’t see the dark matter. You saw the light produced by its crash.
CL: Because you can’t see it!
Because it’s dark matter.
Right, you can’t. That's the whole conundrum here. If we could see it, we wouldn't have to do all this. And this is something that's really important to understand in physics. There are a lot of indirect things like that. Because very seldom do you actually see exactly what you’re looking for.
[gestures to her eyeglasses]
OK. Right now my glasses are on the table. You see them, you think immediately in your mind, “This is a direct detection of my glasses on the table,” correct?
Yes.
CL: Okay, well, I'm going to turn that argument against you. I'm going to say, “Actually, you do not see my glasses. What you see is the light from my glasses arriving into your eyes, which are the detector.”
Okay.
CL: Same thing. We call this direct [detection of dark matter] because what we are looking at is a direct interaction of a dark matter particle with our xenon nucleus. In our case, it's direct because there is a direct collision
And you can see the light.
CL: And I can see the light. The same way that you can look at my glasses because you see the light from my glasses.
Read about the latest results from UAlbany’s contributions to the LUX-ZEPLIN experiment
Check out this video from the Sanford Underground Research Facility to see what the LZ detector looks like and how they got it a mile deep into the mine.
There are more photos here from the Department of Energy’s Lawrence Berkeley National Laboratory, which is leading the project.
Audio editing and production by Scott Freedman
Photos by Patrick Dodson
Written and hosted by Jordan Carleo-Evangelist
[0:01] Host: Welcome to The Short Version, the UAlbany podcast that tackles big ideas, big questions, and big news in less time than it takes to cross the Academic Podium. I'm Jordan Carleo-Evangelist in UAlbany's Office of Communications and Marketing.
[0:18] Cecilia Levy: It's the biggest lesson in humility that you will ever have. Every class I start with, ‘Remember, we know nothing.’ We just don't. We have to accept that.
[0:32] Host: In this episode, we take a field trip to the zoo. Not the animal zoo, with giraffes in lemurs and such. That's a different podcast.
We're going to the Particle Zoo. That's the term physicists like UAlbany’s Cecilia Levy use to describe the many subatomic particles that make up our universe. We're talking electrons, gluons, quarks — the building blocks of all the cosmic stuff that has ever existed.
Here is probably a good time to note that everything we can see and have ever seen amounts to only about five percent of what scientists believe is actually in the universe. The rest — well, they’ve spent decades trying to figure that out.
Much of that everything else is believed to be dark matter and dark energy, which math says must exist for our observations of the stars to make sense. But nobody has directly detected it yet. It's not like a new species of insect you can capture in a mason jar.
Dark matter is invisible because it doesn't interact with light, but we know it's there through its gravitational pull on the galaxies we can see. It interacts so seldom with ordinary matter that nobody's even sure where to look for it.
Cecilia, an associate professor and astroparticle physicist, is part of an international research team that thinks the cosmic quiet of a defunct South Dakota gold mine is as good a place to look as any. She and several UAlbany colleagues are part of the LUX-ZEPLIN project funded by the U.S. Department of Energy. LUX-ZEPLIN, or LZ, is the world's most sensitive dark matter detector, located deep inside the mine. It's a giant vessel filled with seven tons of liquid xenon. They're looking for the telltale flashes of light triggered by collisions with a kind of dark matter known as Weekly Interacting Massive Particles, WIMPs.
The WIMPs haven't shown themselves yet.
Even so, Cecilia explained why the hunt for dark matter is important — even if we never find it — and what it illustrates about humanity, scientific progress, and the way we've methodically built an understanding of the universe. Like a child building a tower of blocks — only some of the blocks we can't see.
Here's our conversation.
[2:05] Host: How did you end up doing this? So if you were 10 or 11 years old — ‘When I grow up, I'm going to be an astroparticle physicist studying dark matter.’ I mean, what was that path? How did you get interested in the things you had to be interested in to end up here?
[3:12] Cecilia Levy: I was nine years old. I loved the stars. I loved the universe. Back in the days you could order these magazines that would come in and there were pictures of the moon and of galaxies, and I was bought a book on space and pretty much I was nine years old and I was going around telling people I was going to be an astrophysicist. And back in the day, usually I was met with laughter, lots of criticism, and a lot of, “You're a girl, why are you doing that? Don't you think there's better things to do for a girl?” I was like, “I'll show you.”
So initially I wanted to be an astrophysicist, and then when I was in college — I knew I was going to go into physics in college — I actually started taking astronomy classes and I was like, well, actually, I'm not that into it. And turns out I actually loved particle physics. And then my first research experience was as an undergrad. And that's why I always tell my undergrads you need to have research experience. This is what can shape the rest of your life or not. But this is when you need to know. And it turns out that I did my first research experience as a senior on dark matter, and I did my PhD on this, and then I came here and then I swept to LZ as a postdoc. And here I am.
[4:37] Host: When we say astroparticles, what are we talking about?
[4:39] Cecilia Levy: When we say astroparticles — we have what we call the Particle Zoo. So all of the particles that make up normal matter, if you want, the stars, everything we can see. But we know that our vision of that is incomplete, and we know that there needs to be some other particles that are there. And one of them, which is the main one here, is called a Weakly Interacting Massive Particle. And that's actually what would make up dark matter.
[5:09] Host: We know that there must be other particles because the laws of physics, as we understand the universe works, and the math tell us —
[5:18] Cecilia Levy: If you look at everything we know now, there's a ton of different things we've observed that we are kind of like, “Huh, this doesn't really work the way we expected it to.” And so we know that our standard model as it is now is not complete. It's broken. No field of physics works in a vacuum. So every field of physics is really linked together. And so at the same time, you are looking at astronomical observations and you see indirect evidence that there is this dark matter. If you look at the universe, what is the universe made of? The universe is made of us — what we call baryonic matter, which is everything we can see. That’s barely 5 percent of the universe. The 95 percent remaining, we really don't know what it is — but we think about 25 percent is dark matter and 70 percent is dark energy. And the reason we know this is because we look at tiny, tiny fluctuations of the universe and pretty much to have the thermal map of the universe and for it to match, we need to have 5 percent normal matter, 25 percent dark matter, 70 percent dark energy. Otherwise, we can't match the data that we observe. So the way I always say it, it's like the wind. So if I ask you: How do you know that there is wind?
[6:46] Host: You feel it
[6:47] Cecilia Levy: Exactly, or you are going to see the leaves move in the trees, but what's the wind made of? If you don't have a way of looking at particles, you see that there has to be wind because leaves are moving. But you can't actually see it directly. The way stars move in the galaxy, there has to be something else, otherwise it doesn't match what we know. You know, Newton's 17th-century physics, okay?
[7:15] Host: We have all this indirect evidence that it exists. And the fact that it would exist explains a lot about how we observe the universe. Why is it important to get that direct evidence? Why are we spending hundreds of millions of dollars in an old gold mine beneath South Dakota to find that direct evidence?
[7:37] Cecilia Levy: That is why we do science. I mean, it's always to say, “Huh, this is weird. Let's explain why it's the way it is.” So that is the most, I think, fundamental question, which is simple curiosity. The real answer is that this is how humanity progresses. This is how we have technology today. Because at some point someone said, “Huh, let's study this.” And I always try to make a bit of an analogy with, say, the electrons. You can imagine back in the days, you could have said, “Why do we care about electrons?” And today I would not be able to record this without electrons. You wouldn't be able to have your cell phone, electricity, whatever. And the applications of dark matter right now — we don't really even know its properties. We are at the infancy of a discovery —we're hoping, of course. The first thing to say is that, “Oh my God, we've detected it. So yes, we've got an answer.” Once we have detected that, we can start thinking about what actually we can use it for, and I am convinced that we cannot possibly fathom all of the use that this is going to have in the future.
[8:49] Host: The LZ experiment in South Dakota is looking for the Weakly Interacting Massive Particles — the WIMPs. Why is it important that it is located where it's located?
[9:01] Cecilia Levy: It is a Weakly Interacting Massive Particle. So they interact almost never with normal matter, but almost never is good enough for us to try to detect it. So the problem is we are on Earth and our detector is not impervious to everything else, so that means that we're going to detect every other known particle to man. And so we go underground so that we shield mostly from cosmic rays. Essentially what we did is we put LZ in a very, very quiet place — as quiet as we can find.
[9:39] Host: A cosmically quiet place.
[9:41] Cecilia Levy: Yes, you have the rock all around it and the earth itself becomes your shield. So that means that if a normal particle, something that we know comes in, it's going to interact with the Earth and not with LZ because by the time it gets to LZ, it will have lost all of its energy. It's not going to be able to interact in there. But dark matter — dark matter does not interact, and so it actually has a very good probability of just going straight through the Earth and actually interacting in our detector.
[10:13] Host: What I love about the LZ stuff is that it illustrates a part of the scientific process that I think the general public who haven't spent their lives in science don't really understand. We tend to think of things in very binary ways like, “Did you find it or did you not find it?” But reading the papers that have come out of the LZ experiment, you realize in the process of not finding it, you learn a lot that helps sort of narrow the search or refine the search. What does that look like from your perspective?
[10:44] Cecilia Levy: What people don't understand is that — and it breaks my heart to have to say it, but it is the truth and we have to accept that — science is excruciatingly slow and boring. And when I say this, it's not that what we find is boring; it's that the day-to-day activities, it just takes really, really, really stubborn people to make it happen. Because, you see, I mean in the dark matter field we've been looking for that thing for, what, 35 years? And we haven't stopped. And the other thing is that you never do things all at once. This is how humankind learns. You are assembling a tower, so you have to start with a very good base, and then you can build on top of that base. And this is humankind’s approach to knowledge. And so from our perspective, it can be frustrating.
These experiments started with a completely different technology. Then that technology had limitations, so then a new technology emerged, and so when we go and we ask for money, well, nobody's going to give us money if we just come out of nowhere saying, “Oh, please give us $75 million to build this giant experiment that is completely untested before, and we have no idea whether or not it's going to work.” Nobody's going to give us money. They would be crazy to do so. So that's why you start little and then you grow again. You are building a tower
[12:14] Host: The day after LZ or some other future experiment detects and confirms direct evidence of dark matter, what does that mean for humanity? What does it mean for our understanding of the way for the universe works? How are things different?
[12:31] Cecilia Levy: Okay — for you, day to day, not a difference. Theorists are going to have a field day, and probably we are going to be uber criticized by the entire physics community, which is, and I really want to be very clear, a good thing. This is as it should be because no one should come in and make an extraordinary claim like this and not have everyone first thing say, “Hold on. Full stop. Prove it to me or I don't believe you.” Because this is the hallmark of good science. This is where our business is not about believing. Our business is about proving. In physics, it would be spectacular. That would be the first time that we find evidence of something beyond really the standard model. It just opens up an entire new era of physics.
[13:20] Host: What if we never find direct evidence of it? Does it undermine the models or is it just, like, no, the models make sense. We know it's there. We don't have direct evidence of it, and that's a bummer, but everything else tells us it's there.
[13:37] Cecilia Levy: The truth is that our models have already changed because initially, with our simplest model, we should have found dark matter ages ago, and we didn't. That means that the simplest modeling, it's not correct. And so that is always that balance. You have to adjust things. And if we don't find dark matter, well, we're going to be very disappointed. But what I really want to point out is that what we will not have found is this particular flavor of dark matter that we were looking for exactly. A null result is awful, but it's also so important and informative. So what I will say is that, yes, of course we want to find dark matter. That is obvious, but if it's not there, we're still learning tons in the process, and we're learning what it's not.
[14:23] Host: Why is it important for the world, the federal government, to fund research like this that doesn't have — even if it's successful — doesn't have an immediate, obvious application at a time when people are sort of skeptical of science and the expense of it? Why is that important?
[14:48] Cecilia Levy: Oh, let me be very candid with you because right now, for tomorrow, it's absolutely nuts for your kids. You want them to live back in the dark ages? Because that's what you're going to do. You don't fund us now — your kids are paying for it. Science has three goals. When you do an investment, if you have a portfolio, are you going to put everything in short-term return? No, you're not. You're going to put short term, you're going to put medium term and you're going to put long term. Exactly the same thing with science. You have to have those three. You have to fund stuff that right now are going to give you immediate application. That's fantastic. But you need to fund stuff now that right now does not have immediate application. But a hundred years from now, yes you will. You're investing in the future.
[15:39] Host: That was Cecilia Levy, an astroparticle physicist and associate professor in UAlbany's Physics Department. Cecilia explained how scientists are hunting for dark matter in a defunct South Dakota gold mine.
If you're like me and you don't quite understand what it would actually look like to find a thing we cannot see and that almost never directly interacts with ordinary stuff, be sure to check out The Longer Version in our show notes.
The Short Version would not be possible without contributions for many people, including audio production and editing by Scott Freedman in the UAlbany Digital Media Studio, located deep — but not quite as deep as LUX-ZEPLIN — inside UAlbany's Podium tunnels.
We'll be back next week with another conversation about something interesting.
I'm Jordan Carleo-Evangelist here at the University at Albany, and this has been The Short Version.