370 | Daniel Akerib on a Faint Hint of Dark Matter

One reason it's been so hard to directly detect the dark matter is that we're not sure what it is. But for a long time, a leading candidate has been "Weakly Interacting Massive Particles" (WIMPs), so an impressive amount of experimental effort has been put into trying to detect them. Recently, a single provocative event appeared in the LUX-ZEPLIN https://en.wikipedia.org/wiki/LZ_experiment (LZ) dark matter experiment. Daniel Akerib, a physicist at LZ, joins us to talk about dark matter and the recent data, which has low statistical significance but is intriguing nevertheless. Blog post with show notes and transcript: https://preposterousuniverse.com/podcast/2026/10/05/370-daniel-akerib-on-a-faint-hint-of-dark-matter/ Support Mindscape on Patreon https://www.patreon.com/seanmcarroll. Daniel Akerib received a Ph. D. in physics from Princeton University. He is currently Professor of Particle Physics and Astrophysics at the SLAC National Accelerator Laboratory and Professor of Physics, by courtesy, in the physics department at Stanford University. He has been searching for dark matter since the early 1990s, first with the Cryogenic Dark Matter Search and more recently with the LUX and LUX-ZEPLIN https://lz.lbl.gov/ projects. SLAC web page https://faculty.slac.stanford.edu/person/daniel-akerib Stanford web page https://physics.stanford.edu/people/daniel-akerib Paper: "Search for dark matter particle interactions in an extended nuclear recoil energy window with the LUX-ZEPLIN (LZ) experiment https://arxiv.org/abs/2609.02823" Wikipedia https://en.wikipedia.org/wiki/Daniel_Akerib

Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.

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• 1:10Sean Carroll?

  Hello, everyone. Welcome to the Mindscape Podcast. I'm your host, Sean Carroll. You might have heard in the news, you know, we're not up on the news here at Mindscape. We don't actually chase after every ambulance and try to fill you in on the latest wrinkle in scientific or other intellectual discovery. But occasionally, it's worth, you know, changing the rules a little bit, doing something a little bit different. So you may have heard that there was a press release out from the LUX-Zeplin experiment, or just the LZ experiment. This is an experiment deep underground, looking for dark matter, looking for the occasional dark matter particle that will come into their detector, bump into a nucleus, and leave a signal. And the new result is, you know, there's many experiments that have been doing this

• 1:55Sean Carroll?

  for quite a while. Now they've seen an event that they can't explain. And we're going to dig into this for two reasons. One is, it might be the dark matter, and that would be very exciting. It's not at all super statistical significant right now. It's, you know, a little hint of a clue. It's absolutely not something you should get overly excited about. But it's also respectable, right? I mean, these are people who know what they're doing. They tried very hard to kill it. This is what good experimenters do. When they get a weird signal, something that they didn't anticipate, or something that would be very exciting, they really put a lot of effort into figuring out, could it have been something else?

• 2:38Sean Carroll?

  And they can't figure out that it could have been anything else. Even though it's only one event, it still kind of sticks out like a sore thumb. So I think it's worth getting on top of that, figuring out, what might it be? Why are we interested in this? That kind of thing. But the other reason is it is a good lesson in how science works. It's how things actually get done to find something and not know whether or not it's the final answer or not. You know, we get a lot of, in the modern age, nonsense out there about how science works. And it's good to like, really hear from a true expert, the process by which we gather

• 3:20Sean Carroll?

  data, and then interpret the data, and try very hard to understand what it could be, and all the different people working on this, and whether or not it turns out to actually be the dark matter. This is top-notch science in action as it's supposed to be done. So today's guest is Daniel Akerib, who is a physicist at SLAC. I think SLAC keeps changing its acronym, but it's the Stanford Linear Accelerator is what it used to be. Anyway, it's the Particle Physics Laboratory, associated with Stanford. And Dan is someone I've known for a long time. He's been working on dark matter searches for a long time. There's different kinds of dark matter, as we'll talk about, different kinds of searches. He and his teams have been looking for WIMPs, weakly interacting massive particles.

• 4:05Sean Carroll?

  For a long time, WIMPs were the leaders as the theoretical candidates for dark matter. They're not the only ones out there. But we've been looking for a long time, and we haven't seen them yet. That doesn't mean they're not there. Maybe they're just out of reach. Maybe this is the first hint that we are seeing them. So I love it as an example of real science, of the care, the level-headedness, the tentativeness that real scientists have to go through every day. There's no answers in the back of the book. You can't hurry the process. You have to sit through it and be careful. So whether they get more data points, whether there are more events, and we figure out this is the dark matter, or whether it someday goes away, or whether it's just a mistake

• 4:50Sean Carroll?

  of some sort, all these are possible, we'll know. It's good science. In the meantime, we'll learn a lot. You folks who are listening here will be on top of what needs to be understood when more information comes in, both from this experiment and from other experiments doing similar things. So let's go.

• 5:25Sean Carroll?

  Dan Akerib, welcome to the Mindscape Podcast.

• 5:28Speaker 02

  Sean, it's great to be here. It's good to see you.

• 5:30Sean Carroll?

  It's been a long time. Dan and I have known each other for a long time, both when we were at different Midwest institutions, but now scattered to different coasts. But it's good to catch up again. Now, you have devoted your life to looking for something that nobody has ever seen. So tell us a little bit about dark matter. What is it? Yeah, let's go very general here. Why do we think it's there?

• 5:53Speaker 02

  Yeah, okay. Right. So very, very generally, either we don't understand the laws of gravity, or there's stuff that's missing. And a simple example that I like to use is that solar system, we understand since Kepler and Isaac Newton, how it fits together. The sun's gravity keeps the planets moving in their circular orbits. The outer planets take a lot longer to orbit because gravity, strength of gravity falls off. It's much weaker out there, and so things move more slowly, just like a ball on a string. You can twirl it hard and fast, tension in the string, or a weaker, more gentle force, things will move more slowly.

• 6:33Speaker 02

  Apply that same physics to the galaxy as a whole, the Milky Way, and things seem to be moving much too fast, given the amount of stuff that is present in stars. So for about 100 years now, there's been this mystery of we see gravitational forces that we can't account for with the mass that should be giving rise to it. And I'm a particle physicist by training, and it was understood at some point, let's say roughly sometime in the 80s or 90s, that ordinary stuff couldn't make up the difference,

• 7:14Speaker 02

  that ordinary matter, protons, neutrons, the periodic table, anything that's been made in an accelerator, can't account for this missing stuff. And so that's kind of cool, because as a particle physicist, I latched on to this hypothesis, which is, there must be a new form of matter to explain the dark matter. And so let's go hunt for new particles. And that sounds out there, but oftentimes, over the course of, you know, nuclear and particle physics, going back to the 20s and 30s, a favored conservation law seemed to be violated. Well, we can't give up on this law. Let's invent a new particle.

• 7:57Speaker 02

  And, you know, over the fullness of time, often those particles were discovered. And so we're working kind of in that tradition. Let's not be so cavalier to throw away the laws of gravity. Let's see if we can find the missing stuff.

• 8:12Sean Carroll?

  Well, you mentioned this possibility that gravity is to blame. Let's sort of get that out of the way. I mean, maybe it's true, like you said, but how seriously do you take that possibility?

• 8:25Speaker 02

  That one could modify the laws of gravity?

• 8:27Sean Carroll?

  Yeah.

• 8:28Speaker 02

  For me, you know, I have the option of just sidestepping that, because that's not my area.

• 8:36Sean Carroll?

  Right.

• 8:36Speaker 02

  I don't come up with new theories of gravity. People have been working on new theories of gravity for 30 years. I would, you know much more about this than I do, so I'm not sure I'm really going to answer your question there.

• 8:51Sean Carroll?

  No, I mean, it's perfectly okay. I guess I'm just asking you, like, personally, do you go to sleep at night thinking, like, what if the dark matter just isn't there?

• 9:00Speaker 02

  Well, I do that anyway, you know, because we've been hunting for 30 years or more. If, when I worry about it not being there, I don't worry about it not being there because I worry that the laws of gravity are different.

• 9:15Sean Carroll?

  Right.

• 9:16Speaker 02

  But that's, part of that is just, you know, you, as a scientist, you place your bets. Based on the coins you have in your pocket, what problems can you approach? Right? And I think, you know, the, the nature of research, if we could go a little bit meta is, you know, it's easy to sit back and say, I wonder what everything's made of, right? That's not very helpful unless you can ask that question in a way that might be answerable with a new theory or with a measurement that you can go make in the laboratory or an observation that you can make. And so I think the choices we make as scientists are, what question looks cool and interesting? And am I a person who with the tools I have and

• 9:59Speaker 02

  the collaborations I'm part of that I can approach that question in a, in a useful way?

• 10:04Sean Carroll?

  By the way, if at any point you're tempted to go meta on science or the scientific process, you have come to the right place. This is where you're allowed to do that. Don't feel guilty about that. But this leads exactly into the next question because you say, okay, there's new particle. Let's put aside the gravity thing. Let's imagine dark matter is the simplest explanation. I mean, plenty of particles are hard to see. So one more isn't that much of an ask. How much can we sort of infer about this? Is it more or less if there's dark matter of the form we suspect, then we more or less know what it has to be like? Or are there many, many, many different possibilities?

• 10:41Speaker 02

  So, so we kind of turn to unanswered questions in particle physics, which in, in many ways arose independently. So there, you know, we have the standard model of particle physics and the experts, which I am not, I'm a practitioner, will tell you that, well, you know, this spectrum of particles that we have and the fundamental forces, six quarks and leptons and da, da, da, da, seems very arbitrary. You know, there's mass scales involved and grand unified theories and it, and it looks in many ways unnatural. So people have come up with theories like supersymmetry and others that maybe have now gone out of fashion,

• 11:21Speaker 02

  where let's fix, you know, let's get a more fundamental theory that seems more natural. And the kind of stupid analogy I like to make is that imagine we have the periodic tables of the elements with all these patterns and ionization potentials, valences and all of that. But we didn't know about quantum mechanics. Then it would say, oh, we have this interesting structure of periodic table, but there must be more to it. There must be a more fundamental theory. And quantum mechanics discovered, explained all of this, right? Chemistry is in a sense understood through the laws of quantum mechanics, right? And so I know this analogy doesn't really hold to strict scrutiny, but the standard

• 12:04Speaker 02

  model of particle physics seems arbitrary in many ways. And so is there a deeper underlying theory that will shed light on the structure of the thing that we see? And when theorists have come up with different, you know, new theories like supersymmetry or technical or whatever, they invariably predict new particles. And the connection that was made, I don't know, 30 plus years ago was that, oh, astrophysicists and cosmologists are looking for this missing mass and particle physicists are looking for these underlying explanations to the standard model. And they seem to be pointing in the same direction. We should go look, right? And that's not the only one. There's, there's, you know,

• 12:47Speaker 02

  in the theory of strong interactions, this is the strong force that holds the nucleus together. There were fundamental symmetries that seemed not to be violated and they seemed like they should have been violated. And so the theory was come up with to solve the strong CP problem. And this was Helen Quinn and Roberto Peche, and they proposed a new symmetry and that new symmetry led to the prediction of another particle called the axion, which behaves quite differently from the one that we're looking for, which is the WIMP, a weakly interacting massive particle. It might be a supersymmetric extension to the standard model. Others are looking for axions, which would appear in detectors in a quite different way. They would be wave like. I'm sure we'll go into this more. We're

• 13:32Speaker 02

  looking for billiard ball scatters. They're looking to tune in the axion radio station. So there's different clues from particle physics that we've been, that we've been looking at. About, I don't know, 10 or 15 years or so ago, as we've been carrying out these searches and nature wasn't yet yielding to our experiments, new ideas came up, which was rather than just be guided by missing, you know, open questions in particle physics that could predict particles, that could be the dark matter. This problem is so fundamental. Our, our axion searchers and our WIMP searches, WIMP searchers, they're doing a wonderful

• 14:14Speaker 02

  job, but nature hasn't yielded yet. And so let's expand the portfolio and let's change the question a little bit and say, is there anything that isn't disallowed, did I get my double negatives right? Isn't disallowed by particle physics that could also produce dark matter? Right? And so let's, let's expand the portfolio in that direction too. And that gave rise to this whole thing called dark sector. There might be dark photons, et cetera, et cetera. And so, you know, it's, it's a wide open field. There are, I don't know, 30 decades of mass of different particles that physicists can envision that might be detectable or could be produced in accelerators, et cetera, to try to, you know, get at this really fundamental problem.

• 14:58Sean Carroll?

  And for better, for worse, different kinds of particles, it sounds like, require different experimental strategies.

• 15:04Speaker 02

  Absolutely. Right? And so as these, you know, theoretical ideas were proposed, of course, they weren't done in a vacuum. It was like, oh, this could be searched for in a beam dump. This could be looked at, you know, at CERN and high energy collisions, et cetera, et cetera. Right? So.

• 15:20Sean Carroll?

  And again, for the audience that's out there and may have heard a little bit about some of these buzzwords, there are WIMPs, like you mentioned, weakly interacting massive particles. And you also briefly mentioned, you know, supersymmetry is an idea that may have something to do with that. So are WIMPs part of supersymmetry or are they related somehow?

• 15:40Speaker 02

  So as a, as a, as a WIMP hunter, we get to be a little bit theory agnostic, right? And we would be sensitive to any particle that could have been produced in the big bang that, that interacts with ordinary matter. It's useful to have specific targets. And so, you know, we, when we run an experiment and we exclude some parameter space, we can say, you know, some particular parameter space of supersymmetric models appears not to explain the dark matter. And, you know, there are many orders of magnitude that we haven't searched. So there are models to hunt. It's a way of connecting with other

• 16:23Speaker 02

  techniques. So there are, you know, what, what we do, we call direct detection. This is a WIMP directly scattering from the nucleus in our detector and registering a signature. WIMPs could also collect in large gravitational bodies at the center of stars, at the center of the galaxy. And WIMPs and anti WIMPs could annihilate and produce a gamma ray signature or some other signature. And so by, by couching it in terms of some specific models, it allows us to compare and contrast with, you know, other, other types of techniques of looking for dark matter. Supersymmetric searches at colliders like at Fermilab and now at the

• 17:06Speaker 02

  LHC. And so they're like, you know, we do or don't see this model. Might you be seeing something that is connected with this?

• 17:14Sean Carroll?

  And I think you mentioned something like 30 years. I mean, you, we, the community have been looking for this for a very long time. Is, was there some feeling that we should have seen it by now? Are we a little bit surprised it's not there yet?

• 17:27Speaker 02

  So yeah, so if we go back to the nineties, I guess, when supersymmetry was, you know, it was right around the corner, right? It was felt that this is a very natural explanation for, you know, these questions in particle physics. It's such a golden opportunity for nature to have picked supersymmetry in the early universe and produced dark matter that we could now detect. And it was thought to be a very natural scale at which to produce dark matter. And we've looked through, I don't know, seven or eight orders of magnitude on the WIMP search side. We are probably now pushed to regimes where, you know, the

• 18:08Speaker 02

  cognoscenti would say, well, if it was natural, it would have kind of showed up by now, right? And I think this is part of what led people to invest real time 10, 15 years ago to say, let's, you know, it might still be there and we're making progress, but maybe that isn't what nature chose. And so let's expand the portfolio. So I think that's, that's a very just natural reaction of the scientific community, right? To say, let's, let's grow the portfolio. Let's keep looking, but let's grow the portfolio.

• 18:38Sean Carroll?

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• 20:33Sean Carroll?

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• 20:39Sean Carroll?

  So the WIMP idea as a candidate for dark matter is something that is still alive and kicking, but the fact that we haven't found it yet hasn't led to despair. It's led to, let's think of what else it could be, because there's plenty of theoretical possibilities.

• 20:53Speaker 02

  That's absolutely right. Yeah.

• 20:55Sean Carroll?

  Yeah. But you're in Wimpland. That's what you're looking for.

• 20:57Speaker 02

  I am in Wimpland. Right. Right.

• 20:59Sean Carroll?

  And how are you doing that?

• 21:03Speaker 02

  How are we searching?

• 21:03Sean Carroll?

  How are you searching? What does the experiment look like?

• 21:06Speaker 02

  Perfect. Right. So I'm a simple-minded guy, Sean. I, billiard ball scattering is such a simple concept. So our detector is made of many, many, many, 10 tons, if you will, of xenon billiard balls, condensed into a liquid.

• 21:27Sean Carroll?

  Sorry, when you say xenon billiard balls, you mean the actual atoms?

• 21:31Speaker 02

  I mean xenon, actual atoms of xenon. And we're going to think of them as microscopic billiard balls in the context of the experiment. Dark matter being, having very rare interactions. You want to stick as much instrumented target material in its way in the hopes that dark matter is going through your detector. By having more detector mass, you'll improve the chances that a dark matter particle will hit a xenon nucleus. That xenon nucleus will recoil and register a signature. So the current experiment that we're running, the LUXZEPLIN or LZ experiment in South Dakota, which has been taking data for, I don't know, when did we start taking data, 2023?

• 22:15Speaker 02

  Is comprised of the inner chamber, seven tons of liquid xenon. So picture a titanium vessel, about a meter and a half in diameter, about a meter and a half tall of this liquefied xenon. So it's about minus 100 Celsius in temperature.

• 22:32Sean Carroll?

  Okay.

• 22:32Speaker 02

  Easily cooled with liquid nitrogen as a cooling thing that you can just buy industrially.

• 22:40Speaker 02

  And this xenon is about three times the density of water. And when particles interact with it, it produces small flashes of light, and some of the xenon gets ionized. So electrons are freed up. And with a special system of electrodes, we can drift those electrons and pull them into the gas phase at the top of the liquid surface.

• 23:05Speaker 02

  We get scintillation light from the event where, you know, that first interaction. So light and charge. The charge turns into a second light signal by recording these light signatures with arrays of photomultiplier tubes. We can record event by event what happened in the detector and where. And by measuring these two signals separately, we can learn things about what, you know, what sort of particle may have interacted. Many of the interactions are mundane. Residual radioactivity in the environment of the experiment.

• 23:42Speaker 02

  Background neutrons from radioactivity that might scatter multiple times or leave signatures in other parts of our detector system. So all of this is about trying to, you know, find a needle in a haystack. A very weak signal of unknown strength in the face of enormous backgrounds. We put the experiment a mile underground in a former gold mine in South Dakota. We surround it with eight meters of purified water. When we have additional detectors outside this titanium tank to look for, you know, residual backgrounds, gamma rays from radioactivity, neutrons, et cetera. Then we even, within the xenon tank,

• 24:26Speaker 02

  we can locate exactly where the event took place. So a meter and a half in diameter, we can locate the event down to a few millimeters. If it occurred near the edges of this tank, like, let's say, the outer 10 centimeters of the xenon, then we can say, oh, too high a chance. It's due to radioactivity. So let's only focus our dark matter search on the inner four or five tons. And we'll use that outer ton or two as a shield, as an active shield. Okay? So now we're trying to create this very, this inner sanctum that is very quiet in terms of radioactivity, radiation being able to penetrate, et cetera. And then we wait and we look for a special signature.

• 25:09Sean Carroll?

  One thing I just got to remark on is you say something like seven tons of xenon. And I'm, I, my imagination thinks that's the size of a house. But it's something a meter across. It's not that big.

• 25:20Speaker 02

  Right. You can, yeah, sort of outstretch your arms and touch the edges of the detector.

• 25:25Sean Carroll?

  What fraction of the world's xenon population are you using?

• 25:28Speaker 02

  Yeah. So, so the annual production of xenon worldwide is somewhere between 60 and 100 tons.

• 25:38Sean Carroll?

  Okay.

• 25:38Speaker 02

  So we're using a tenth of world's production. We bought it over a few years. Try, but you want to buy it without spiking the price.

• 25:48Sean Carroll?

  Yeah.

• 25:49Speaker 02

  It can get pricey depending on industrial uses. It's about one in, one in 10 million atoms in the atmosphere is a xenon atom. And so you liquefy massive amounts of air for industrial uses. And you, and you distill out the rare gases, xenon, krypton, and I think neon also is from distillation.

• 26:15Sean Carroll?

  I know there are supply chain issues in the world today. Do you need to worry about your xenon supplier?

• 26:20Speaker 02

  Right, right now, it's okay. I mean, you buy the xenon once and you, you know, you safeguard it. You have all these redundant systems. So if you lose your cooling power and the xenon starts to boil, you turn on your backup generators. You know, you, you, you do everything you can to never return it to the atmosphere.

• 26:44Sean Carroll?

  But it doesn't go bad.

• 26:46Speaker 02

  It doesn't go bad. No, no, no. It's, it's not consumed. Yeah.

• 26:51Sean Carroll?

  And so you, so you have this, you know, meter and a half or whatever sized tub of xenon. You've protected it from, say, layers of shielding. You put it a mile underground, and, which is amazing. But also, there's, there's a history of that. Like, talk about this idea of putting physics labs underground.

• 27:10Speaker 02

  Right, right. So, so I guess one of the, one of the early ones was done by a guy named Ray Davis, who was interested in detecting neutrinos from the sun. And I think, you know, I don't know what the original motivation was. Maybe it was just test the standard solar model. So nuclear interactions in the sun produce heavier elements, protons fused to make, you know, heavy isotopes of hydrogen and helium. And these, these interactions produce neutrinos. So very weakly interacting particles. In a sense, you could think of them as quite similar to wimps, except they're very light.

• 27:54Speaker 02

  And so Ray set out to build a big experiment. You know, figured out how much target mass he would need. And this was, I don't know, was this, I don't remember the numbers, but many tons of chlorine in some liquid form. I think it was like dry cleaning fluid. And that was a cheap industrial way to buy chlorine. And he staged it in, actually, in the same cavern that we're working in today. It was now called the Davis Cavern.

• 28:26Sean Carroll?

  Oh, nice.

• 28:27Speaker 02

  Fast forward, Ray was quite successful in detecting neutrinos from the sun. But he uncovered a big mystery. He only detected one-third the amount that was predicted from the standard solar model. And the way that he, that he collect, he measured the rate was that

• 28:47Speaker 02

  on Earth, in radioactive, you know, if you have a piece of material that's radioactive, it can undergo beta decay. A neutron in a nucleus will turn into a proton, emit an electron and a neutrino.

• 29:02Speaker 02

  Ray used the reverse process. He had chlorine atoms in his detector. They would absorb a neutrino from the sun. A neutron would turn into a proton. And that meant that that chlorine atom, happily in some molecule in the cleaning fluid, would turn into an argon atom, a noble gas, that would suddenly, in a sense, be freed up from that molecule that it was part of. And Ray could drift a dozen or so argon atoms from this enormous tank of cleaning fluid every month. But he only uncovered about a third the number that was predicted from the solar model. So this became a big mystery in its own right. Everyone's like, Ray, maybe you don't understand your detector.

• 29:45Speaker 02

  Maybe you lost some of your 10 argon atoms a month or whatever it was. Ray stuck to his guns. He persevered. In the end, the snow experiment in Sudbury used a different technique to measure neutrinos from the sun. And they, they uncovered the neutrinos that were missing from Ray's experiment that the sun was producing. But along the way, they changed flavor. So Ray's detector wasn't sensitive to them. And this, you know, the experiments combined explained that neutrinos have flavor and they change flavor on their journey from the sun. And this is a whole field in its

• 30:24Speaker 02

  own right. And we did now mount large experiments to to study how neutrinos change flavor. So this was arguably the first big, successful, groundbreaking, underground experiment. And the reason Ray did his experiment underground was that, you know, cosmic rays, you know, are high-energy particles that are, you know, from from space, maybe they're from supernova remnants, whatnot. They create an enormous amount of high-energy radiation at their surface. If you want to conduct a very sensitive experiment to to look for very rare processes, you don't want to do it at the surface. It would be like trying to do astronomy during

• 31:05Speaker 02

  the day. Yes, the stars are on during the day, but there's an enormous amount of scattered sunlight. So let's do astronomy at night. So for us, the equivalent is let's do, let's make our measurements deep underground where we're shielded, or at least some of our backgrounds, cosmic rays are shielded by this mile of earth or whatever.

• 31:24Sean Carroll?

  And I love the fact that it's in a mine because, you know, physics is not cheap. It costs a lot of money and we spend a lot of money on it. But I think people don't understand the extent to which physicists try their best to do things on the cheap whenever they can. Like, you could have built an underground, you could have dug underground, but you're like, no, that costs money.

• 31:43Speaker 02

  That's right. That's right. We are frugal. And, you know, and there's never, you know, there's just just more good ideas to pursue in science than there is money to pursue them. And, you know, so there's this just natural competition of ideas and getting funding and all. That's a whole, that's a whole different subject. Yeah. But, yeah, this was, this was found space, right?

• 32:07Sean Carroll?

  Yeah. Talk a little bit about then the history of looking for dark matter. I mean, you mentioned 30 years, but your experiment's not been running for that long. So what are their previous experiments you're building on? Are they the same thing but smaller? Or do technologies change?

• 32:22Speaker 02

  So, so technology's certainly changed over time. And in fact, prior to physicists searching for the rare process of dark matter interactions, physicists had been searching for another rare process called neutrino-less double beta decay. So we talked a little bit about, you know, neutron decays into a proton, produces neutrinos and electrons. There's a process that hasn't been discovered yet called neutrino-less double beta decay. Let me back up a second. That, so there are certain isotopes in nature which are forbidden

• 33:03Speaker 02

  from undergoing a single beta decay, because that would require them to decay into a nucleus that is heavier. And that's not allowed by energy conservation. Energy and mass are equivalent, E equals mc squared. So, an object of a given mass can't decay into something heavier, because that would violate energy conservation. So there are certain nuclei, germanium is one, germanium-76, which can't undergo single beta decay. But if it, it makes a double hop, it can do two neutrino beta decay and end up at a lighter nucleus. So that is allowed by energy conservation and the standard model, et cetera. Xenon,

• 33:48Speaker 02

  interestingly, also has a double beta decay nucleus. And both germanium and xenon happen to be also good materials from which to make very sensitive detectors. So around the time that searching for dark matter in the form of WIMPs was starting to become a thing, physicists were already operating germanium detectors to look for this double beta decay process. And so the first dark matter experiments were co-opted germanium double beta decay experiments. And so instead of trying to look for this signature at the beta decay

• 34:30Speaker 02

  endpoint, it was like, well, let's reconfigure this. We expect WIMP interactions to be rare, like double beta decay. So, wow, it's great. You guys have already built experiments that are deep underground, that are shielded, that have very sensitive detectors with low radioactivity. This is kind of just what we need, except you're looking out at, in in our energy units, two MEV of energy, and we want to look at something that is about, I don't know, 100 times lower energy signals. We want to look at 10 keV scale. And so, the electronics, the readout was reconfigured to now look for ultra-low threshold. And so, handful of double

• 35:15Speaker 02

  beta decay experiments became the first dark matter experiments.

• 35:19Sean Carroll?

  Okay. That makes perfect sense. I didn't actually know that. That's very useful. And, okay, you're on this thing called LUX-Zeplin. Tell us a little bit about that, because we know that, we've all heard stories that at the Large Hadron Collider, there's 5,000 people on an experiment. I've never been on collaborations that big. What about you guys?

• 35:39Speaker 02

  So, when I started searching for dark matter, I, I had joined an experiment called CLEO. This was electron-positron collisions at an accelerator at Cornell. And when I joined, it was about 100 people working on it. And two years later, it had, it had, it had grown to about 200 people. And I don't know if anyone has studied this, but the number of meetings seems to be scale with, I don't know if it's the log, or the square root, or whatever, the number of collaborators.

• 36:11Sean Carroll?

  Maybe linearly, yeah.

• 36:12Speaker 02

  Yeah. And it just felt like, wow, I seem to be spending less and less time in the lab and more and more time in meetings. And so, just the way that I was working felt less satisfying. And I, for personal reasons, I had, was motivated to move to the West Coast. And I looked around to see, you know, what was going on that was interesting. And I hit on a group that was searching for dark matter. And, wow, it was only three institutions, and it was about 30 people. And they were still looking for something that could lead to a fundamental discovery in particle physics. So that sociologically became very appealing.

• 36:55Speaker 02

  And so I jumped into this, this group that had, had just been part of these germanium converted from beta decay into dark matter experiments. And then they were launching a new type of detector, which is still operating today in a collaboration now called Super CDMS. Back then, it was CDMS-1, cryogenic dark matter search. And it was, you know, using germanium hockey pucks cooled to ultra-low temperatures to look for, you know, in a macroscopic chunk of material, a rise in the temperature of the device due to a single particle interaction.

• 37:32Sean Carroll?

  Yeah.

• 37:32Speaker 02

  Right. So I worked up part of that program for about 15 years. And we made a lot of progress. We were at, you know, many times over the course of that period, world-leading in the search for WIMPs, we had done more sensitive, longer time exposures than any other team. But then a new technology was coming online. In about 2007, teams that had been developing liquid xenon detectors for gamma-ray astrophysics, and also for dark matter, were starting to see, oh, possibility of using this type of detector to search for dark matter. And the first, first couple of detectors that ran were, I think, on the

• 38:16Speaker 02

  order of 10 kilograms, right? And we were, we were, we were approaching that mass with our germanium millikalvin devices. But when the, when the, when the Xenon-10 and the, and the, I think it was, was it Zeppelin-2 or Zeppelin-3 detector, these things started coming online and basically blew us out of the water. Because they could also make these detectors to have very low natural radioactivity. And they seemed to perform remarkably well at, at, at being able to detect signals at very low energy. And basically, in 2007, Xenon 10 had a better,

• 39:00Speaker 02

  you know, more sensitive search for dark matter than these germanium experiments. And so me not being an inventor of technology, but an end user, thought it's time to change technology, right? And for the last 15 years, it's been the liquid xenon detectors that have been in the lead. And there were three main collaborations. Lux and Zeppelin merged to form LZ. The Xenon program, which has been operating detectors at the Gran Sasso Laboratory about an hour from Rome. And the Panda X collaboration, a Chinese collaboration that has been operating detectors in China.

• 39:42Speaker 02

  And those, those three collaborations have sort of been, over time, been leapfrogging each other. I think each collaboration has, at one point in time, laid claim to the world's most sensitive, you know, time exposure for, for dark matter. And so that's kind of cool because it's not so much that you wanted to do the longest time exposure that didn't see something, but rather, since you're now, oh, you know, the next six months of our data will be more sensitive than anything that's come before it, which means funding agency, we have discovery potential, right?

• 40:23Speaker 02

  It means, you know, if, if dark matter in the form of WIMPs was just around the corner, then we could be the ones to get, you know, that first glimmer of it.

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• 41:30Sean Carroll?

  And now you've found it, right?

• 41:32Speaker 02

  Oh, my God. We had our first glimmer of a hint of a maybe of something. And we don't know what that something is yet.

• 41:40Sean Carroll?

  Tell us about the new event that's causing

• 41:43Speaker 02

  Yeah, oh, my gosh. Right, right. So, let me take a breath there.

• 41:49Sean Carroll?

  Please.

• 41:50Speaker 02

  So, there had been a theoretical bias that WIMPs would produce these events at very low energy. About 10 years or so ago, theorists were like, well, let's see if we can construct a more general set of models that might also cause us to look in the experiments that we're building in other places in other ways. And so for me, these are just buzzwords. I know they mean more to you than they do to me. And so effective field theories and non-relativistic effective field theories were basically saying,

• 42:28Speaker 02

  what is the most general set of ways that WIMPs could interact with xenon nuclei or germanium nuclei or what have you? And what that led to were different types of recoil spectra, right? So we have this idea that a WIMP scatters from a nucleus. The WIMPs have their own distribution of velocities in the halo of the Milky Way. They're in some gravitational potential, so just like the sun, the solar system orbiting the center of the galaxy, each individual WIMP particle is on its own circular orbit around the center of the galaxy. So there's a velocity distribution.

• 43:12Speaker 02

  But then the question is, well, how do WIMPs interact with nuclei? If they're the most simple-minded thing, billiard ball scattering, then even simple calculations that I can follow show you that the energy spectrum of WIMPs would look like a falling exponential.

• 43:29Sean Carroll?

  Okay, hold on a second. I don't know what an energy spectrum is.

• 43:33Speaker 02

  Okay, so

• 43:34Sean Carroll?

  I mean, I do, but let's

• 43:36Speaker 02

  Yeah, yeah, yeah, yeah, yeah. Right. If, if you were to, if you were to measure 100 WIMP recoils in your detector and you were to measure the energy of each of those recoils, you would make a graph of number of events versus energy. And if you took the logarithm of the number of events in each energy bin, it would look like a falling, a, a, a, a straight line that's falling. If you didn't take the logarithm, it would look like an exponential decay.

• 44:04Sean Carroll?

  Right.

• 44:05Speaker 02

  Okay.

• 44:06Sean Carroll?

  So it's, but my point was, the spectrum is not like of an event, like one event doesn't have a spectrum. The spectrum is

• 44:14Speaker 02

  Absolutely right.

• 44:15Sean Carroll?

  The histogram of all the events.

• 44:16Speaker 02

  Of all the events.

• 44:17Sean Carroll?

  So many of them, yeah.

• 44:18Speaker 02

  Right, right. So, one hopes, right. So, um, so, so what the theorist said was that, you know, look, look more generally. Don't just look at this low-energy regime. Our more general ways that WIMPs can couple to the xenon detector, the xenon nuclei in your detector, show that there are other models, other types of coupling, if we can include velocity dependence and spin and what have you, that you might get events that go out to significantly larger energies. So in our natural units, we use KEV, kiloelectron volts. We'd been looking down at sort of the 10 KEV scale.

• 45:01Speaker 02

  And so now we've been for some time motivated to look out to a few hundred KEV. So in our experiment, we form these analysis teams. We have the low-energy nuclear recoil group. We have the high-energy nuclear recoil group, the low-energy electron recoil group, et cetera. Because with each of these groups, you know, you're saying, well, this is the model, signal model that we want to study. These are the peculiar backgrounds that we'll have to worry about if we're to see a signal. These are the specific calibrations we'll have to do to calibrate our detector out in that regime, et cetera, et cetera. So it's almost like there are four experiments built into this one single experiment.

• 45:44Sean Carroll?

  Okay.

• 45:45Speaker 02

  Okay. So we'd, we'd set limits previously on these searches out to a few hundred KEV, which is kind of the limits of our calibration. It's hard to calibrate for practical reasons beyond that regime. There are probably people thinking about how to do better there as we speak. So in, in the data set that we'd accumulated in 2023 and 2024, the team that was searching for these high-energy nuclear recoils found an event. And it lied exactly on this nuclear, nuclear recoil band that we calibrated by bringing

• 46:29Speaker 02

  up neutron sources to the detector. And it was sufficiently far away from residual backgrounds from radioactivity that produce so-called electron recoils. And we also had to study, you know, when you're looking for a rare process, you have to think not just single events that could mimic the thing you're looking for, but are there conspiracies where two events could arrive in the detector overlapping? And maybe one of them was on the edge of the detector and lost some charge or lost some light or whatever, and these events unwittingly conspired to populate your nuclear

• 47:13Speaker 02

  recoil band just by chance, right? So, you know, we have to be open to all those possibilities. All of these backgrounds were poured over.

• 47:21Sean Carroll?

  I also want to, let me, let me just highlight this thing that you mentioned very quickly, that you carry a neutron source over to the experiment

• 47:31Speaker 02

  Right.

• 47:32Sean Carroll?

  And then pull it away. And so basically, you're testing, like, are we detecting it when we know events really happen?

• 47:37Speaker 02

  That's right. That's right. So, so we can't bring up a test beam of WIMPs

• 47:42Sean Carroll?

  Right.

• 47:43Speaker 02

  Because that doesn't exist. But we can bring, because, because our theoretical bias is that WIMPs would scatter from a nucleus in the detector. It's a big, heavy target. There's underlying quantum mechanics that says this is the signal to look for. This is embedded in our signal model. The way that a nucleus recoils in the detector leaves a very characteristic signature. And to demonstrate our understanding of that signature, we bring up a source of neutrons. So this could be a beam of neutrons created with an instrument called a DD generator, or it could be a fission source, radioactive isotope that produces neutrons.

• 48:25Speaker 02

  Or it could be a so-called alpha N source where alphas get captured from one ray. Anyway, so we have all these things in our toolkit. So we, we, we, we take our, our neutron source, you know, out of the vault, and we bring it near the experiment. And, and since neutrons are neutral particles, they won't create these electron recoils, which are background, but they will scatter, billiard ball scatter from xenon nuclei. And so that gives us a way of calibrating the response of the detector. And it basically maps out this region in the measured quantities that we have that says this is where you should look for a nuclear recoil.

• 49:07Sean Carroll?

  Right. And so just so people know, it's not that you almost never see events. You see all sorts of events all the time, and you're trying to pick out the weird-looking ones.

• 49:17Speaker 02

  We're trying to pick out things that are away from the edges. We're trying to pick out things that are consistent with a nuclear recoil, things that have just the right ratio of the primary scintillation and the secondary scintillation signal, because those, the ratio of those two signals is a characteristic signature for us. Electron recoils, gamma ray comes in and scatters, it produces a certain ratio of charge to light. Nuclear recoils produce a lower ratio of charge to light, and so that's, that's our signature. And we can map out that response using neutron calibration sources. Yep.

• 49:58Sean Carroll?

  Good. Great. Sorry for interrupting. I think you were mentioning that you were inspired by theorists to look for different kinds of things, and this is what maybe led you to the recent excitement.

• 50:08Speaker 02

  Yes. Yeah. So, so we found an event out at this sort of higher energy regime.

• 50:16Speaker 02

  And we could explain it with any of the backgrounds that we considered, and that's represents a lot of head scratching and a lot of experience. And so this is, this is all the different ways that we can think of for a conspiracy to happen of accidental coincidence of two events overlapping that migrate out of the background region into the signal region.

• 50:40Sean Carroll?

  And the event itself is like two or three years old?

• 50:43Speaker 02

  It happened in 2023. In fact, we even joked about, we need to look up who was on shift and get them t-shirts to say, I was on shift and have, like, that's, that's the whole t-shirt. I was on shift. So.

• 51:00Sean Carroll?

  And so what is so special about this event? Like, you know, bottom line of course.

• 51:06Speaker 02

  It's consistent with dark matter.

• 51:08Sean Carroll?

  Okay.

• 51:09Speaker 02

  And it's not consistent with any known background, but one event is not really enough to hang your hat on, right? So you do the statistical analysis and you ask, you know, how, you know, rigorous statistical treatment of the backgrounds and the signal and what you expect and what you modeled, et cetera, et cetera, et cetera.

• 51:32Speaker 02

  And the, I'll just give you the, you know,

• 51:36Sean Carroll?

  Yeah.

• 51:36Speaker 02

  The expression in the paper, the global significance of this detection was 2.8 sigma, which is below the threshold for, you know, hand evidence, discovery, et cetera. Three sigma, four sigma, five sigma. How, you know, how much should we pay attention to this? And, and the reason is, is that one in a hundred experiments will have a three sigma fluctuation.

• 51:59Sean Carroll?

  Right.

• 51:59Speaker 02

  And particle physicists do hundreds of experiments per year. Right. I mean, if, when you go across the whole community. So when the community says, I have a three sigma result, you go, well, that's nice.

• 52:10Sean Carroll?

  Yeah.

• 52:11Speaker 02

  Take more data.

• 52:13Sean Carroll?

  In psychology, this would be huge.

• 52:15Speaker 02

  Right. No, we each have to assume that our three sigma or our sub three sigma result might just be a fluctuation.

• 52:23Sean Carroll?

  Right.

• 52:24Speaker 02

  And many things have fluctuated away.

• 52:27Sean Carroll?

  Yeah.

• 52:27Speaker 02

  When you subject them to more scrutiny, you know, and, and, you know, there may be backgrounds that we haven't thought of and modeled yet, right? And so not only will taking more data give us more stats, more statistics, but the backgrounds that we can think of that may have conspired if we got the level wrong or whatever, or there's a different way that those backgrounds could come together, you know, that maybe we quote unquote got unlucky that they landed right on the nuclear recoil band and we've made all this fuss for nothing. More events of this class don't know about this very thin special nuclear recoil band.

• 53:10Sean Carroll?

  Sure.

• 53:10Speaker 02

  So they might pepper other parts of this distribution.

• 53:13Sean Carroll?

  Good.

• 53:13Speaker 02

  Right? And that would teach us, okay, guys, you know, we were excited, but it does look like it's something mundane, didn't know about the nuclear recoil band. Now let's try to understand what that background was that we missed, because then we want to, you know, get rid of that background upfront in any future analyses, right? So that's one possibility is that we see more events, but they're not in the right place.

• 53:42Sean Carroll?

  Right.

• 53:42Speaker 02

  Stand, stand down, right? Chill out, right? Okay. On the other hand, we could see more events that lie right on that band, and then that would be, you know, holy crap squared.

• 53:54Sean Carroll?

  Right.

• 53:54Speaker 02

  Right? Or we might see nothing. There might be nothing more. This might just remain a mystery. Full stop. Right? This has happened before.

• 54:04Sean Carroll?

  Just to give the folks a visual picture here, I'll try to include it in the show notes if I remember, but the plot is pretty suggestive, right? I mean, even though it's only one event, you have a plot and you see all the background events. They're very clear what the background is, and then you got this one little bugger.

• 54:21Speaker 02

  Yeah, there's a, there's a big smear.

• 54:23Sean Carroll?

  Yeah.

• 54:23Speaker 02

  And then there's weight space, and then there's this event. You go, that gets your attention. Yeah.

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• 55:29Sean Carroll?

  Do you have a favorite idea, if it is a background, like, it's not neutrinos, that's, we know about those, we would have figured those out, like, do you have a hunch as to maybe what it could be?

• 55:43Speaker 02

  I don't.

• 55:43Sean Carroll?

  No. That's what makes it interesting. Yeah.

• 55:45Speaker 02

  That's what makes it, because we, you know, we, we threw a lot of, lot of brain power at this, right? A lot of studies, you know, part of, so if, if, by the way, if we hadn't seen anything, we also would have published that. We may not have made a press release, but, you know, every new null result, we publish. So certainly like, well, we'd identify this as a data set of interest, but we saw something, we're certainly going to publish that. It gets it out there. There are other experiments. There's the xenon enton experiment.

• 56:20Sean Carroll?

  Right.

• 56:20Speaker 02

  There's the Panda X experiment. This will certainly motivate them to go and look in their data if they haven't already, or maybe they're, you know, maybe they're part way through their own analysis. And, yeah, so we'll see, you know, if they see anything.

• 56:39Sean Carroll?

  I know that one of the hilarious things that I learned by hanging out with the LIGO folks when they discovered their signal was that they tried their best to fool themselves. They would inject fake data into the pipeline and look at it. Is that something that you do?

• 56:56Speaker 02

  So we do, we do a process called salting. There's, so you can, you can blind or you can salt.

• 57:02Sean Carroll?

  Okay.

• 57:03Speaker 02

  So I think something that your listeners may be familiar with is the idea of a, of a drug study. You do a double-blind study. There's a, the active drug, there's the placebo. Neither the patients nor the doctors know who in the study is getting the active drug versus the placebo, so that you can approach the results in an unbiased way as possible.

• 57:26Sean Carroll?

  Yeah.

• 57:27Speaker 02

  Right. So we like to, you know, do our version of that, so we can, we can blind ourselves to the signal region, make all of the decisions that we're going to do to determine whether, you know, the big reveal, you know, doesn't suffer from bias. So you can, you can blind yourself while you make all these decisions and you have calibration and you have sidebands and you have all these studies that you can do and all these, you make all these final decisions. This is what a signal is, looks like, and if it shows up, it shows up. If it doesn't, it doesn't. The other thing that you can do is called salting. So we can, we can have a small working group within our collaboration, salt the

• 58:08Speaker 02

  data by manufacturing events that will show up to everyone else as if they were signal.

• 58:16Speaker 02

  And then only after the analyzers have done everything they're going to do to look for signal in the data, do you then give them the magic recipe to check, is this, are these salt events or are these real? And so, you know, we had a physics analysis meeting when, you know, the, the core analysis team had subjected themselves to 100 questions from the rest of the collaboration. Did you check this? Did you look at that? What about that distribution? Blah, blah, blah, blah, blah. Okay, we're done. Everyone is satisfied that we've done all we can do. Now it's time to unsalt. And so, drumroll, unsulting. It was

• 59:00Speaker 02

  not a salt event. The dot in the plot was not a salt event. Now, we're not super happy with the way that the salting was done for this result, so we weren't putting a lot of emphasis on it for this analysis.

• 59:12Sean Carroll?

  Okay.

• 59:13Speaker 02

  And part of that was just, you know, the happenstance of when the salting was done, and then we did more refined nuclear recoil calibrations, and it was like, hmm, that, that event looks like it was maybe from the old calibration, right? It isn't where you would, you know, maybe it was sea salt and we should have used rock salt, or yeah, I'm just kind of.

• 59:36Sean Carroll?

  Right.

• 59:37Speaker 02

  Uh, poking fun at it, but still, it's so far away from the backgrounds that we can explain. So I think, you know, some of our new data, some of it is salted post-improved calibration. So, you know, we'll see.

• 59:55Sean Carroll?

  And so let's let ourselves imagine that it's actually dark matter. We're allowed, like, I'm allowed, certainly.

• 1:00:03Speaker 02

  You give me position, you give me permission.

• 1:00:04Sean Carroll?

  You give me permission. When would you expect to get some confirmation?

• 1:00:09Speaker 02

  So we have, you know, we, the, the data that we analyzed was 2023, 2024. We have three times as much data recorded that we haven't looked at yet.

• 1:00:22Speaker 02

  Because we wanted to

• 1:00:23Sean Carroll?

  Look at it. What are you doing on the podcast?

• 1:00:25Speaker 02

  We will, we will. We chunked out this one chunk and we're like, let's go to town on that while we continue to take data.

• 1:00:31Sean Carroll?

  It does take time.

• 1:00:32Speaker 02

  It takes time. So now we have our lessons learned. We have our algorithm. So there are active discussions within the collaboration now to say, okay, guys, you know,

• 1:00:46Speaker 02

  do we go slow and super deliberate? Do we go in a mad rush?

• 1:00:50Sean Carroll?

  Yeah.

• 1:00:50Speaker 02

  Something in between? Do we want, you know, we, I don't know, I don't want to, I can't even say tip our hand because I don't think we've decided yet.

• 1:01:00Sean Carroll?

  Mad rush, do the mad rush.

• 1:01:01Speaker 02

  This result was, this result was, you know, was reported a week ago yesterday.

• 1:01:08Sean Carroll?

  Right.

• 1:01:08Speaker 02

  Right? So the, the team that led the analysis frankly is exhausted.

• 1:01:15Speaker 02

  These guys were putting in some serious long days. You know, the team that was writing the paper, the team that was reviewing the paper, the rest of us trying to, to kibitz and, you know, I mean, the first paper draft had 300 comments. Right? So, you know, this is a, this is a very deliberate process, right?

• 1:01:33Sean Carroll?

  Yeah.

• 1:01:33Speaker 02

  And so, you know, we don't want to blow it. Right? You know, there's this far side cartoon, uh, symphony orchestra. And there's this guy in the back with two big symbols and his thought bubble says, don't screw up. Don't screw up. Don't screw up. And the caption is Roger screws up.

• 1:01:57Speaker 02

  So, you know, we got to get it right. It's science. We have to get it right. So.

• 1:02:02Sean Carroll?

  Okay. One, here is a like.

• 1:02:04Speaker 02

  Well, let's say we get it right.

• 1:02:05Sean Carroll?

  Yeah.

• 1:02:06Speaker 02

  And let's say there's three more events and let's say they rely right on the nuclear recoil band. Bah, we're super excited. This is, this is a game changer. This is beginning to maybe, just maybe start to answer this question that's been on the table for a hundred years.

• 1:02:24Sean Carroll?

  And it sounds like it will be confirmable by other experiments before too long. If it's real.

• 1:02:31Speaker 02

  Don't trust a single experiment. Right? Right?

• 1:02:33Sean Carroll?

  Don't trust a single event. Don't trust.

• 1:02:35Speaker 02

  Don't trust a single event. Yeah. And, you know, everything in science is provisional.

• 1:02:40Sean Carroll?

  Yeah.

• 1:02:40Speaker 02

  Right? This can be falsified. This could not show up again. This could go in the dustbin. We just don't know. We have to, we have to just keep at it.

• 1:02:50Sean Carroll?

  An important question is if it is dark matter.

• 1:02:54Speaker 02

  Yeah.

• 1:02:54Sean Carroll?

  Is this, does it fit nicely into our theoretical expectations for what dark matter would look like?

• 1:03:00Speaker 02

  Absolutely. Yes. Yeah.

• 1:03:03Sean Carroll?

  It's just what you would have expected?

• 1:03:05Speaker 02

  Well, you know, again, it's out of these higher energies. So we were taught, think more broadly, but yes, this can easily be, uh, you know, a, a dark matter particle that has, that, well, we, we have to study to death, frankly.

• 1:03:20Sean Carroll?

  Yeah.

• 1:03:20Speaker 02

  So for us, that means keep taking data, uh, and, you know, one event turns into three, turns into 20. Then we get to start, you know, populating that energy spectrum.

• 1:03:34Sean Carroll?

  Yeah.

• 1:03:34Speaker 02

  Which contains information. You'd like to do an even bigger experiment that could detect hundreds of events. If this is, if one event per 200, you know, whatever, four ton years of exposure is the rate. And of course the statistics on one event are all over the place, fluctuation wise. Um, you'd like to study it in great detail. You'd also like to see that event show up in a collider experiment. Because, you know, if the mass is a TV, uh, which is our best fit with huge air bars, if it's even something, that should be accessible in colliders, right? And so you want to study, you want to study it in detail in the lab.

• 1:04:18Speaker 02

  You want to study it in detail. It's astrophysical signature. Then you want to start interacting with your observer friends again. Hey, Risa, these simulations that you make of how galaxies form. If the dark matter happens to be a one TV particle with these couplings to ordinary matter, blah, blah, blah. Does that help you understand your models? Does that help you tune your models, et cetera? You, you don't see dark matter in some galaxies or dwarfs, whatever. I mean, can we, can we start to knit that whole story together? I mean, if what we see is real, it's just, it's defined the work we need to do over the next 30 years. Right?

• 1:05:04Sean Carroll?

  Yeah. Is it, is it something that, I know it's only one event. Does it tell us anything about the kind of dark matter it might be? Like, do we know what particle it is?

• 1:05:14Speaker 02

  We don't know what particle it is. So much we don't know about it, but, um, and now I'm going to poke a little fun at my, at my theory friends. If you would like to read about a couple hundred of the possibilities, you could, you could go online and look at some of the recent papers. It, I think it's fair to say it's, it could still be just about anything.

• 1:05:40Sean Carroll?

  I've heard the word Higgsino.

• 1:05:44Speaker 02

  Yep. Right?

• 1:05:47Sean Carroll?

  I mean, there are, I guess I want to get across the impression that, um, even though it doesn't tell us what particle it is, it, it implies something about the kind of particles. Some kinds of particles would be more like this.

• 1:05:58Speaker 02

  We know that it's, yeah, this, if this were real, this is, this is a heavy particle with weak interactions. Right? Weak interactions because it's taken four ton years of exposure to finally show up.

• 1:06:10Sean Carroll?

  Right.

• 1:06:10Speaker 02

  Right? Right? So we know it's, it's weak interaction. We know it's massive. That's about all we could say based on one event. And there's still huge air bars on its coupling strength, on its mass, et cetera. But, you know, give us 10 more events.

• 1:06:28Sean Carroll?

  Yeah, then it's a different story.

• 1:06:29Speaker 02

  And we, it's a different story, right?

• 1:06:31Sean Carroll?

  But basically what you're saying, that a hundred different theorists saw this one event and said, they wrote a paper saying, well, it could be this.

• 1:06:40Speaker 02

  You know, I mean.

• 1:06:41Sean Carroll?

  I love theorists.

• 1:06:42Speaker 02

  Theorists are great. They're very creative. They help us figure out what to look for and they help us figure out what it might mean. Right? It's very much that partnership and, and rightfully so, each theorist is excited about their own ideas.

• 1:06:57Sean Carroll?

  Yeah.

• 1:06:57Speaker 02

  And so, cool. They're like, I've been thinking about this for 30 years. And if this particle is consistent with the theory that I've been thinking about, you're darn tootin', I'm going to write a paper.

• 1:07:09Sean Carroll?

  You're going to write a paper, of course.

• 1:07:10Speaker 02

  In fact, I'd already written the paper. Now I just.

• 1:07:13Sean Carroll?

  Plug in the numbers.

• 1:07:13Speaker 02

  Put in the parameter. I plug in the numbers.

• 1:07:17Sean Carroll?

  I mean, talk about maybe, um, the way that this is happening. And again, we're still in the mode, like we're pretending it's real, right? Like, we don't know. We don't know.

• 1:07:26Speaker 02

  That's right. That's right.

• 1:07:27Sean Carroll?

  Do you expect something like this to sneak up on you? Like you see one event and then you wait and you see another event? Or, or would you have expected to turn on the detector and there they are?

• 1:07:38Speaker 02

  So, think about this as being a super fancy Geiger counter. So turn on a Geiger counter and it goes tick, tick, tick, tick, tick, tick, tick.

• 1:07:53Speaker 02

  Tick. Right? I mean, this is, this is just a random. It's a random process. Right? And this, this fools most people. If you're going to flip a coin 50 times. Right? You're, you're very likely going to get a stretch where you got like six heads in a row or something like that. And when, when you ask people to make up a random distribution of coin flips, they, they invariably don't make enough in a row of heads or of tails. Right? So we're, we're not very good at this, right? Thinking about statistics. I mean, everyone's had this experience with a Geiger counter trying to measure a low rate. Wait, is it on? Oh, yeah.

• 1:08:38Speaker 02

  You know, so it's, again, coming back to one event, right? So, and any, anyone with basic stats can do this. One event in, you know, 220 days of exposure. Right? If I now say, okay, I've got, I don't know what the number is. Let's say 600 days in the can. You could make a 10% to 90% confidence envelope to say, this is how many events you should see as, as you increase your exposure. And it gets very wide, very fast.

• 1:09:15Sean Carroll?

  Have you done, have you done that?

• 1:09:17Speaker 02

  Yeah, of course we've done that.

• 1:09:18Sean Carroll?

  But I mean, so what do we expect? What's the range?

• 1:09:21Speaker 02

  It just gives us, I, I forget. I don't know. I mean.

• 1:09:24Sean Carroll?

  It's wide. It's a wide.

• 1:09:26Speaker 02

  It's wide, right? I mean, if we, you know, there's a, there's a good, there's a reasonable chance that we see nothing in three times the stats. There's a reasonable chance that we see something in three times the stats.

• 1:09:40Sean Carroll?

  Right.

• 1:09:40Speaker 02

  And that, that prediction is only worth, you know, the paper it's printed on is probably not even printed. Um, because we're going to look, right? That's the answer.

• 1:09:53Sean Carroll?

  And, and the important thing to keep in mind is even if you see nothing, it might still be real, right? You might've gotten lucky very early, right?

• 1:10:01Speaker 02

  That's right. It might've been that tick and then nothing else for eight seconds.

• 1:10:04Sean Carroll?

  So you got to keep going after that.

• 1:10:06Speaker 02

  We have to going, right?

• 1:10:08Sean Carroll?

  And you mentioned, we're going to try to connect it to particle accelerators. What about, you mentioned indirect detection, looking in the sky? Does this have implications for what to look for in the sky?

• 1:10:20Speaker 02

  So I'm not so, I'm not so expert on this.

• 1:10:25Speaker 02

  What, what is cutting edge indirect detection now?

• 1:10:31Speaker 02

  That would, I feel like that would have to be modeled. In fact, maybe there's already a paper out there that says, okay, let's say, no, because people have been thinking about this. And so it is now an exercise of let's plug in the numbers and ask, you know, what would a signal from, you know, the center of the galaxy look like? If this is a one TV particle or 10 TV particle, et cetera, then we can say something about its abundance. And therefore, you know, how many might have collected in the center of the galaxy and what different signatures should we look for? I mean, people may go back to the data in the Fermi gamma ray telescope and say, did,

• 1:11:13Speaker 02

  did we look, you know, if this is, if this is the dark matter, did we, did we look, you know, is there a more pinpoint search to make to look for this thing rather than a broad search? You know, maybe that's happening. I don't know. Yeah.

• 1:11:28Sean Carroll?

  And you mentioned the LHC as some place we will look, but this is presumably, if it is a one TV particle, so the Higgs boson for comparison is point one two five TV. It's lighter. Presumably this is a lot of motivation for building even bigger particle accelerators.

• 1:11:49Speaker 02

  Absolutely. And certainly, you know, we're, we're in this shutdown now, the LHC, the machine is being upgraded, the detectors are being upgraded. But, you know, this, this, this thing that, because are we still pretending that it's real?

• 1:12:07Sean Carroll?

  Yeah, let's pretend.

• 1:12:08Speaker 02

  Yeah, okay. So

• 1:12:13Speaker 02

  that, that this, this may be a clue as to the sector where, where dark matter is. And this one particle wouldn't be the whole story of the particle physics. If this is supersymmetry, it has a lot of cousins. And so people will be looking in their current data, you know, making a more targeted search. If I inform my search based on this, let's also jump into the Dan Sean room and pretend it's real, then how, how should I refine my search to see if any, any cousins of this dark matter particle are in my data? Right? That's something that, that, you know, you can do now.

• 1:12:51Sean Carroll?

  I'm presuming that essentially no data that your experiment could get would tell us, oh, it's supersymmetry. Like that requires much more detailed data.

• 1:13:01Speaker 02

  Right. And, and, and we have both, you know, the, the, the, the benefit and the deficit of being a quite generic search. We're looking for billiard ball scattering. If we get a spectrum of events, we could say a little bit more about it. But that isn't, you know, supersymmetry is something that will be discovered in an accelerator.

• 1:13:20Sean Carroll?

  Yeah.

• 1:13:21Speaker 02

  Right? And they will work out the couplings and they will say, oh, this seems to be, we're finding the cousins of your, your dark matter particle that scattered from your xenon nucleus in June of 2023. And wow, that's so cool because now we know how the WIMP would have interacted or the supersymmetric particle would have interacted with other supersymmetric partners and ordinary matter in the early universe when the universe was, you know, 10 to the whatever minus 37 seconds old. And this is how dark matter would have been produced. Right? That, you know, a journalist asked me, did you guys solve the dark matter? Well, like, not even close. Right?

• 1:14:06Speaker 02

  I mean, to really wrap it up and put a bow on it. Right? I mean, some of your listeners may be familiar with, you know, the Steven Weinberg book of a previous generation called The First Three Minutes. We now understand, you know, based on laboratory measurements and observations and theoretical modeling and the big bang expansion, we have a concordant picture of the ratio of the light elements is produced, you know, in the first three minutes of the big bang. The ratio of hydrogen to helium and deuterium, et cetera, et cetera. Right? We know the laboratory physics. We have, you know, by winding the Hubble expansion backwards, we knew the temperature

• 1:14:49Speaker 02

  of the universe at certain time, and there was, this was the ratio of protons to neutrons, and this was the nuclear binding energy. And this is how many would have fused to make helium, et cetera.

• 1:15:01Speaker 02

  You know, maybe this is the work of the next 30 years to, to do the detailed laboratory studies and the observations and the model building to be able to know that what went bump in the night in June of 2023 actually is a dark matter particle because we have all of the, we can tie that whole story together.

• 1:15:21Sean Carroll?

  Yeah.

• 1:15:22Speaker 02

  And literally calculate the relic density based on the physics of this species. That would be amazing.

• 1:15:29Sean Carroll?

  Okay, we can drop the reality distortion field and stop pretending. That's okay.

• 1:15:35Speaker 02

  Back to work.

• 1:15:36Sean Carroll?

  As a final, no, not quite back to work yet. I want to ask you a final question to sort of, you know, you've done the hard work here. If it does turn out to be real.

• 1:15:45Speaker 02

  Yeah.

• 1:15:46Sean Carroll?

  How will you feel? And also if it doesn't turn out to be real, if you never see another one again, how would you feel?

• 1:15:53Speaker 02

  So, okay, so if it turns out to be real, I will, I will feel elation. It will be amazing just to be part of a discovery team. The thing that, you know, 100 year old mystery, it's just like mind boggling. I don't, actually, I don't know how I'll feel.

• 1:16:12Sean Carroll?

  A little bit of relief?

• 1:16:13Speaker 02

  A validation, relief. Hey, family members, we're not crazy, it turns out. Right? You know, I don't know, it'd just be amazing to be part of that enterprise. Right? I've been hunting for weird, rare things since my PhD experiment. And it would be amazing to be part of a discovery. That would just be the coolest thing.

• 1:16:42Speaker 02

  If it goes away, if it's, you know, we see four more events and none of them are on the nuclear recoil band, then, you know, the, the, the pitch that we'd been making is still there. Right? Looking for dark matter and the ways that we've been doing with the tools that we've been using, none of that motivation has changed. It's all still there and we will still do our damnedest to make the case for, you know, not just continuing the experiment we're doing, but building the next one. And we've, you know, put some pretty concrete ideas out there as to what we think we need to build and why. And it's gone through, you know, the, the peer review process that our field engages

• 1:17:24Speaker 02

  in, which is what are the most interesting ideas that we could pursue over the next decade? And let's prioritize them based on cost and breadth of program, et cetera, et cetera, et cetera. And, you know, this one survived that process and was recommended to go forward, but it did so in budget scenarios that have not yet been realized.

• 1:17:49Sean Carroll?

  And it's not just a matter of getting 100 tons of xenon and doing the same thing again.

• 1:17:55Speaker 02

  In, in a way it is. It's, it's building a bigger instrument with more xenon, but also beating down the backgrounds. We talked a little bit about the backgrounds. So if you're going to build a 10 times bigger thing and you're going to run it for five times as long, you have to push your backgrounds down by a factor of 150. Otherwise you're going to make a really good measure of Krypton 85 decays or, you know, whatever. Choose, choose your background. And the fun thing sociologically is that, so our new, our new proposed experiment is called XLZD. I'm going to shamelessly use some time here for PR.

• 1:18:33Sean Carroll?

  Please.

• 1:18:34Speaker 02

  We have merged with our current competitors. So we're currently competing with a xenon N ton collaboration, which is a partnership between US groups, European groups, Japan, and they've been running a quite similar experiment to what we're operating. And with, you know, similar techniques, similar exposure, similar timelines.

• 1:18:58Speaker 02

  And a few years ago, we joined forces with them to form XLZD, xenon, lux, Zeplin, Darwin, crappy name, sorry, apologies. Think XKCD, XLZD helps you pronounce it.

• 1:19:15Speaker 02

  So we've merged our teams to pitch and build this new experiment and, you know, draw from the, the broader pool of expertise, you know, some things that xenon N ton did worked better than what choices we made in LZ and vice versa. And so that's just kind of cool to have, you know, sort of greatest hits from both teams to pool together. And of course, funding agencies like when you do this, because then you're sharing around the cost. And, and, you know, we've kind of come full circle now. Xenon is a double beta decay nucleus, as I mentioned early on, and we think that this

• 1:19:56Speaker 02

  instrument that, you know, was originally pitched for dark matter is now basically it's being pitched for both types of science. You know, we think we basically build the same instrument and you can use it to both do dark matter and double beta decay in, in xenon 136.

• 1:20:16Sean Carroll?

  You know, I think that we all owe you some thanks for devoting your career to this. I mean, it'd be great if it happens. I'm rooting for you. I hope you found it. It, it, you know, it does take a lot of time and a little bit of, more than a little bit of perseverance.

• 1:20:31Speaker 02

  I enjoy the hunt.

• 1:20:32Sean Carroll?

  Yeah.

• 1:20:33Speaker 02

  You know? Yeah.

• 1:20:35Sean Carroll?

  And maybe you found it. So we'll see, we'll check back.

• 1:20:37Speaker 02

  Okay.

• 1:20:38Sean Carroll?

  If you, if you actually found dark matter and we're sure of it, I never, ever have the same person on Mindscape twice, but you know, we'll have you back to tell us if you really did find the dark matter.

• 1:20:47Speaker 02

  You know, to share the wealth, I have, I have, I have 200 collaborators.

• 1:20:51Sean Carroll?

  That's true. But okay, I guess we give them, throw them some flowers as well. But this was great. Dan Akerib, thanks very much for being on the Mindscape podcast.

• 1:20:59Speaker 02

  That was a lot of fun, Sean. Thank you so much.

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