Speaker 1Hello, 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 Zeppelin 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 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. 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 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 Acrib 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. For a long time, WIMPs were the leader. 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 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 that are doing similar things. So, let's go. Dan Akrib, welcome to the Mindscape Podcast.
Speaker 2Sean, it's great to be here.
Speaker 1It's good to see you. 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? Let's go very general here. Why do we think it's there?
Speaker 2Right. So, 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 general force, things will move more slowly. Apply that same physics to the light. Or apply that same physics to the light. Or 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, 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 favorite conservation law seemed to be violated. Well, we can't give up on this law. Let's invent a new particle, 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.
Speaker 1Well, 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?
Speaker 2That one could modify the laws of gravity? Yeah. For me, you know, I have the option of just sidestepping that, because that's not my area. I don't come up with new theories of gravity. People have been working on new theories of gravity for 30 years. You know much more about this than I do. So I'm not sure I'm really going to answer your question there.
Speaker 1No, 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?
Speaker 2Well, I do that anyway, you know, because we've been hunting for 30 years or more. 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, right? But that's -- part of that is just, you know, 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 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, well, question looks cool and interesting, and am I a person who, with the tools I have and the collaborations I'm part of, that I can approach that question in a useful way?
Speaker 1By 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 a 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? Is 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?
Speaker 2So we kind of turn to unanswered questions in particle physics, which in many ways arose independently. So we have the standard model of particle physics and the experts, which I am not, I'm a practitioner, will tell you that, well, this spectrum of particles that we have and the fundamental forces, like, you know, like six quarks and leptons, da-da-da-da-da, seems very arbitrary. evolved and grand unified theories and it and and it looks in many ways unnatural so people have come up with uh theories like supersymmetry and others that maybe have now gone out of fashion where let's fix you know let's get a more fundamental theory uh that seems more natural and the kind of stupid analogy i like to make is that imagine we have the periodic tables of 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 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 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 in the theory of strong interactions this is the strong force that holds the nucleus together uh there were fundamental symmetries that seemed not to be violated and they seem 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 looking for billiard ball scatters they're looking to tune in the axion radio station um so there's different clues from particle physics uh that we've been that we've been looking at um 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 uh to our experiments new ideas came up which was rather than just be guided by missing out on the reality of what we're looking for we're looking for 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 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 if did i get my double negatives right isn't disallowed by particle physics that could also produce dark matter right and so let's go to the next question and i'm going to go to the next question and i'm 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 etc etc 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 etc to try to you know get at this really fundamental problem and
Speaker 1for better for worse different kinds of particles it sounds like require different experimental
Speaker 2strategies 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 etc etc right so and and
Speaker 1again 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 that there's a lot of stuff that's going on in the big bang and you 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
Speaker 2so as it 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 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 techniques so there are um you know what the what we do we call direct detection this is a wimp directly scattering from a nucleus in our detector and registering a signature um 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 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 and
Speaker 1i 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
Speaker 2so yeah so if we go back to the 90s 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 universe and produced dark matter that we could now detect um 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 whim search side we are probably now pushed to regimes where you know the 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 uh but let's grow the
Speaker 1portfolio so the wimp idea as a candidate for dark matter is something that is still alive and kicking but the fact 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
Speaker 2possibilities that's absolutely right yeah yeah
Speaker 1but you're in the wimp land that's what you're looking for i am in the wimp land and i'm in the wimp land right right and how are you doing that yeah how are we
Speaker 2searching or how are we searching what
Speaker 1does the experiment
Speaker 2look like perfect right so so i am a simple-minded guy sean i billiard ball scattering is such a simple concept um so our detector is made of of many many many 10 tons if you will of xenon billiard balls you know condensed into a liquid uh so sorry when you say xenon
Speaker 1balls you mean the actual xenon actual atoms
Speaker 2xenon and we're gonna think of them as microscopic billiard balls in the context of the experiment uh 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 luck zeppelin or lz experiment in south dakota which has been taking data for uh i don't know when did we start taking data 2023 um 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 okay easily cooled with liquid nitrogen as a cooling thing that you can just buy industrially 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 um we get scintillation light from the event where you know that first interaction 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. By measuring these two signals separately, we can learn things about what sort of particle may have interacted. Many of the interactions are mundane, residual radioactivity in the environment of the experiment, 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, we can locate exactly where the event took place. So 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 that 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. OK, 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.
Speaker 1Yeah. 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 like a meter across. It's not that big.
Speaker 2You can sort of outstretch your arms and touch the edges of the detector.
Speaker 1What fraction of the world's xenon population are you using?
Speaker 2Yeah. So so the the annual production of xenon worldwide is somewhere between. Between 60 and 100 tons, right? So we're using a tenth of world production. We bought it over a few years. Try it. Do you want to buy it without spiking the price? Yeah, it can get pricey depending on industrial uses. It's 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.
Speaker 1I know there are supply chain issues in the world today. Do you need to worry about your xenon supplier?
Speaker 2Right. Right now, it's OK. I mean, you buy the xenon once and you you know, you you safeguard it. You have all these redundant systems. So if you lose your cooling. Power, 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. And it doesn't go bad. It doesn't go bad. No, no, no. It's it's not consumed.
Speaker 1Yeah, yeah. So you so you have this, you know, meter and a half or whatever size tub of xenon. You've protected it from layers of shielding. You put it a mile underground and which is amazing. But also. There's there's a history of that. Like, you know, talk about this idea of putting physics labs underground.
Speaker 2Right. Right. So so I guess one of the one of the early ones was done by a guy named Ray Davis, who is 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 fuse 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. 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. 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's now called the Davis Cavern. Oh, nice. Forward, Ray was quite successful in detecting neutrinos from the sun, but he discovered 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 on Earth in radioactive, you know, if you have a piece of material that's radioactive, it can undergo beta decay and a neutron and a nucleus will turn into a proton, emit an electron and a neutrino. 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 was like, Ray, maybe you don't understand your detective. Maybe you lost some of your ten 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. 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 the whole field in its own right. And we did now mount large experiments 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 created an enormous amount of high energy radiation at the surface. If you want to conduct a very sensitive experiment 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 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.
Speaker 1And 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.
Speaker 2That's right. That's right. We are frugal and, you know, and there's never you know, there's 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 and so, yeah, this was this was found space, right? Yeah. Talk a little bit about the
Speaker 1history of looking for dark matter. I mean, you mentioned 30 years, but your experiments 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?
Speaker 2So so technology has 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 is 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 from undergoing a single beta decay because that would require them to decay into a neutron. 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 uh 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 etc xenon 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 uh 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 endpoint uh 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 to 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 beta decay experiments became the first dark matter experiments
Speaker 1okay and so that was i didn't actually know that that's very useful and okay you're on this thing called lux zeppelin um 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 an experiment but i've heard stories that at the large hadron collider have been on collaborations that big what about you guys
Speaker 2so when i started searching for dark matter uh i i had joined an experiment called clio uh this was uh electron positron collisions at a 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 um i don't know if anyone has studied this but the number of meetings seems to be scale with i don't know if the log or the square root or whatever the number of collaborators maybe linearly 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 right uh and so just the way that i was working felt less satisfying uh 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 and so i jumped into this this group that had had just been part of these germanium converted from beta to k into dark matter experiments and then they were launching a new type of detector uh which is still operating today in a collaboration now called super cdms back then it was cdms1 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 temp arise in the temperature of the device due to a single particle interaction yeah 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 uh but then a new technology was coming online in about 2007 uh teams that had been developing liquid xenon detectors for um gamma ray astrophysics and also for dark matter so it was really interesting to see oh possibility of using this type of detector to search for dark matter and the first first uh couple of detectors that ran were uh i think on the order of 10 kilograms right and we were we were we were approaching that mass with our germanium millikelvin devices uh but when the when the when the xenon 10 and the and the i think 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 rate activity and they seem to perform remarkably well at at being able to detect signals at very low energy and basically in 2007 xenon 10 had a better you know more sensitive search for dark matter than these germanium experiments um 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 uh in the lead and there were three main collaborations lux and zeppelin merge to form lz the xenon program uh which has been operating detectors the gran sasso laboratory about an hour from rome and the panda x collaboration a chinese collaboration that has been uh operating detectors in china 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 uh for dark matter um and so it's kind of cool because it's not so much that you wanted to do the long 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 it means you know if if dark matter in the form of whims was just around the corner then we could be the ones to get you know that first glimmer of it and
Speaker 1now you found it
Speaker 2right 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 tell us about the new event that's causing yes oh my gosh right right so let me take a breath there please so that there had been a theoretical bias uh that would produce these events at very low energy um about 10 years or so ago the 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 what is the most general set of 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 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 this the energy spectrum of wimps would look like a falling exponential okay i don't
Speaker 1know what an energy
Speaker 2spectrum is okay so i mean i do but yeah yeah yeah yeah right if if you were to if you were to measure a hundred 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 straight line that's falling if you didn't take the logarithm it would look like an exponential decay right okay
Speaker 1the My point was the spectrum is not, like, of an event. Like, one event doesn't have a spectrum. The spectrum is the histogram of all the events because you're going to get so many of them.
Speaker 2Right. So, one hopes. Right. So, what the theorist said was that, you know, look more generally. Don't just look at this low-energy regime or more general ways that WIMPs can couple to 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. Right. Right. Right. We've been looking down at sort of the 10-KEV scale. And so, now we've been, for some time, motivated to look out to a few hundred KEV. So, in our experiment, we formed 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, 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. Okay. Okay. So, 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 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 recoil. It was a nuclear recoil band that we calibrated by bringing 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 recoil band just by chance, right? So, you know, we have to be open to all those possibilities. So, all of these backgrounds were
Speaker 1poured over, and in the end… Very quickly, did you carry a neutron source over to the experiment and then pull it away? And so, basically, you're testing, like, are we detecting it when we know events really happen?
Speaker 2That's right. That's right. So, we can't bring up a test beam of WIMPs because that doesn't exist. But we can bring… Because our theoretical bias is that WIMPs would scatter from the nucleus in the detector. It's a big, heavy target. There's underlying quantum mechanics that says this is… 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, a radioactive isotope that produces neutrons, or it could be a so-called alpha. Anyway, so we have all these things in our toolkit. So, we take our neutron source, you know, out of the vault, and we bring it near the experiment. And since neutrons are neutral particles, they won't create these electron recoils, which are a 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.
Speaker 1Right. 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.
Speaker 2We'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 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 our signature. And we can map out that response using neutron calibration sources.
Speaker 1Good. 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.
Speaker 2Yes. Yeah. So, we found an event out at this sort of higher energy regime. And we couldn't explain it with any of the backgrounds that we considered. And that represents a lot of head scratching and a lot of experience. So, this is all. This is all the different ways that we can think of for a conspiracy to happen. An accidental coincidence of two events overlapping that migrate out of the background region into the signal region.
Speaker 1And the event itself is like two or three years old.
Speaker 2It 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. That's the whole t-shirt. I was on shift. And so, what is
Speaker 1so special about this event? Bottom line, of course.
Speaker 2It's consistent with dark matter. Okay. 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 signals. Right. And what you expect and what you modeled, et cetera, et cetera, et cetera. And the, I'll just give you the, you know. Yeah. The expression in the paper, the global significance of this detection was 2.8 sigma, which is below the threshold for, you know, hint, evidence, discovery, et cetera. Three sigma, four sigma, five sigma. How, you know, how much should we pay attention to this? And the reason is, is that one in a hundred experiments. We'll have a three sigma fluctuation. Right. 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. Yeah. Take more data. Right. Because maybe. This would be huge. Right. No, we each have to assume that our three sigma or our sub three sigma result might just be a fluctuation. Right. And many things have fluctuated away. Yeah. 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. People 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. Sure. So they might pepper other parts of this distribution. Good. 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 up front in any future analysis. Right. So that's one possibility is that we see more events, but they're not in the right place. Right. Stand down. Right. 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. Right. Or we might see nothing. There might be nothing more. This might just remain a mystery. Full stop. Right. And this has happened before.
Speaker 1Just to give the folks a visual picture here, I'll try to include it in the chat. show notes if i remember but the plot is pretty suggestive right i mean you have even though it's only one event you have a plot you see all the background events they're very clear what the background is and then you have this one little bugger yeah there's a
Speaker 2big smear yeah and then there's white space and then there's this event you go that gets your attention yeah do you have
Speaker 1a favorite idea if it is a background like it's not neutrinos that's we know about those we would figure those out like do you have a hunch as to maybe what it
Speaker 2could be i don't know
Speaker 1that's what makes it interesting
Speaker 2yeah that's what makes it because we've you know we we threw a lot of a lot of brain power at this right um 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 would have been a new result and we would have been able to figure it out we published so it's 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 and ton experiment 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 um and yeah so we'll see you know if they see anything i know that
Speaker 1one of the hilarious things that i learned by hanging out with the ligo folks when they discovered their signal was that they uh tried their best to fool themselves they would inject fake data into the pipeline and look at it is that something that you
Speaker 2do so we do we do a process called salting there's so you can you can blind or you consult okay so i think something that your listeners may be familiar with is the fact that you can do a double blind study and you can do a double blind study and you can do a double blind study 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 yeah 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 or not we're going to be able to do that so we can we can blind ourselves to the signal region make all of the decisions that we're going to determine whether you know the big reveal uh 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 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 data by manufacturing events that will show up to everyone else as if they were signal 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 that 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 okay we're done everyone is satisfied that we've done all we can do now it's time to unsalt and so drum roll unsalting it was not a salt event the dot and 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 okay 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 use rock salt or yeah i'm just kind of right uh poking fun at it but still it's so far away from the backgrounds that we can explain um so i think you know some of our new data some of it is salted post improved calibration so you know we'll see and so let's
Speaker 1let ourselves imagine that it's actually dark matter we were allowed like i'm allowed so you give me position
Speaker 2you give me permission i give you
Speaker 1permission um when would you expect to get some confirmation
Speaker 2so 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 because we wanted it what are you doing we chunked out this one chunk and we're like let's go to town on that while we continue to take data it does take time it takes time so now we have our lessons learned we have our rhythm so there are active discussions within the collaboration now to say okay guys you know do we go slow and super deliberate do we go in a mad rush yeah 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 i mean
Speaker 1rush do the mad rush i'm going to vote
Speaker 2for a mad rush this result was you know a week ago yesterday right right so the the team that led the analysis frankly is exhausted right 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 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 cymbals and his thought bubble says don't screw up don't screw up don't screw up and the caption is roger screws up right so you know we got to get it right it's science we have to get it right so
Speaker 1okay one here is a like let's say we
Speaker 2get it right and let's say there's three more events and let's say they rely right on the nuclear recoil band but 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 100 years
Speaker 1and it sounds like it will be confirmable by other experiments before too long if it's real don't
Speaker 2trust a single experiment right right don't trust a single
Speaker 1event don't trust don't trust a single event
Speaker 2and you know everything in science is provisional yeah 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
Speaker 1an important question is if it is dark matter yeah is this does it fit nicely into our theoretical expectations for what dark matter
Speaker 2would look like absolutely yes yeah it's just what you would have expected 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 dark matter particle that has that well we have to study to death frankly right 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 which contains 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 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 etc use you don't see dark matter in some galaxies there's 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
Speaker 1yeah is it is it something that i know it's only one event does it tell us us anything about the kind of dark matter it might be like do we know what particle it is
Speaker 2we 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 can 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
Speaker 1i've heard the word higzino yep right 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 we know that it's
Speaker 2yeah 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 right right right so we know 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 error bars on its coupling strength on its mass etc but you know give us 10 more events yeah then it's a different story right basically what you're saying that 100
Speaker 1different theorists saw this one and said they wrote a paper saying well it could
Speaker 2be this you know i mean i
Speaker 1love theorists theorists
Speaker 2are 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 yeah 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 it's a weak interaction then it's going to be it's going to help us figure out what to look for in the future so i think that's a very good point i think that's a very good point i think that's a very good point i mean talk about maybe um
Speaker 1the 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 that's right um do you expect something like this to sneak up on you like you see one event and then you wait maybe you see another event or or would you have expected to turn on the detector and there
Speaker 2they are 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 tick right i mean this is this is just a random it's a random process right and this this fools most people if right and this this fools most people if right and this this fools most people if right you're very likely going to get a right you're very likely going to get a right you're very likely going to get a stretch where you got like six heads in stretch where you got like six heads in stretch where you got like six heads in a row or something like that when when a row or something like that when when a row or something like that when when you ask people to make up a random you ask people to make up a random you ask people to make up a random distribution of coin flips distribution of coin flips distribution of coin flips they they invariably don't make they they invariably don't make they they invariably don't make enough in a row of heads or of tails enough in a row of heads or of tails enough in a row of heads or of tails right so we're we're not very good at right so we're we're not very good at right so we're we're not very good at this right thinking about statistics i this right thinking about statistics i this right thinking about statistics i mean everyone's had this experience with mean everyone's had this experience with mean everyone's had this experience with the geiger counter trying to measure a the geiger counter trying to measure a the geiger counter trying to measure a low rate wait is it on low rate wait is it on low rate wait is it on oh yeah oh yeah oh yeah you know you know you know so so so it's it's it's again coming back to one event again coming back to one event again coming back to one event right so right so right so and any anyone with basic stats can do and any anyone with basic stats can do and any anyone with basic stats can do this one event in you know 220 days of this one event in you know 220 days of this one event in you know 220 days of exposure right if i now say okay i've exposure right if i now say okay i've exposure right if i now say okay i've got got got i don't know what the number is let's i don't know what the number is let's i don't know what the number is let's say 600 days say 600 days say 600 days in the can you could make a 10 to 90 in the can you could make a 10 to 90 in the can you could make a 10 to 90 confidence confidence confidence envelope envelope envelope to say this is how many events you to say this is how many events you to say this is how many events you should see as as you increase your should see as as you increase your should see as as you increase your exposure and it gets very wide very fast exposure and it gets very wide very fast exposure and it gets very wide very fast have you done have you done that have you done have you done that have you done have you done that yeah of course we do
Speaker 1yeah of course we do yeah of course we do but i mean so what do we expect what's but i mean so what do we expect what's
Speaker 2but i mean so what do we expect what's the range the range the range i i forget i don't know it's wide it's i i forget i don't know it's wide it's i i forget i don't know it's wide it's wide right i mean wide right i mean wide right i mean if we you know there's a there's a good if we you know there's a there's a good if we you know there's a good there's a reasonable chance that we see there's a reasonable chance that we see there's a reasonable chance that we see nothing nothing nothing in three times the stats there's a in three times the stats there's a in three times the stats there's a reasonable chance that we see something reasonable chance that we see something reasonable chance that we see something in three times the stats right and that in three times the stats right and that in three times the stats right and that that prediction is only worth that prediction is only worth that prediction is only worth you know the paper it's printed on it's you know the paper it's printed on it's you know the paper it's printed on it's probably not even printed probably not even printed probably not even printed um because we're gonna look right um because we're gonna look right um because we're gonna look right that's the answer the answer is and the
Speaker 1that's the answer the answer is and the that's the answer the answer is and the important thing to keep in mind is even important thing to keep in mind is even important thing to keep in mind is even if you see nothing if you see nothing if you see nothing it might still be real right you might it might still be real right you might it might still be real right you might have gotten lucky really have gotten lucky really
Speaker 2have gotten lucky really right that's right it might have been that right that's right it might have been that right that's right it might have been that tick and then nothing else for eight tick and then nothing else for eight
Speaker 1tick and then nothing else for eight seconds so you got to keep going right
Speaker 2seconds so you got to keep going right seconds so you got to keep going right after that we have to keep going right after that we have to keep going right so expert on this um so expert on this um so expert on this um what what is cutting edge indirect what what is cutting edge indirect what what is cutting edge indirect detection now detection now detection now um that would i feel like that would have um that would i feel like that would have um that would i feel like that would have to be modeled in fact maybe there's to be modeled in fact maybe there's to be modeled in fact maybe there's already a paper out there that says okay already a paper out there that says okay already a paper out there that says okay let's say no because people have been let's say no because people have been let's say no because people have been thinking about this and so it is now an thinking about this and so it is now an thinking about this and so it is now an exercise of let's plug in the numbers exercise of let's plug in the numbers exercise of let's plug in the numbers and ask and ask and ask you know what would a signal from you you know what would a signal from you you know what would a signal from you know know know the center of the galaxy look like if the center of the galaxy look like if the center of the galaxy look like if this is a this is a this is a 1tv particle or 10 tv particle etc 1tv particle or 10 tv particle etc 1tv particle or 10 tv particle etc then we can say something about its then we can say something about its then we can say something about its abundance abundance abundance and therefore and therefore and therefore you know how many might have collected you know how many might have collected you know how many might have collected in the center of the galaxy and what in the center of the galaxy and what in the center of the galaxy and what different signatures should we look for different signatures should we look for different signatures should we look for i mean people may go back to i mean people may go back to i mean people may go back to the data in the fermi gamery telescope the data in the fermi gamery telescope the data in the fermi gamery telescope and say and say and say did did we look you know if this is if did did we look you know if this is if did did we look you know if this is if this is the dark matter did we this is the dark matter did we this is the dark matter did we did we look you know is there a more did we look you know is there a more did we look you know is there a more pinpoint search to make to look for this pinpoint search to make to look for this pinpoint search to make to look for this thing rather than a broad search i you thing rather than a broad search i you thing rather than a broad search i you know maybe that's happening i don't know know maybe that's happening i don't know
Speaker 1know know yeah and um you mentioned uh yeah and um you mentioned uh yeah and um you mentioned uh the lhc as some place we will look but the lhc as some place we will look but the lhc as some place we will look but this is presumably this is presumably this is presumably if it is a one tev particle so the higgs if it is a one tev particle so the higgs if it is a one tev particle so the higgs boson boson boson for comparison is 0.125 tev for comparison is 0.125 tev for comparison is 0.125 tev that's the lighter presumably this is a that's the lighter presumably this is a that's the lighter presumably this is a lot of motivation for building even lot of motivation for building even lot of motivation for building even bigger particle accelerators
Speaker 2bigger particle accelerators bigger particle accelerators absolutely and and certainly you know absolutely and and certainly you know absolutely and and certainly you know we're we're in this shutdown now the we're we're in this shutdown now the we're we're in this shutdown now the lhc lhc lhc uh the machine is being upgraded the uh the machine is being upgraded the uh the machine is being upgraded the detectors are being upgraded detectors are being upgraded detectors are being upgraded um but um but um but you know this you know this you know this this this thing that because are we still this this thing that because are we still this this thing that because are we still pretending that it's real yeah let's pretending that it's real yeah let's pretending that it's real yeah let's pretend yeah okay pretend yeah okay pretend yeah okay um so that that this that that this that that this this may be a clue as to the sector this may be a clue as to the sector this may be a clue as to the sector where where dark matter is and this one where where dark matter is and this one where where dark matter is and this one particle wouldn't be the whole story of particle wouldn't be the whole story of particle wouldn't be the whole story of the particle physics if this is the particle physics if this is the particle physics if this is supersymmetry it has a lot of cousins supersymmetry it has a lot of cousins and so people will be looking in their and so people will be looking in their and so people will be looking in their current data current data current data you know making a more targeted search you know making a more targeted search you know making a more targeted search if i inform my search based on this if i inform my search based on this if i inform my search based on this let's also jump into the dan sean room let's also jump into the dan sean room let's also jump into the dan sean room and pretend it's real and pretend it's real and pretend it's real then how how should i refine my search then how how should i refine my search then how how should i refine my search to see if any any cousins of this dark to see if any any cousins of this dark to see if any any cousins of this dark matter particle matter particle matter particle are in my data right that's something are in my data right that's something are in my data right that's something that that you know you can do now i'm
Speaker 1that that you know you can do now i'm that that you know you can do now i'm presuming that essentially no data that presuming that essentially no data that presuming that essentially no data that your experiment could get would tell us your experiment could get would tell us your experiment could get would tell us oh it's supersymmetry like that that oh it's supersymmetry like that that oh it's supersymmetry like that that requires much more detailed data
Speaker 2requires much more detailed data requires much more detailed data right and and we have both you know the right and and we have both you know the right and and we have both you know the the benefit and the deficit of being a the benefit and the deficit of being a the benefit and the deficit of being a quite generic search we're looking for quite generic search we're looking for quite generic search we're looking for billiard ball scattering if we get a billiard ball scattering if we get a billiard ball scattering if we get a spectrum of events we could say a little spectrum of events we could say a little spectrum of events we could say a little bit more about it but that isn't you bit more about it but that isn't you bit more about it but that isn't you know supersymmetry is something that know supersymmetry is something that know supersymmetry is something that will be discovered in an accelerator will be discovered in an accelerator will be discovered in an accelerator yeah right and they will work out the yeah right and they will work out the yeah right and they will work out the couplings and they will say oh couplings and they will say oh couplings and they will say oh this seems to be we're finding the cousins this seems to be we're finding the cousins this seems to be we're finding the cousins of you the your dark matter particle of you the your dark matter particle of you the your dark matter particle that scattered from your xenon nucleus that scattered from your xenon nucleus that scattered from your xenon nucleus in june of 2023 in june of 2023 in june of 2023 and and and wow that's so cool because now we know wow that's so cool because now we know wow that's so cool because now we know how the wimp would have interacted or how the wimp would have interacted or how the wimp would have interacted or the supersymmetric particle would have the supersymmetric particle would have the supersymmetric particle would have interacted interacted interacted with other supersymmetric partners and with other supersymmetric partners and ordinary matter in the early universe ordinary matter in the early universe ordinary matter in the early universe when the universe was when the universe was when the universe was you know 10 to the whatever you know 10 to the whatever you know 10 to the whatever minus 37 seconds old minus 37 seconds old minus 37 seconds old and this is how dark matter would have and this is how dark matter would have and this is how dark matter would have been produced been produced been produced right that you know right that you know right that you know the journalist asked me did you guys the journalist asked me did you guys the journalist asked me did you guys solve the dark matter like not even solve the dark matter like not even solve the dark matter like not even close close close right i mean to really wrap it up and right i mean to really wrap it up and right i mean to really wrap it up and put a bow on it right i mean some of put a bow on it right i mean some of put a bow on it right i mean some of your listeners may be familiar with your listeners may be familiar with your listeners may be familiar with you know the steven weinberg book of a you know the steven weinberg book of a you know the steven weinberg book of a previous generation called the first previous generation called the first previous generation called the first three minutes three minutes three minutes we now understand you know based on we now understand you know based on we now understand you know based on laboratory measurements and laboratory measurements and laboratory measurements and observations and theoretical modeling observations and theoretical modeling observations and theoretical modeling and the big bang expansion and the big bang expansion and the big bang expansion we have a concordant picture of we have a concordant picture of the ratio of the light elements is the ratio of the light elements is the ratio of the light elements is produced you know in the first three produced you know in the first three produced you know in the first three minutes of the big bang minutes of the big bang minutes of the big bang the ratio of hydrogen to helium and the ratio of hydrogen to helium and the ratio of hydrogen to helium and deuterium etc etc deuterium etc etc deuterium etc etc right we know the laboratory physics right we know the laboratory physics right we know the laboratory physics we have you know we have you know we have you know by winding the hubble expansion by winding the hubble expansion by winding the hubble expansion backwards backwards backwards we knew the temperature of the universe we knew the temperature of the universe we knew the temperature of the universe at a certain time and there was this was at a certain time and there was this was at a certain time and there was this was the ratio of protons to neutrons and the ratio of protons to neutrons and the ratio of protons to neutrons and this was the nuclear binding energy this was the nuclear binding energy this was the nuclear binding energy and this is how many would have fused to and this is how many would have fused to and this is how many would have fused to make helium etc make helium etc make helium etc you know maybe this is the work of the you know maybe this is the work of the you know maybe this is the work of the next 30 years to to do the detailed next 30 years to to do the detailed next 30 years to to do the detailed laboratory studies and the observations laboratory studies and the observations laboratory studies and the observations and the 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 yeah and literally calculate the relic density based on the physics of this species that would be amazing okay we can
Speaker 1drop the reality distortion field and stop pretending that's okay back to work as a final 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 yeah how will you feel and also if it doesn't turn out to be real if you never see another one again how would
Speaker 2you feel 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 a little bit of relief like 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 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've 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 it happen we're going 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 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 etc etc etc 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 and it's not
Speaker 1just a matter of getting 100 tons of xenon and doing the same thing
Speaker 2again 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 background so if you're going to build a 10 times bigger thing you're going to have 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 we have merged with our current competitors that we're currently competing with the xenon n ton collaboration which is a partnership between some u.s groups european groups japan and they've been running a quite similar experiment to what we're operating um and with you know similar techniques similar exposure similar timelines um and a few years ago we joined forces with them to form xlzd xenon lux zeppelin darwin crappy name sorry apologies think xkcd xlzd helps you pronounce it um 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 um so that's just kind of cool to have you know sort of greatest hits from both teams to pull together and of course funding agencies like when you do this because then you're sharing around the cost and um 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 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 that it's going to be a great opportunity to build this instrument and we think we basically build the same instrument and you can use it to both do dark matter and double beta decay in
Speaker 1in xenon 136 you know you know i think that we all um 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 uh you know it does take a lot of time and a little bit
Speaker 2of more than a little
Speaker 1bit
Speaker 2of perseverance i enjoy the
Speaker 1hunt yeah you know yeah we'll see we'll check back okay 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 you know to share the wealth i
Speaker 2have i have i have 200 collaborators so that's true
Speaker 1but okay i guess we get them throw them some flowers as well but uh this was great dan akrib thanks very much for being on the mindscape podcast that was a lot of
Speaker 2fun sean thank you so much you
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