Daniel and Kelly’s Extraordinary Universe - Did the XENON experiment just discover a weird axion?

Episode Date: August 6, 2020

Recent results from the XENON experiment could be the first hint of something groundbreaking... or it could be nothing. Learn more about your ad-choices at https://www.iheartpodcastnetwork.comSee omnystudio.com/listener for privacy information.

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I'm Dr. Joy Hardin-Bradford, host of the Therapy for Black Girls podcast. I know how overwhelming it can feel if flying makes you anxious. In session 418 of the Therapy for Black Girls podcast, Dr. Angela Neal-Barnett and I discuss flight anxiety. What is not a norm is to allow it to prevent you from doing the things that you want to do. the things that she were meant to do. Listen to therapy for black girls on the IHeart Radio app, Apple Podcasts, or wherever you get your podcast.

Hi, it's Honey German, and I'm back with season two of my podcast. Grazacios, come again. We got you when it comes to the latest in music and entertainment with interviews with some of your favorite Latin artists and celebrities. You didn't have to audition? No, I didn't audition. I haven't auditioned in like over 25 years. Oh, wow.

That's a real G-talk right there. Oh, yeah. We'll talk about all that's viral. and trending, with a little bit of chisement and a whole lot of laughs. And of course, the great bevras you've come to expect. Listen to the new season of Dacias Come Again on the IHeartRadio app, Apple Podcasts, or wherever you get your podcast. Hey, Daniel, did you guys find dark matter yet? Not yet.

Still looking. It's been what, a few decades? Yeah, embarrassingly, more than a few. I'm just wondering, you know, are you sure you're doing it right? Well, you know, I think we're doing our best, but there's always a chance we're messing it up. Just asking because, you know, maybe it's time to bring in some engineers to take over and help you out. Oh, yeah, we could use a few cartoonists maybe lighten the mood over there. Oh, I see.

You need someone to make light of dark matter? Hi, I'm Horham, a cartoonist and the creator of PhD comics. Hi, I'm Daniel Weitzen. I'm a particle physicist, and I'm desperately seeking dark matter. Sounds like a movie from the 80s. Desperately seeking dark matter. I hope it is, because in the end of those movies, they always find what they're looking for. So that means that in about two hours, I'll discover dark matter. There'll be some ups and downs. But in those movies, they don't always find what they were expecting.

That's true. Sometimes it turns out the friends they made along the way are the real dark matter. Anyways, welcome to our podcast. Daniel and Jorge Explain the Universe, a production of IHeard Radio. In which we take you on a rom-com journey throughout the entire universe, hoping you'll fall in love with the biggest mysteries and the smallest mysteries, the craziest things that are out there, because the curiosity of scientists is your curiosity. The things you wonder about are the things that scientists today are still trying to understand.

Yeah, and sometimes we cover not just the science itself, but when scientists discover something new and how they went about doing it. That's right. And my favorite version of these stories is when somebody builds something new to look for something, A, and then they accidentally stumble across something totally different, which blows their minds and changes our understanding of the universe. Those are my favorite stories. That happens a lot? Like, you're looking for one thing, but you discover something else? That happens almost every time we turn on a new kind of eyeball to the universe. The universe is so filled with surprises that every time we create a new technology that lets us listen to the universe or look at the universe in a new way, we see something weird. You know, you got the cosmic microwave background radiation, this small hiss of background noise that filled the antenna in New Jersey. Those guys were definitely not looking for, but ended up being pretty good evidence for the Big Bang.

You got particles discovered here and there when nobody was expecting them. Every time we turn on a new telescope, we see some new kind of star or galaxy or black hole or weird stuff that we didn't expect. It's a wonderful experience. And what's the standard protocol? Do you like pretend you weren't looking for A and you were looking for B the whole time? Or do you pretend that it was all part of the, you know, master plan? No, it's the best kind of discovery, the unanticipated discovery. As an experimentalist, you're not interested in going to find what somebody else predicted.

Because then, hey, they get the Nobel Prize. It was really their idea. You're just checking the box. You're an experimentalist if you want to be an explorer. You want to go out in the universe and discover something new. And so, yeah, you're going to follow the map and get ideas from the theorists. But the fantasy is to find something weird, something new which changes our very understanding of the universe. And so, frankly, there's a bit of sometimes an overreaction.

Like, hey, I found something I don't understand. Maybe it's a crazy new discovery. Yeah, I'm sure McGillen and Lewis and Clark. and all those explorers would often say, whoops, what's this? Maybe it's dark matter. In the anachronistic science fiction time travel movie that I'm pitching Netflix, that's totally a scene.

Is it a romantic comedy as well? Of course. Everything has to be a rom-com these days. That's right. It's the Marvel formula. That's right. Dark matters of dark matter. But anyways, we're talking today about one such experiment

that was looking for one thing and may have accidentally or inadvertently found something else, maybe of more importance. That's right. This is an experiment that released the results a few weeks ago, and we got questions from listeners about what does this mean? And it's also made a real buzz in the particle physics community. Here's examples of what some prominent particle physicists said. Neal Weiner, a dark matter physicist at NYU, said, I'm trying to be calm here, but it's hard not to be hyperbolic. If this is real, calling it a game changer would be an understatement.

Whoa. Yeah. So people got pretty excited about this. That is pretty hyperbolic. Mike Turner, famous physicist at UChicago, former head of the NSF, said, quote, I really want to believe it, but I think it will probably break my heart. Oh, just like a good romantic comedy. It sucks you in, it makes you fall in love with it, and then it crushes you.

Ah, so there's a lot of buzz about this, huh? People are tentatively excited. People are tentatively excited. They want this to be something new, something fantastic. fantastic, something fundamental. On the other hand, of course, it could just be nothing. It could be to the experiment list, don't quite understand what their machine is doing. So all these professional, prominent particle physics professors are pausing their expectations?

Well, you know, they are pretty particular about claiming discovery. So you have to really cross a threshold before people believe you've found something new. All right. So today on the podcast, we'll be asking the question. Did the Xenon experiment just discover an axiom? Now, that's a lot of X's for one sentence. There's a lot of X's and a lot of Ons. Xenon, Axion.

Those are two words which sounds pretty science-y. You know, the X just kind of pushes it over. That's right, you know. And the Xion experiment is pretty cool. It's actually well-named because basically it's a huge tub of Xenon. And it's sitting in a mine underground in Italy looking for dark matter. and everybody's been waiting to hear what it says. Like, will it find dark matter?

And so it was already an exciting moment for particle physics when we knew they were going to announce their results. And so everybody was pretty surprised at what they ended up announcing. But anyways, we were wondering, as always, how much of this incredible potential discovery had made it out there to the public, how aware people are about this question. And so Daniel, as usual,

went out there into the wilds of the internet to get people's reactions to the question, did the xenon experiment just discover an axi? That's right. And if you're interested in participating in our virtual person on the street interviews and lending your speculation to our podcast, please write to us to questions at danielanhorpe.com. We are always looking for and welcoming volunteers.

Think about it for a second. Do the word xenon and axiom mean anything to you? If someone asks you this question, here's what people had to say. I have no idea what an axione is. But I do know that Xenon experiment has to do something with finding dark matter. I have no idea what the Xenon experiment is. I have no idea what Axions are. Just listen to that fascinating episode this week.

However, I have not heard about this one either. I'm not entirely sure, but I don't think so. The word Xenon just reminds me of Zeno Warrior Princess, so I have no idea what that is. Honestly, I thought Intel were just making computer hardware, not physics experiments. But what do I know? Maybe their new scene on CPUs have somehow discovered a deep truth of the universe. I have no idea of what either of them is. But whenever we hear statements in science with a question mark in the end,

then the answer is most likely no. All right. I'm with the person who said, it sounds like Xena, the warrior princess. I bet she made a lot of discoveries in her time, you know. She was an explorer, for sure. And a trailblazer. How to slice a person in half in one. swoop. I think she destroyed the dark crystal at some point in maybe one episode. I think you're

crossing your universes there. Is there a dark crystal in Xenon? Well, I was a little surprised that none of our listeners had heard of this result in science because it was on the New York Times and all sorts of websites and definitely a few listeners wrote in to ask us. But I guess it hadn't penetrated as deeply as I thought. So this may be the first time you're hearing about this fascinating result, in which case, I'm glad that we get to explain it to you. Yeah. Was it like front page of the New York Times or, you know, there's kind of a lot going on these days. There is a lot going on these days. I don't get the New York Times physical copy, so I can't really tell how prominently it is.

And I definitely dig down to read the science underneath all the crazy politics and medical pandemic news just to sort of escape that crazy university. Just to kind of sorbet your palate a little bit. Also, it's your profession. And I'm curious and I'm hoping that they will discover it. And I heard about it professionally also, you know, through particle physics. that's something exciting was coming. And so I was waiting to hear about this result. And by that you mean Twitter.

You heard it on Twitter. I'm not going to give away our totally secret mechanisms for communicating important scientific advances. Scientific. It probably is secret because nobody's following you. Just kidding. All right. So let's dig into it. So Daniel, a potentially amazing and groundbreaking and world-turning result.

has just been found in an experiment in this world recently a few weeks ago. So step us through it. What is the Xenon experiment, first of all? So the Xenon experiment is basically a huge tub of xenon cooled down sitting underground. And you might wonder like, why would you want to do that? Who would want to chill a bunch of Xenon down to very cold temperatures? And the reason is that it's looking for a very shy particle. It's hoping to spot one particle of dark matter flying through the earth.

and banging in to one of these xenon atoms. Interesting. So paint a picture for us. How big of a tub are we talking about? Is it like a pool or is it like a bathtub or is it more like a bucket? It's like a really big bucket, maybe like a hot tub size. I mean, xenon's pretty heavy stuff. This is about three metric tons of xenon.

And so, you know, it's about as tall as a person and maybe a meter in diameter. And so, you know, it's enough to like flash freeze Han Solo probably. Now we're talking language I can understand. And that was definitely a rom-com. I mean, if Star Wars is not a rom-com, I don't know what is. I know. Exactly. Exactly.

Anyway, the Zonon experiment was not trying to, you know, capture and freeze people who are on the run from interstellar bounty hunters. Instead, it was trying to capture a signal of dark matter. This stuff that fills the universe, but so far has been frustratingly invisible to us. I see. And why Xenon? Zon is one of the noble gases, right?

That's right. It's one of the noble gases. And we use xenon because if dark matter bumps into something, it's going to be a very small signal. And so what we want is a very big pile of very quiet matter that otherwise isn't doing anything. So that if we get a little signal, if dark matter comes in and happens to bump into one of these nuclei, we can tell. If you just got like a huge tub of hydrogen, there's all sorts of crazy stuff going on all the time. And if dark matter comes in and bumps a hydrogen atom, you wouldn't even notice. But a big pool of xenon just sitting there. there mostly does nothing. And so if something is able to penetrate a mile underground and bump into one of these xenon atoms, then you might notice. It's pretty chill. It doesn't, I guess, it's cold, so it's not moving, and it's also not very reactive, I guess, is what you're saying. Exactly.

And that's why we use these noble gases. Other teams we're thinking about using liquid argon, for example, but xenon really has the best combination of being available, not being crazy expensive, and giving off the right kind of signal when it does get bumped. Right. And it also fit the acronym better. Exactly. And that's how we make these choices, really, in the end. It's about PR.

It would have been awkward if the Xenon experiment used Argon, wouldn't it? Maybe nobody had to know. It could be a big cover-up. You know, this is Xenon Gate. It's all covered up anyways. Yeah. And, you know, we build this device because we're looking for a particular thing. We know that dark matter is out there. We know that it has matter, that it has gravity, that's some kind of stuff. but we don't really know very much else about it.

We hope that it also can do something else, that is that it can bump into normal matter and sometimes interact with it using some sort of force that's not grabbed. We know dark matter doesn't feel electromagnetism, so it can't be that force. We know it doesn't feel the strong force. We know it doesn't feel the weak force.

If it felt one of those forces, we would have seen it already. So we're hoping, beyond hope, that it also has some new kind of dark force, and it can use that to bump into normal matter. Interesting, huh. And we don't know that it does. It's just a guess.

It's just a hope. It's like, well, if it is this thing and it has this new force, then maybe we could see it this way. So xenon, this experiment really is built on sort of a lot of assumptions. Like, let's build the kind of thing that could see this very particular kind of particle. You could be wrong. Like, it could be that maybe dark matter only interacts through gravity, in which case, even this giant tub of xenon wouldn't see it or interact with it or catch it. Exactly. We have only very weak arguments.

to suggest that dark matter is a particle and that it can interact with normal matter in any way other than gravity. We've never seen it. We certainly have never proven that it can interact non-gravitationally. We're just sort of hoping it does because if it doesn't, we have no chance at ever figuring out what kind of particle it is. Oh, I see. Because gravity is so weak that you can only use it to study like enormous galaxy size blobs

of dark matter. So we're hoping it's there. And, you know, in particle physics, we often play the game of finding a negative result. like, if we build this thing and we don't see it, that means, hey, if dark matter is a particle, it doesn't have this kind of interaction. We can still learn something about what dark matter doesn't do. It's not nearly as exciting, but, you know, it's still new territory scientifically. You're still checking a box and hoping to get a clearer picture. Yeah, exactly. But, you know, sometimes you build this device to look for one very particular kind of particle and it spots

something else. You know, in some sense it's very specific. It's looking for this kind of particle, a wimp, a weakly interacting massive particle that we think dark matter might be. But on the other hand, it's just a very sensitive, very quiet detector that could notice some other weird new thing flying through the universe. So it's kind of a last attempt at trying to feel or touch dark matter. Because if it doesn't work, then it tells you that maybe we'll never interact with dark matter. Yeah. And this is sort of the like seventh step in the succession of these detectors.

They started with a very small little container of xenon just to see. if it worked. And it did. But the smaller amount of xenon you have, the less sensitive you are. So then they scaled up and they scaled up and they scaled up. And this is the first time they've had a detector that's like more than a ton of xenon. And as they were running this one, they're simultaneously building a bigger one. And the reason is that you want to run longer and you want more xenon because that gives you more chances to find it. So this is like Xenon XL and now they're thinking about Xenon XXL. Yeah. This is xenon one ton, and pretty soon they're coming with xenon n ton, which means like several tons of z.

And then there's competition. There's one in the U.S. called LZ, and another one in China called Panda X, and everybody's racing to build the biggest amount of xenon. And who has the coolest name for their device? Definitely Pandex wins that one. All right, so the idea is that you have this tub of xenon. It's chill. It's not very reactive. and the scenario is that maybe a dark matter particle will come in and bump into a xenon atom and then what, like move it or cause it to flash or wiggle? What's the scenario under which you might detect dark matter?

Yeah, so we're not terribly sensitive to it. All we can see is depositions of energy. Like a particle comes in and bumps the xenon. We can't see the particle that came in at all. All we can see is that the nucleus recoiled, like the xenon got pushed a little bit. And as you said, it deposits some energy and what it makes is that the xenon absorbs that energy from the little push

and then it gives it off again. It doesn't like to hold on to it. So it usually gives off a little photon. It gives off a little flash of light. And so this is one reason why we choose xenon is has really nice scintillation properties. Basically, you excite any of the xenon atoms and they form a little molecule pairs of xenons. Those are excited like wiggling back and forth and then they relax back down to two individual xenon atoms and give off a photon. And then you can capture those tiny little dark flashes of light with photo multiplier too. Interesting. Very scintillating for sure and tantalizing. I mean basically you have like a bathtub a mile underground in the dark where the camera attached to it and you're waiting for little

flashes of light. Which could mean ironically dark matter. Exactly. It could be shedding light on dark matter. All right. Well, that's what it was built for. But recently they announced that they saw something else and maybe even more interesting than dark matter. So let's get into that. But first, let's take a quick break. I'm Dr. Scott Barry Kaufman, host of the psychology podcast. Here's a clip from an upcoming conversation about exploring human potential. I was going to schools to try to teach kids these skills and I get eye rolling from teachers or I get students who would be like, it's easier to punch someone in the face. When you think about,

emotion regulation, like, you're not going to choose an adapted strategy, which is more effortful to use, unless you think there's a good outcome as a result of it, if it's going to be beneficial to you. Because it's easy to say, like, go you, go blank yourself, right? It's easy. It's easy to just drink the extra beer. It's easy to ignore, to suppress, seeing a colleague who's bothering you and just, like, walk the other way. Avoidance is easier. Ignoring is easier. Denials is easier. Drinking is easier. Yelling, screaming is easy. Complex It's problem solving, meditating, you know, takes effort. Listen to the psychology podcast on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts.

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Tennis is full of compelling stories of late. Have you heard about Icon Venus Williams' recent wildcard bids or the young Canadian, Victoria Mboko, making a name for herself? How about Naomi Osaka getting back to form? To hear this and more, listen to Good Game with Sarah Spain, an Iheart women's sports production in partnership with deep blue sports and entertainment on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. By Capital One, founding partner of IHeart Women's Sports. All right, Daniel, we're talking about the Xenon experiment that was built to detect dark matter. So it's a giant tub of Xenon. It's sitting there, chilling, waiting for dark matter.

But then they saw something that maybe is not dark matter. Yeah, so first of all, they've been looking for dark matter for a while and not seen. it and other folks been looking for dark matter for a while and not seeing it. And people started to get worried like, well, maybe it's not there. Or maybe it's different from what we expected because these experiments are really good at seeing dark matter if it has a certain amount of mass, something between like 10 and, you know, maybe 200 giga electron volts, which is about the mass of a proton. If dark matter was much lighter than that, it might not have enough energy to bump into

these xenon atoms and excite them. It might be there. It might be flying through your detector. It might be bumping to the xenon atoms, but not giving them enough energy to give off that flash of light. So people were worried about that scenario. So they pivoted and they said, well, let's use the same detector, but try to figure out a way to use it to look for lighter mass dark matter. And the way they do that is instead of looking for the xenon nucleus, the protons and neutrons that heavy blob at the center of the atom, they said, let's look for it bumping into the electron, because the electron is really light, has very, very little mass. Oh, I see. So they developed a technology to look for electron recoils instead of nuclear recoils. Those are different. Those are different.

They give different signatures. Like if you bump an electron off of a xenon, all of a sudden you have a charged particle inside this pool of xenon. and they have an electric field which will pull that electron out of this tub of xenon and measure it. So they can tell that signature separately from the nuclear recoil. Like a single electron or a single ion? You can detect that? You can detect that because it triggers a little shower. It makes more of itself and that lets you detect it. Actually, you know, the nuclear recoil and an electron recoil will give you scintillation light plus some ionization from the electrons. And so it's a game of like, you know, the ratios. You can tell them apart. It's a bit technical.

But they can tell an electron recoil apart from a nuclear recoil. Like are you hitting the center of the atom or are you bouncing off one of the electrons on the side of it? But does that require the dark matter to be like a certain energy? Like what if dark matter is also pretty chill and just, you know, it doesn't feel like interacting with Xenot? But it's there. And it could interact, but it's just chill. Yeah. Well, that's one of the issues that we already know that dark matter is chill.

We talked about this on a podcast pretty recently. dark matter we know is cold meaning that's not moving at relativistic speeds and so it doesn't have a whole lot of energy which is why it has to be kind of massive in order to deposit some energy we know that it's not carrying a lot of kinetic energy otherwise but I guess I mean like what if it's there

it's interacting with the xenon but not at a you know high enough energy or something you know it's just like gently bumping into the zenon it could be but remember that the earth is moving around the sun and so we expect to have some velocity relative to the dark matter.

Unless the dark matter happens to also be swirling around the sun at the same rate, there should be basically a dark matter wind at all times. It's not really possible to have no velocity relative to the dark matter. Not heard those words before. Dark matter wins. Yeah, in fact, there's a whole another generation of dark matter detectors.

They're going to try to look for directional dark matter, not just like, is dark matter coming in at all, but is it coming in this direction, or is it coming in that direction, is coming up from above or below, to try to be a little bit more sensitive to it. Like catching the ether. Yeah. And if you do see dark matter, you expect it to have modulation by season. Like it should be going this way in the spring and that way

in the fall. You really are sort of moving through a cloud of dark matter. Well, but back to this experiment. So they gave up on trying to detect it with the nucleus of the xenon atoms. And so they switched to detecting it with the electrons of the xenon atoms. And then they found something unexpected. Yeah. And you know, give up is a bit strong. These experiments are big and they have different teams. So they have sort of like a bifurcated strategy. They're still looking for the xenon nuclear recoils, but now they added this other way to look for dark matter to look for the electron recoils. And so they look for it and they ran this thing for a couple of years and they've been analyzing the data. And, you know, it's not like you can just see one electron recoil and be

like, aha, I found dark matter because there are other things that can also kick an electron. You know, like you're like a mile underground and you're surrounded by weird minerals and this lead and Krypton and stuff down there. And sometimes one of those atoms will decay radioactively and it'll get through your shielding and it'll kick one of your electrons. So what you have to do is a careful calculation of like

how often do you expect that to happen? And so you know like, well, we expect that to happen in this case 232 times on average when we run this experiment and then you can compare that to what you see. Do you see more than that or not? I see. And so that's what they did.

they, you know, I guess they had calibrated it. They measured, you know, the stuff outside of the box and the tub, and then they compared it to what they saw inside of the tub, and that was different. Yeah, the way they calibrated is actually they shoot radiation. They bring radioactive sources near it to verify that they can see them, and then they move them away to verify that the signal disappears. So they can use that to verify, like, how sensitive they are to these radioactive measurements. And then they use other ways to measure, like, how much lead in krypton.

is surrounding our experiment. So they do a lot of work to really calibrate. And that's the name of the game in these experiments where you're looking for like very small number of signals is beating down the background, suppressing all these other things that can look like your dark matter. And then also understanding them very, very precisely, calibrating very, very carefully.

Just like with LIGO and all those other very sensitive experiments, it's all about making a very quiet experiment and understanding how quiet it is. Right. Kind of like eliminating all the noise or, taken into account all of the noise. Exactly. Right.

So they got more hits than they expected of something. They saw more of these scintillations, these photon events, than they expected by a good number. That's right. So they expected 232 and they analyzed all their data and they got 285, which is something like 50 more than they expected. And they think they understand that number 232 pretty well. Like, they're pretty confident in that number. So it's pretty unlikely for, you know, lead and krypton to explain all these scintillations. Like, it could just be random chance.

I mean, everything is quantum mechanical and there are fluctuations. And they've done the calculations. But the probability of this just being like a fluctuation is like 2 in 10,000. Wow. But, you know, it still seems pretty amazing to me that it's a pretty small number. I mean, you know, 232 data points on a massive. experiment with significance about the universe doesn't seem like a lot. Like I would expect thousands or millions of data points, kind of like you have in the

particle collider. Yeah, it's a whole different kind of world, though. I mean, they are doing their best to make this really quiet because they expect a very rare signal, you know. And so if you're hunting for unicorns in the forest of Siberia, you scan a huge forest and you try to make your filter really, really picky. So you find the unicorns, they're not just like drowning in ordinary horses. With random horns in their foreheads.

Exactly. But you're right. Yeah, these data points are pretty rare. I mean, they ran for a couple of years, which means they get like one piece of data every day or two. That's crazy. The other side of this experiment, the nuclear recoil one is even quieter because those events are even harder to mimic.

And I remember times when they ran for two years and they saw two events. And they expected one. And they were like, ooh, interesting. What is this second event? Are you kidding? Seriously. Wow. And they get to know they're doing like, this is this event and that event.

They have names and relationships with these events. Where were you when we found the second blitz? Exactly. Exactly. So this is actually kind of a big number for a dark matter experiment. They're used to dealing with events like less than 10. Oh, my goodness. But because they went over to the electron side of things, they have larger background, so they see more events.

Okay, so I guess the idea is they were looking for dark matter and waiting for dark matter to interact with the xenon. and give off these events and they saw more than they expected even with dark matter or more than they expected from like a baseline, no dark matter scenario? Yeah, they saw more than they expected from the no dark matter scenario. Okay.

Right? But the signal they see is kind of weird. It's not the signal you would expect to see from dark matter. It peaks at a very, very low electron energy, like just above where they're able to measure. That's where all these events are piling up. Oh, I see.

So that's the mystery. That's the weird thing. They don't think this is dark matter that they're seeing. This doesn't look like dark matter. So they built this device to look for dark matter. It's very quiet. It's very beautiful. And they analyze the data and they see something in there,

which they can't explain using normal standard model physics and radioactive decays. But it also can't be described by dark matter. But how do you know it's not dark matter? Because we don't know what dark matter is. The signal that they see in the Xenon experiment can't be explained by dark matter whims that they were looking for. To give electrons a kick in the way that they see would require a really fast moving particle. And we think dark matter is cold. We think it's slow moving.

All right. So you're saying that they feel pretty sure that it's not dark matter then. Yeah, it just doesn't look like the dark matter signal that they expect. I mean, they don't have a whole lot of handles on this data. You know, what they can do is look at the energy of the electrons that are kicked off. And they have a prediction for what that looks like if it's dark matter. And they have a prediction for what that looks like if there's no dark matter. And it doesn't agree with either of those scenarios.

Interesting. The energy distribution they see can't be explained by a dark matter particle. It has to be like a third scenario, something else. That's right. It has to be something else. And so they came up with a few crazy ideas, which if they're real, could explain the signal and would like totally blow up physics. What?

They're like, it's unicorns. Essentially, they went for physics unicorn. All right, let's get into what it could be, what kind of new. And unexpected or groundbreaking types of physics could explain these results. But first, let's take another quick break. I'm Dr. Scott Barry Kaufman, host of the psychology podcast. Here's a clip from an upcoming conversation about exploring human potential. I was going to schools to try to teach kids these skills, and I get eye rolling from teachers

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Presented by Capital One, founding partner of IHeart Women's Sports. All right, Daniel, so the Xenon experiment did not find dark matter as they built it, but it found something else. It found electron signatures at an energy level that doesn't match with the predictions of dark matter, so it could be something else. That's right. And so they played around, they said, well, what is this? Like, you know, what could this be?

Could it be something else? Are there any other ideas out there? Any things we weren't looking for, but might be able to explain this weird signature that we do see? And they suggest in their paper a few possible explanations. They have several ideas for what this could be. They do. They have several ideas, which range from like totally crazy to super boring. I see.

From like pink unicorns to. like, you know, gnarwls that's just somehow migrated to the forest. No, it's like pink unicorns too. Actually, maybe we didn't tighten the knobs well enough. Oh, really? Huh. Well, let's start with the most boring one. Okay.

The most boring is that it's not just xenon in the tank. Like, they try to make it pure xenon. They really work hard. There's a lot of really smart people doing this experiment. But if instead of being pure xenon, it has just like a few atoms of tritium. Tritium is an isotope of hydrogen and is unstable. If you have like three atoms of tritium per kilogram of xenon, then it can decay to helium three giving off an electron, which looks exactly like this signature. Oh, I see. And they not only could it be a contamination,

but they can pinpoint what kind of contamination it could be. Yes. And it's very hard to measure the amount of tritium in xenon. It's very hard to get it pure. And it's very hard to isolate the tritium. And so they're working on that. They're using all sorts of clever techniques to try to isolate the tritium and measure it separately, et cetera, et cetera. But this all could just be a bunch of puffery around a little bit of contamination in their xenon. Okay. Is that common to have tridium accidentally in your xenon? I mean, I don't have any firsthand experience.

I don't have any xenon in my house that I've purchased. But yeah, I mean, xenon is naturally occurring and it's filtered out of the air and in the process of gathering. of gathering xenons, sometimes impurities come in. And so it's pretty hard to get like really, really pure xenon. So it's something they were aware of, obviously, something they were worried about. And it is something that they can use to explain this signature without invoking crazy new pink unicorn particles.

So they're working on that. I see. But wouldn't that over time decrease, like as all the tritium decays, that would go down eventually? I suppose it would. But, you know, this would be enough tritium in there to provide this signal.

I mean, the tritium does have a pretty long half-life. Oh, I see. All right. So that's the most boring. Sorry, least exciting, most boring explanation for this result. And so what's the next most exciting? The next most exciting is that maybe they saw a weird kind of neutrino. Like, we know the neutrinos are out there.

We've ruled out neutrinos as dark matter because we know dark matter, if it's a particle, has to be pretty heavy and move pretty slow. We know that because of the way it's shaped the whole structure of the, universe. So we know that neutrinos are out there, but there's not enough of them to explain the dark matter and they have too much energy. But people thought, you know, this huge device that we've built is also a good way to see neutrinos. Like if a neutrino flies through here and bounces into one of these electrons, then we could see that. Right. That's how they find neutrinos in the first place, right? Like a big tub of something chill. Yeah, exactly. Big tub of something chill is a good

way to find shy particles, especially you put it underground so you don't get bombarded by muons and all sorts of other stuff from cosmic rays. And so it's very similar technology to all the neutrino experiments. Like we talked about the Dune experiment, which is fundamentally very similar to this experiment. Equally cool acronym, yes. Equally cool acronym. But to make this signature, this sort of like weird spike in their electron spectrum, they need a particular kind of neutrino that we've never seen, which is a neutrino with a little magnetic field.

What? Like a non-neutral neutrino. Yeah, neutrinos don't have electric charge. And we think that the reason that particles have a magnetic field is because they have both electric charge and this weird quantum spin. So it's not like they're actually spinning, but there's some like weird particle analogy to spinning with charge. It gives you a little magnetic field. And we talked last week on the podcast about how a muon has a little magnetic dipole in north and a south. And you can measure it really precisely to learn secrets to the universe.

Well, neutrinos, we don't think they have them. But if there was a kind of neutrino, which did have a magnetic field, it would give you this kind of signature. So a new neutrino? A new kind of neutrino, yeah, a neutrino that has a little magnetic field. I see. And is that even allowable in the sort of loss of physics, or would this totally be new and break that down? This would be totally new. It would be crazy.

You would have to really rework the whole standard model to allow for a neutrino that had any sort of like electromagnetic interactions. It would break a lot of stuff. But that's exciting, right? That's like, hey, that's what we're doing this for. We're doing this to break our understanding. So we can rebuild it, right? That's what experimentalists are hoping to do is to find something new and crazy. You're like, break it, break it.

Exactly. But, you know, it's got to be real. And when you think about this kind of a new idea, you have to think like, well, if that existed, would we see it somewhere else? Is there another way we could or should have spotted this? You know, are you just trying to explain the fact that you didn't really? to get pure xenon and make it sound dramatic. If these new kind of neutrinos existed, they would interact with the xenon in a way that could maybe explain this weird data.

Yes. Okay. So that sounds pretty, I don't know, interesting and groundbreaking, but you're saying that there's a third possibility, which is even crazier. That's right. And so there's another idea, which is maybe they didn't see dark matter. Maybe they didn't see neutrinos. Maybe what they saw were this weird particle called axions.

Yeah. We talked about axions a couple of episodes ago, right? They're detergent particles, right? They clean up the other molecules. Yeah, they name of them. And atoms, right? There you go. They do all the dirty work of the universe.

Yeah, they're a crazy particle invented to solve a problem in theoretical physics. You know, why two things seem to balance, and we don't know why. And they invented this axion to give those things balance. And then as a bonus, people realized, hey, wait a second, maybe axions could be the dark matter. And we talked about it on the podcast a few weeks ago. And axions, if they exist, they're sort of like a photon, but they have a little bit of mass. But they're really, really not very heavy. They're like a tiny little bit of mass, like one one thousandths of an electron bolt, which is very small, given that like an electron is like half a million electron volts.

So these things, if they exist, would be like, you know, a billion times less mass than the electron. Right. But you still sort of think of them as a heavy photon, like a photon with mass. That's right. And if axioms are out there, then in order to be the dark matter, they need to not be moving very fast, right? Dark matter is cold. And axioms are very, very low mass. And so this experiment couldn't see dark matter axioms.

But they said, all right, well, we can't see dark matter axions. What if there's a new weird kind of axiom? Like one that's made in the sun and shot out with a lot of energy. So like a hot axion. Wow. Sounds like a reach. It's a bit of a reexion. Yeah, it's a bit of a reach.

Like, let's put on all of our idea hats, everyone, because we're going to lose funding if we don't come up with some cool ideas. Hey, I thought you'd be impressed. It's sort of like, you know, physics engineering. They're like, all right, what if we take a piece of this idea, and we staple it to that idea? And then we hang the whole thing on this third idea, and it sort of, you know, does what we need to do.

Is that why you think of engineering? Yeah. In the view of engineering, like, that's seen from Apollo 13, where they're like, what if we use duct tape to glue this tube over here? Exactly. Is that not the high water mark for engineering? I'll let that pass. So you're saying this is like creative physics here, creative, you know, problem.

Yes, yes, exactly. They're coming up. They're like, what can we do to explain this weird signal in an exciting way? Because who wants to write a boring paper about tritium? We want to write a paper saying maybe we discover this crazy new thing that nobody ever thought could exist, but we might have broken open the universe. Meaning, because if you do find this axiom, this new

kind of potential hypothetical particle, so it's like a hypothesis on a hypothesis, right? So if you do find it, that would break the loss of physics? Well, it would be hard to explain because nobody knows why axions would be produced in the sun. And if they were produced in the sun

in order to have enough speed to be seen by the Xenon experiment, then it would cool down the sun. Like it would be pumping out a lot of energy. And we would expect stars in the sky to fade out much faster than we see. So the solar axon is sort of already disfavored by lots of things in physics. It's sort of contradicted by astrophysical measurements already. I see. It'd be too weird. If it does exist, it means we need to re-understand how stars work, which is, hey, that's

exciting. And we need to understand why stars are making this axiom and why this axiom exist in this way. So it would be a pretty big discovery. If solar axions were real, it would make us rethink a lot of stuff. I see. You have to rethink not just the standard model, but also like how stars work. Yeah. And we've gotten pretty good at understanding how stars work. You know, we have a good model for how they burn and how they die and the various kinds of stars that are out there. And so this would throw a wrench in like a pretty well-established field. Right. So that's pretty exciting to be at a time when, you know, an experiment like this that's high profile finds something unexpected and it could be some pretty amazing things.

Yeah, it could be. But you know, my personal opinion is that this is a big reach. You know, they see something weird in their data. That's cool, but you know, we see weird stuff in our data all the time. And usually it's because we didn't really understand the backgrounds. We didn't really understand the performance of our instrument. There was something weird going on. It was miscalibrated or some other source of these events that we didn't anticipate.

And so you got to be really skeptical. And that's why we have a really high threshold for believing that something there is new. Like, first of all, you'd have to see it in another experiment. An independent experiment would have to see the same thing, hopefully using slightly different technologies or, you know, being differently sensitive to sources of bias. And the other thing that makes me wonder about this is if you have a chance to Google it and to look at the data, you see that it all sort of piles up right on the edge of where they can see. You know, they can see electrons down to a certain energy and then below that, they just can't detect them.

And all of these things pile up right on the edge of where they're able to see, which always makes me suspicious. Like, do you really know what's going on at the very extreme ends of your detector? So it just makes me wonder if really, in the end, this is an issue of understanding your detector response. I see. Because I guess there's a secret option D, which is that it's just nothing. Yeah, that is just nothing. Which is just like they just didn't calibrate it well or, you know, it's different than they were expecting because what they were expecting was wrong. Yeah, and I don't mean that they didn't.

didn't do their jobs well, or that they're not smart. This is super duper hard. They're doing something nobody else has ever done before. They're not just like ordering something from Amazon and turning it on, right? They're pushing the bounds. Which is basically engineering, Daniel. It sounds like that's what you think. You said it.

No, engineering would be ordering six different weird things from Amazon and making them do something else. That would be awesome engineering. But now, these folks are pushing the boundaries of what can be done. They've won the race to get like the one. one-ton experiment up and running and working and with this new clever technique. And so I'm not criticizing them at all, but often when you're on the bleeding edge, you don't understand the data that comes in at first and it takes a while to figure it out and to really damp it down.

So that's where they are. And they don't know if this signature means, hey, the universe is telling you a deep secret that it's been waiting to reveal for 14 billion years or, you know, you got to twist that knob a little harder because the experiment's not quite tightened up. Right, and you get to chill that xenon a little bit more. Yeah, it could be. But fortunately, we do have more experiments coming. There's an experiment in the U.S. that's coming up.

It's called LZ, and it's got basically the same strategy, a big tub of liquid xenon. And there's one in China called Panda X. That's underground. It's huge. And so if this is real, they should also see it and we'll hear more from them soon. Right. Is it a requirement that they need to use some of the letters from the end of the alphabet, like Z, X, X. Why? Well, you know, it's xenon. And so they got to have an X in there somewhere, right? Because Xs are awesome.

They mark the spot out here. Yeah, exactly. They're exciting. All right. Well, it sounds like stay tuned is the answer to this question. But it's got physicists excited. And it could mean that we need to rethink our signs that we have about the universe, you know, part of either standard model or how stars work or what kinds of neutrinas there could be. So that's pretty exciting. Or it could be just that we need more data. Yeah, I hope it's something new. I hope that it breaks physics and teaches us something about the universe.

I'm pretty skeptical, frankly, that it's anything real. But stay tuned and keep an open mind and an open heart because that's why we do this stuff. We're asking the universe questions and we got to listen to what it tells us. All right. Well, we hope that answered the question. And we hope it provided some interesting things to think about for those of you who had not heard of this experiment.

So stay tuned for more exciting news. That's right, because this dark tub liquid underground might be shedding light on dark matter. There might be a unicorn bathing in it. That is a very strange mental image. A big unicorn with an X painted on its chest. I don't think that's ethical treatment of unicorns to put them in a dark bathtub a mile underground. Maybe that's what they like. Daniel, maybe we have to rethink our understanding of unicorns. I think the Society for the Ethical Protection of Unicorns is going to be writing you an email.

What's the acronym for that society? I'll pass that one on to my creative partner. All right. Well, we hope you enjoyed that. Thanks for joining us. See you next time. Thanks for listening. And remember that Daniel and Jorge Explain the Universe is a production of I-Hawks,

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