Detecting Dark Matter with Patrick Huber

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Travis

I think there are some questions that have been common throughout all of human history, and one of those I believe is, are there forces that play in our universe that we simply can't see?

So when I hear discussions about dark matter, I can't help but wonder if this may provide an answer to that question. But of course it also brings more questions to my mind. Like, what is dark matter? How do we go about searching for it? And of course, what does that mean if we are able to find it? Well, thankfully Virginia Tech's Patrick Hoover is an expert in this very subject and was kind enough to field these questions from me. Hoover is the William E. Hasinger Jr. Senior Faculty Fellow in the College of Science, a professor of physics and the director of the Center for Neutrino Physics. His research focuses on neutrino physics and he has helped build an internationally recognized program that has implications for both basic science and applications to global and national security. So Patrick and I talked a little bit about what neutrino physics even are because frankly I had no idea and the role they play in the search for dark matter. He explained to how they're currently looking at ancient rocks as one way to help locate this and shared some practical applications for energy that have come from this research as well as some philosophical implications of this search. So if you are curious about what's actually out there, I think this podcast will have a lot to offer you. I'm Travis Williams, this is Virginia Tech's Curious Conversations.

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Patrick

So I'm Patrick Huber. I'm a professor in the physics department and the director of the Center for Neutrino Physics.

Travis

What type of physics was that?

Patrick

Neutrino physics.

Travis

What is Neutrino physics?

Patrick

Neutrinos basically occur in nature whenever neutrons change into protons. So this is in radioactive beta decay and in the fusion inside the sun. And apart from that, you rarely ever will observe any consequences of neutrinos in your daily life. But obviously fusion in the sun is very important to us because ultimately all energy on the earth comes from fusion either in the sun or maybe from previous generations of stars. And also all elements in universe heavier than let's say helium or lithium come from fusion reaction stars. So it's quite important for us to be here to have neutrinos, although we usually don't notice them.

Travis

Yeah, it sounds like they're really at the core of a lot of things we probably just never think about about them though.

Patrick

Yes, that's correct. The other thing which is sort of fun about neutrinos is since they are very weakly interacting, this is the reason we're usually not feeling them in our daily lives, is that they can travel from environments where usually we do not get direct signals. So the sun, the light from the sun, which reaches us is produced about a hundred thousand years before it reaches the earth. takes the energy made in the sun a hundred thousand years to get from the center of the sun where it's made to the surface. The neutrinos which are made in this reaction take only eight minutes. So they can get out of the sun right away and they just travel straight to earth. And as a matter of fact, we can measure these neutrinos. This was one of the big achievements of neutrino physics. So in the second half of the 20th century to really learn how to catch neutrinos from the sun and measure them.

Travis

That is fascinating. want to ask how you do that, but I'm afraid that that might not be something I don't know. that super complicated?

Patrick

So the first real solar neutrino experiment was done by Ray Davis in the sixties. And so what he did is he took a big tank, sort of a few hundred tons of basically dry cleaning fluid inside a mountain. And that dry cleaning fluid for the purposes of neutrinos is made out of chlorine. And when a neutrino from the sun hits chlorine and chlorine atom, every once in a while, will convert the chlorine into an argon atom. Argon is a noble gas. so what...Ray Davis was able to do is he basically was able to extract the argon atoms, which are made in this big map of chlorine solution and count them. And the amazing thing is that he was able to basically count something like 30 atoms per month. You need to put this perspective. So this is a tank, 700 tons of stuff. And he was able to extract 30 argon atoms a month and count them and get it right.

Travis

Wow. That's incredible. that is, that is fascinating. And I don't know. think I saw another quote from you though. It said when you stumbled into something, you were like, this is insane. And I want to do it. Yes. I can't remember exactly what it was.

Patrick

Yes. Now this, this is about this whole poly detection thing, which is sorta, okay. So to set the bar is, I'm a neutrino physicist, so counting 30 atoms out of 700 tons of stuff is something people did 60 years ago. So that's the base level of craziness we start from in my field. And then we put things on top of that. And so this, this idea of poly detection is, is fundamentally that, that people have

 

had for many years the idea that maybe interactions of dark matter or any other particle for that matter could leave an imprint in old minerals, like let's say diamond. And the diamond is typically one or two billion years old. And the question is, can you find traces of events which happened inside this diamond a billion years ago? And answer is maybe, but why would you want to do this? And one of the reasons is that...We're looking for dark matter now for nearly a hundred years. So dark matter is essentially, we're calling it dark matter because we know it does not attract this light in the way other stuff does. Otherwise we would have seen it in the universe. And so we think we know about dark matter because if you look at how far stars fly around the center of the galaxy, you find out that stars which are very far away from the center fly much faster then they should. So a star which goes very fast, feels a very large centrifugal force. So if you're riding on a carousel, right, there's a force which pulls you to the outside. And the further away you're from the center, from the carousel, the faster this force should be pulling. And so you need a stronger force to hold you back in. And if you just hit the visible stuff in the galaxy, the stars, then the stars on the outside of the galaxy couldn't be going as fast as they are, or otherwise they would just be flown out because there's simply not enough gravitational pull to keep them in. And one solution is to argue, well, there's other stuff which exerts gravitational pull, which we just can't see. And that's what we call dark matter. And this doesn't happen only in our own galaxy. It happens in all galaxies where we can do this measurement. And it also happens between galaxies. So if galaxies fly around each other, they also fly much faster around each other, then can be accounted for by just accounting for the gravitational attraction of the visible stuff. And so the disaster physical evidence for dark matter has started to accumulate basically since the early, this was thirties of the last century. that, you know, in the second half of 20th century, it became quite obvious that we're missing a large chunk of the stuff out there. And today we can say with good confidence that basically for each, you know, kilogram of regular stuff we have, there should be about five kilograms of dark matter out there in the universe. And we have no clue what it is.

Travis

Wow. So we don't know what it is. And it sounds like that we basically know that it's impacting things. Like we see it. We see an imprint of something. Right. Something's doing something. And we're like, what is it?

Patrick

Yes. So it's not something we've just made up because we're bored. So we're really seeing the impact on the universe on grand scales, right? Galaxies and clusters of galaxies and so on and so forth. But already at the scale of the solar system, we're not really seeing any traces of it. So if I look at the orbits of the planets around the sun or anything like that, I'm not seeing evidence for that. And that's in principle quite consistent because in the solar system, you know, I have a lot of matter concentrated in a small space like the sun, for instance. And so this, you know, thin soup of dark matter, which flows through the solar system is in comparison, just not very much. And so that's why you're not seeing a direct gravitational impact of that. But the hope is that we can find dark matter. So as I just indicated, the sun flies around the center of the galaxy, takes about 220 million years to, you know, do one revolution.

But we're moving very fast. So our velocity is about 210 kilometers per second. That's really quite fast if you think about that. And the idea is that the dark matter in the galaxy stands still. So we will be racing through this sea of dark matter at a speed of roughly 200 kilometers per second. And that means that if we collide with one of these dark matter particles, it's like basically being hit by rain when you're running, right? So the rain comes down, you run through the rain. And you feel the impact of the rain also on your car windshield, instance, fairly strongly because you're moving relative to the rain. And that's the same idea for dark matter that we therefore could maybe see these little bumps dark matter is causing in our detectors. And of course the dark matter is much smaller, much lighter than a raindrop. So the strength of this impact is very, very tiny. And people have built very sophisticated, very sensitive particle detectors, which try to look for this. And so now you can think about the probability for a single dark matter particle to interact with a given atom in your detector is essentially set by particle physics. And that's a quantity we'd actually like to measure. don't know what this number should be, but this number is fixed. We cannot change it, right? Somebody set this number when the universe was created. And so to up our chances to see one of these interactions, what we can do is we can either use A huge number of atoms, right? If I use a million atoms, the chance is a million times higher. If I use a billion atoms, the chance is a billion times higher. And the same applies for the amount of time I watch. If I watch for one second, I have a certain probability, but if I watch for a thousand seconds, it's a thousand times higher. And so what you now can do is you can assemble one billion grams of detector and observe for one year, or you can take one gram detector and observe for a billion years. Same probability. Now the problem is that, that of course our detectors don't last a billion years. So you need to find something which is a billion years old. And on earth, the only things which are a billion years old are minerals. And then the problem becomes, you obviously couldn't wire up your mineral a billion years ago and take data for a billion years. So you need to find some effect inside the crystal, which has recorded what has happened over a billion years time scale. The only real thing is it has to record in its own crystal structure. So in a crystal, all atoms are supposed to be a very specific place. That's the very definition of a crystal. And so the idea is that you would be looking for atoms which are slightly out of place. And hopefully that gives you a hint that a dark matter interaction has occurred, let's say 500 million years ago.

Travis

So what you're doing is you're looking for dark matter now in these old rocks. Kind of like, I mean, almost sounds like you're looking kind of like for dinosaur prints for like.

Patrick

Exactly. That's why it's also called paleodetection because this is like paleontology where you look basically for dinosaur bones in some mud sediment, which was a river millions of years ago. And that's essentially very much similar, but on a much smaller scale, right? We're not trying to dig up bones, which you can hold in your hand. You're trying to find displaced atoms inside a thing, maybe the size of a diamond. So you're all doing this on an atomic scale. And that's why it's challenging. So you cannot use a shovel and a brush to do this. So you have to find technologies which allow you to find relatively small changes in a very large volume of material. And large volume means that you're looking for a few displaced atoms in something the scale of a few grams maybe. So, that's really the challenge. And that's really what our research is doing. We're trying to develop one specific method, which may be able to achieve this. And whether this then works really for poly detection is a separate problem because for poly detection, even if you have a technology which can, you know, find evidence for these dark matter interactions in the mineral, the other things which happened to this mineral over a billion years, as you can imagine. It gets heated up, it gets cooled down, it moves its depth by many hundreds of kilometers maybe over this period of a billion years. And you need to be able to distinguish all these changes from the changes made by dark matter. To make life more complicated, there's of course also natural radioactivity even deep inside the earth. Much of that natural radioactivity will change the crystal in the very same way dark matter would. And so that means that really...pulling off this poly detection is a complex multidisciplinary problem. We really need to not only understand your particle physics and your material science, but you really also need to understand the geoscience, the history of the samples you're looking at. so this is also the reason it's a really interesting problem. And so we are really at the first step of this thing saying, okay, let's forget about all these complications, but just in principle, can we prove that crystals can faithfully record this information over a long period of time?

Travis

Yeah, it sounds like you would need to use a lot of you need to lean into like your geologists and people like that know the fossil record and have all that stuff. Traditionally, how did people look for dark matter? Was it just a matter of using sensors and looking at?

Patrick

So there are basically right now two big ways we're doing this. One is called direct dark matter detection. The other one is called indirect dark matter detection. So let's start with indirect dark matter detection. Indirect dark matter detection tries to find fingerprints of dark matter in the cosmos. So we're not building experiments on earth, but we're looking out into the universe and try to find signatures of dark matter being there. So one of the most widely used ones is if dark matter can be made in the early universe, it means that it needs to be somehow interacting with ordinary stuff and maybe also interacts with itself. And if it interacts with itself, can happen? The two dark matter particles in the universe collide with each other and for instance, produce an X-ray photon. Okay. And that X-ray photon would have exactly the energy which corresponds to the mass of these dark matter particles. And so now you would be looking for a mono energetic X-rays that is in an X-ray spectrum of the universe. You're looking for lines, right?

These lines should happen more in places where there's more dark matter because dark matter needs to meet dark matter. So places which have a high density of dark matter should shine brighter in that specific line or color of x-rays, if you would like, than places which have a lot less matter. And places which have a lot matter usually are these cores of galaxies, like the center of our own galaxy. A lot of stuff is happening there. There also should be a lot of dark matter. And so you should see a relatively strong signal from there. And what has happened in this field, so every 10 years, somebody claims, we have seen a line from some place which should have a lot of dark matter. And then the astrophysicists come in and say, look, if you really look closely, there are some heretofore unknown astrophysical object, which actually makes your line and it's not dark matter. But the hope is that one day we find a signature where really, you know, I have to say, yes, this is dark matter and cannot be anything else. And the trick would be that you're not finding it in only one place, but you need to find the same line in many different places, which astrophysically are very different, but sort of in terms of dark matter, you understand how they correlate. To be sure the origin was dark matter, not just like in the most recent example with the galactic center excess, it was a population of unknown pulsars, which created that specific line. And that's sort of right now the consensus explanation. And then...Direct dark matter detection is, as the name indicates, direct. You're building a detector which is sensitive enough to really see individual dark matter particles slamming into your detector. There's a wide range of technologies people are exploring. It depends a lot on what kind of dark matter you're looking for because we know very little about dark matter. So we don't know what the mass of the dark matter particles should be. We don't know what type of specific interactions they would have. And so depending on that, different detection strategies are being pursued. One of the of most mature technologies, which is sort of a fancy word, maybe saying for one of the oldest technologies we're using is liquid noble gas detectors. So the idea is that you fill a vat with let's say liquid xenon. The dark matter comes in, it traps in, the xenon produces light and charges, and we have instrumentation which can read out this light and charge quite sensitively. And you put these things usually on the mountain to shield them from cosmic rays from above. And then you run the experiment for a few years and then you do a very careful job. And then you say, oh, we didn't find anything. And then you put a limit on dark matter. saying, okay, we now know dark matter cannot live in this part of our parameter space because if it would be there, we should have seen it. And then you build a bigger one and then you do the same thing again. And then you put a more stringent limit. And the hope of course, is that someday you stop putting limits, but you say, look. Here it is, we found it. But of course, so far this hasn't happened yet.

Travis

Yeah, it sounds like you're trying the indirect method, looking for the fingerprints, but inside these rocks.

Patrick

Right. In some sense, it's sort of a mixture of direct and indirect. It's more like direct experience because it's here on earth, but it's also more like the indirect ones because instead of looking far out into the universe, we look basically back into the past, which means that in both cases you have to deal with the problem that a lot of effects are not controlling. Right? If I'm building the detector and I have it in my lab, I can control what happens to it and I simply can make a better detector to exclude any radical influences.

And that's what the direct detection experiments do very, very successfully. If I use these indirect techniques, be it out in the universe or deep underground than a hundred million years ago, I have to develop techniques which allow me to distinguish non-dark matter changes to my environment, non-dark matter signals from the dark matter signal. And that is really the hard part in doing this.

Travis

Yeah, it sounds like, guess if you are able though to find it and start to identify some traces of these, I would assume that it was kind of like, kind of like what dinosaurs were, then you're able to piece together what that might be, like what characteristics it might have from these imprints. Right.

Patrick

And that's exactly one of the things which is also nice about this sort of looking back into the past. We are quite certain that the sort of amount of dark matter in the vicinity of the earth is not constant throughout the history of the universe. So for instance, galaxies undergo mergers. So they hit other galaxies and that's a pretty violent process, which usually leads to the formation of new stars and things like that. But also the dark matter clouds interact with each other.

 

And you therefore can have locally huge enhancements of the dark matter density. Our own galaxy has probably undergone several collisions throughout its multi-billion year history. And if you can look back in time, we might be able to find evidence of the history of our own galaxy in this way. And so you really get a new dimension which you do not have available with these other techniques.

Travis

That is fascinating. Is there just dark matter around us all the time? Like when I go to the store, am I just in all the stuff's happening and I don't even realize it.

Patrick

That is correct. So in terms of dark matter, you have a density, let me get this right. So the density of regular stuff in the universe on average is like one atom of hydrogen per cubic meter. And you have about five times as much dark matter around you. they're depending on the dark matter mass. So per cubic meter, there should be about five of these dark matter particles around you at any time.

Travis

So there's, there's even more around me than I guess, some of the actual stuff I know. Yes. That is fascinating. Yes. Yeah. Well, I read that one of the things that has come as kind of part of this project is some interest in energy and, and creation of energy from maybe some of the stuff that, that you are discovering, or maybe just some interest from, was it the U S department of energy in what you're doing?

Patrick

So the point is that when we do experiments to see whether we can see dark matter in print, since we don't have dark matter in the lab, I mean, we can't make it and we don't know what it is. We have to use a proxy to test whether our detection methods work and the proxy we use for that are neutrons. The neutrons behave for our purposes very much like dark matter. They come into the side of the crystal, change the crystal structure and lead. And so that's why neutrons for us are an extremely useful tool to develop these technologies. And while we don't know whether our detectors are good enough for dark matter, we know they're good enough for neutrons. That's how we show that they work. Neutron detection is now something which has a huge interest outside of a pure lab environment because neutrons are involved in any form of nuclear energy production. so nuclear energy, course, is a two-faced...animal, you would like, there's the peaceful use of nuclear energy, but then of course, there's also the military application of nuclear energy. And since nuclear weapons have been created, people realized that this is a technology which needs to be tightly controlled, given its huge potential for harm. And so there is a system of international nuclear non-proliferation safeguards in place, which tries to ascertain that that countries are not randomly embarking on nuclear weapons programs. And there's also, of course, an effort underway to eventually abolish nuclear weapons, which will require, of course, also verification regime. And that's where neutrons come in because neutrons can be used to, for instance, see whether a certain container contains a nuclear warhead or not. And that's where our detectors are interesting because our detectors do not require electricity, right? It's just a piece of rock, basically. And so if you want to go to a highly restricted facility, people usually don't let you bring your cell phone, for instance, if they haven't been to one of these facilities, you have to hand in all the electronic devices at the entrance because you never can be sure what kind of information electronic device will record and transmit. But if you just bring a piece of rock, then that's much easier for the owner of the facility to make sure that this rock really only records what you say, it records namely neutrons. And you can bring it out afterwards and analyze the neutron signatures from that. And this is really where we have sort of interests. So the Department of Energy, I'm not sure how familiar your listeners are with that. It's called the Department of Energy. It has been renamed many times. It started as the Atomic Energy Commission. And what it really is, its main purpose is and what the bulk of its budget does, it's in charge of the nuclear arms in the United States produces them, it tests them and also is responsible for storage and everything related to nuclear weapons. And then the Department of Energy also has an office of science, which for instance, funds directly dark matter research. So this is very fundamental curiosity during research, but that's a tiny fraction of its total budget. The bulk of the budget is really related to nuclear weapons and also the ability to detect whether other countries are producing nuclear weapons. That part of DOE is called the National Nuclear Security Administration. And our research is indeed supported by that part of DOE for this application as neutron detectors. And so that's quite interesting that you're going out to find dark matter and in the end you really start to look at things like arms control.

Travis

Yeah, I wouldn't have drawn that connection immediately if you told me that, but it is fascinating and it makes sense that some of what you're doing, like it has some, I guess, translational value in that sense.

Patrick

Also to me came as a surprise, but it's in some sense quite natural because dark matter detection or for that matter also pre-detection is a very hard detection problem. It's really very challenging to make detectors which can do this. And since they're of really pushing the edges of the detector technology, it's not necessarily surprising that these advances in detector technology then have real world applications. This has happened many times before. So when you go to the dentist today and get a dental x-ray, Nobody is developing a film anymore, but you're seeing the image of the doc dentists sees the image right away on the screen. And also dental x-rays today can be done with a fraction of radiation dose for the patient than they were done 50 years ago. These detectors in a dental x-ray machine are a side product of, you know, 70 years of particle physics, trying to build better particle detectors. So this has happened before and it's quite a natural thing to happen. And since we are pushing on the direction where we're really making essentially better neutron detectors and a lot of the things we involve use neutrons for calibration. It's not so surprising that you have this connection for somebody who's sort of steeped in technology. But of course the questions you're asking are very different. I mean, it's one question to ask is Iran building nuclear weapons and the other question is to ask, know, what is the universe made out of? The philosophic and motivations are very, very different, but it turns out In the lab you have to solve the exactly same problem.

Travis

It reminds me of hearing historically about some of the things that we have now because of the space race and the technology that was developed. You get these kind of like nice offshoots of, well, this can do this and it's very helpful for people. Yes, exactly. Take it and run with it. Well, I'm curious when you, when you're talking, when we're talking about dark matter, I guess I'm curious, like maybe from a theoretical standpoint, what implications might it have if we can figure out what dark matter is?

Patrick

Exactly.

So in some sense, it's, think all cultures have this, this philosophical need, if you would like, a spiritual need to ask questions about what the universe really is. Where do we come from? Where are we going? Why is it there? What is made out of? And this has inspired religion, the bulk of the arts, science. So it's really behind a lot of human endeavors, which go beyond just finding food and surviving. And I think in that. that sense, not knowing what the majority of the universe is made out of, to me as a scientist is deeply disturbing, right? We have been at sort of modern science for some 300 years and we've learned a lot of things. We've learned how to make, for instance, mobile phones like the one we're using to record this, are enormously complex devices, but yet there is, you know, five times more stuff out there in the universe, which we have no clue what it is. And also it's not a problem, which is new. We have known it's out there for a really, really long time and haven't been able to figure out what it is.

And the question then is of course, if it's really only, you know, featureless stuff, which just fills a missing gap in our understanding, then maybe it's not so interesting, but nobody says it has to be featureless stuff. It could be that they're dark stars, there could be dark planets, there could be dark civilizations. We don't know. Okay. And I find this quite mind boggling. So it may be that they're, you know, Somebody quote in the dark universe is having the same conversation right now where they say, well, we don't know what all these stars are for, but we can see them. I find this quite fascinating.

Travis

Yeah, are they calling us the light universe then? know

Patrick

that's what we don't know.

Travis

So you're just trying to figure out, right?

Patrick

Yeah, exactly. That's the idea. Exactly. so the possibility to even say something like this is quite astonishing. You can say, look, there could be five times as many planetary systems in the dark sector. They could have civilizations. Maybe the reason we never have seen, made contact with extraterrestrials is because they live in this dark sector and we simply can't see that. Right? I mean, who knows?

Travis

That's a lot to think about, but I think you're right in so many ways. It does seem to be the age old question that no matter where you're from in the world, you're kind of asking like, where are we from? What is this made of? know, wanting to know more about us.

Patrick

I think that that's really human nature. And I think the only thing which has changed over time is we have a tendency to find answers for that, right? So in the Neolithic age, they built things, they're cave paintings from 6,000 years ago, which clearly indicated that people were thinking about things which went beyond pure survival. People built things like Stonehenge to do basically astronomy. People built things like the pyramids to secure the afterlife. And, you know, we shot people to the moon. because we felt like this is an important thing to do. And I think the search for dark matter falls right in that line of thinking. We're just framing this in a much more mathematical language, but the underlying question is the same. The methods have changed, and as I always tell my students, people a thousand years ago were not less smart than we are, they just had less technology.

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Travis

And thanks to Patrick for helping us better understand neutrino physics and the search for dark matter. If you or someone you know would make for a great curious conversation, email me at traviskw@vt.edu. I'm Travis Williams and this has been Virginia Tech's Curious Conversations.

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