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Big Ideas
Neutrinos, Matter, and Antimatter: The Yin Yang of the Big Bang

What happened to all of the universe's antimatter? Can a particle be its own anti-particle? And how do you build an experiment to find out? In this program, particle physicists reveal their hunt for a neutrino event so rare, it happens to a single atom at most once every 10,000,000,000,000,000,000,000,000 years: far longer than the current age of the universe. If they find it, it could explain no less than the existence of our matter-filled universe. This program is part of the Big Ideas Series, made possible with support from the John Templeton Foundation.Learn More

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NATALIE WOLCHOVER: Hi everybody. I’m so glad to see so many people here because I think we’re gonna have a very interesting and unusual discussion about antimatter.

WOLCHOVER: Our first guest is an assistant professor at MIT who focuses on answering big questions about the universe by developing novel particle detectors. So please welcome Lindley Winslow.

WOLCHOVER: The next participant is an assistant professor of physics at the University of Massachusetts Amherst who conducts research in experimental, nuclear, and particle physics. Please welcome Andrea Pocar.

WOLCHOVER: Our final guest this afternoon is also a physics professor at MIT. She’s a member of the IceCube experiment, which is located at the South Pole. Please welcome Janet Conrad.

WOLCHOVER: Thanks so much for being here. Talk to all of us. I guess Janet, maybe you could get us started by telling us how antimatter was discovered and what’s so strange about it.

JANET CONRAD: So antimatter was actually discovered in the 1920s, and it wasn’t expected at all. At that point in time they had a pretty nice description of how the world worked. They had certain building blocks, and there was no extra need for any kind of a extra particle, particularly one that looks exactly like the matter particle except that it has a opposite electric charge. And so it was a real shock when they actually saw this. They saw this in something which is called a cloud chamber. When a particle goes through them coming from cosmic rays, you can actually see the particles.

CONRAD: Field. And if you have a particular kind of charge, say a plus charge, it’ll bend in one direction. And if you have a negative charge, it’ll bend in the other direction. And so that’s what they were seeing when they discovered antimatter, and it was not expected at all.

WOLCHOVER: Yeah. So they immediately … Did they immediately know that there was this mystery of why … where is all the antimatter?

CONRAD: It took a long time to actually understand that every particle that we have in our standard model, and we have a lot of particles in the standard model, actually have, apparently, an antimatter partner.


CONRAD: Or I think if you think about the standard model and all of the particles in it, at least we are certain that all of the particles that have electric charge have an antimatter partner. There is a special particle that is … I’m very, very fond of called neutrinos. It’s my favorite particle. And neutrinos… The word neutrinos means little neutral one, and neutral is a good name for it. It has no electric charge associated with it. And so that’s where the mystery of whether neutrinos have a distinct antimatter partner or not actually comes from.

WOLCHOVER: So we know that something caused this, but could you talk about the conditions that this mechanism has to meet in order to favor matter?

CONRAD: Right. We have a very big problem. So if you ask anybody what’s their favorite equation out there, it’s E = mc2. You can ask anybody what their favorite equation is, and that’s the one that they will come up with. E = mc2 tells you that energy can be turned into particles. It turns out that they have to be turned into particles, the antiparticles, in equal amounts.

CONRAD: So if you think about it, if I am producing something that has electric charge out of something that had no electric charge, I had better produce the opposite charge also so that everything will balance out. So that means that whenever we produce particles out of energy, we also get an equal number of the antimatter with it, and that’s a big problem if you live in a universe that is clearly only matter. So I like to say that the biggest crime that ever happened is that somebody stole all our antimatter. It’s completely gone, and that’s a lot to steal.

CONRAD: So we have to think about what it is that can actually make this happen. And so we have to introduce into our theory some kind of a strange behavior among the particles that will give you a matter, antimatter imbalance. So it tells you that somehow the antiparticles must be behaving differently from the particles, and there are not so many places within our standard model where that can actually happen. But it turns out the neutrinos are one of the places where you could actually fit that in.

WOLCHOVER: So maybe Lindley, could you tell us about … well, I guess first about Majorana particles, which the neutrino might be one. So what are Majorana particles and what do they have to do with this big question that we’re trying to answer?

LINDLEY WINSLOW: So a Majorana particle is a particle that’s it’s own antiparticle. And so you could figure out that there might be some sort of mechanism where if this is happening you could make more matter than antimatter. And so as Janet was sort of alluding to, because neutrinos don’t have any electric charge, there’s nothing really to tell you whether they’re their own antiparticle. It’s not obvious. And as an experimentalist, it’s the not obvious thing that you wanna go poke at.

WOLCHOVER: So why is it that if you have a particle that is both a matter and antimatter particle, it’s the same thing, then why would that help with our problem where we’re actually trying to generate an asymmetry?

WINSLOW: So if you have a particle that’s its own antiparticle, then you couldn’t make a process happen where you make more matter than antimatter. So you don’t conserve, as Janet said, the matter and the matter in the reaction.


WINSLOW: You make just a little bit more.

CONRAD: With most of the particles … most of the cases of the particles in the standard model, you need a particle and an antiparticle to collide, and then that produces energy or something like that and the whole thing disappears. But if the neutrino is its own antiparticle, then effectively you can have an antineutrino and antineutrino actually annihilate and disappear. It’s because there isn’t any real different between an antineutrino and a neutrino in this picture.


ANDREA POCAR: Right. Or another way of seeing it maybe is if a neutrino comes in produced by some process, in its flight it transforms into what we call antineutrino and then produces a reaction that produces the matter of the other kind. And so again, its transformation in that case can occur.

WINSLOW: It’s a very weird thing. Neutrinos are being a little bit weird, and we still like to poke at them somewhere.

CONRAD: That’s actually what makes neutrinos so special.

WINSLOW: I think that’s why we love them so much is they like to do funny things.

CONRAD: Because they’re doing things that the rest of the particles are not allowed to do.

WOLCHOVER: Constantly, right? They’re constantly doing things that surprise everyone.

CONRAD: They’re very independent.


CONRAD: Right.

WOLCHOVER: So the person who first proposed this idea that neutrinos might be Majorana particles was in fact Ettore Majorana who was a very strange character. Could any of you tell us … maybe Andrea, could you tell us about him? Who?

POCAR: Yeah. He’s a fellow Italian, so maybe I’m talk about that.

WINSLOW: You’re especially … Yeah.

POCAR: So he was Sicilian, and he was a genius since a young age. And he got into Fermi’s group in Rome right at the very early age of the nuclear era when the nucleus was starting to be understood. And apparently there are stories that he showed up in his group and the very first day he was given the, you know, what the status of experiments were in the lab in Rome and mysteries of calculations that couldn’t be completed, that were difficult, couldn’t match up what the measurements were saying. And apparently he showed up the next day with … saying … telling the people there that they had done a good job because everything that they had calculated so far was correct in only one night apparently. This is a mixture of legend and reality. But he was for the few who actually knew him, he was very precocious, very independent. He liked to work alone. And the mystery is that he vanished. Maybe this is a telltale of the things he was studying in a way.

WOLCHOVER: So I’m wondering what evidence we have that neutrino is a Majorana particle or how we would find evidence that it is.

WINSLOW:So obviously, since we haven’t answered this yet it doesn’t … It’s pretty hard to answer this question. So the idea that the field is really going after is to look for this rare process called neutrinoless double beta decay.


.And so the mechanism is that the two electrons get spit out with their two neutrinos. And you can either think of it as what Janet said earlier, that the two neutrinos annihilate because they’re Majorana particles, or that because they’re Majorana particles they transform into the other one and kind of get sucked back into the decay. And so if we saw this process of two electrons coming out and no neutrinos, then we’ve seen evidence that the neutrino’s a Majorana particle, and that would be really exciting.


WINSLOW: Yeah. And there’s our Majorana neutrinos.

WOLCHOVER: Yeah. So the little-

WINSLOW: And now the …

WOLCHOVER: Wiggly line is they’re coming out then …

WINSLOW: And there we go.

WOLCHOVER: Oh, yeah.

WINSLOW: And so if they’re Majorana, you can just complete the line there.


CONRAD: So you see there that now we have two electrons coming out, right? And we have lost the antineutrinos that were coming out, that the antimatter is not coming out of that decay.

WINSLOW: Yeah. So this process made matter and no antimatter, and so that is why we are so excited about it.

WOLCHOVER: And so who figured this out?

POCAR: It was figured out in the 30s already. The 30s was a time when things moved extremely quickly from figuring out what a beta decay is, actually is, to figuring out, well, if that occurs, then we will have the two electron decay as well with neutrinos coming out. But then if neutrinos might have this property, they might, as we say, annihilate and maybe this other process exists. It hasn’t been found yet.

CONRAD: But just to … Yeah. To make it very clear, the process on this side has been seen. The process on this side is the one that we are looking for.

WOLCHOVER: So you’re both looking for it.

CONRAD: Mm-hmm.



WOLCHOVER: So maybe for-

CONRAD: Yeah, I like my neutrinos. I like to see my neutrinos. Yeah.

WINSLOW: So we’re actually no neutrinos.

CONRAD: They’re the no neutrinos. I am the neutrinos.

POCAR: Oh, I do both.

CONRAD: You do both. That’s true.

POCAR: Just to be safe.

WOLCHOVER: So this process over here only happens on average once every 10 to the 21 years in a typical nucleus. But then this one … How rare is this one over here?

POCAR: That’s at least 10000 or 100000 times slower at least. We only have limits on its occurrence.

CONRAD: And Lindley’s holding the record on that limit, right?

POCAR: Lindley is holding the record on that.

WINSLOW: Currently a 10 to the 26, but I … So next week is the big meeting for all of neutrino physics, and I understand that we’re about to lose it to another … the third competitor between. The one experiment that’s not represented on…

WOLCHOVER: What is this record? Or how do you-

WINSLOW: 10 to the 26 years.

WOLCHOVER: So what does that mean that that’s a record?

WINSLOW: That is … We haven’t seen anything, and so we know it has to happen less than one time in 10 to the 26 years.

WOLCHOVER: Okay. In a typical nucleus? Yeah.

WINSLOW: So to give you an idea for how rare this is, the experiments that Andrea and I are building now, we’re going to have one ton of nuclear material. And we expect five decays a year if it is at that 10 to the 26 year half-life.


WINSLOW: So five-

POCAR: And I’ve realized that sounded a little scary. It’s actually regular atoms that we put together. It’s not nuclear material in the sense of a fuel from a reactor or anything like that. It’s special. It’s isotopically identified pure, but it’s not dangerous.

WOLCHOVER: And everybody uses a different type of nuclear material, right? People have theories like, oh, this one’s gonna be better for this purpose or …

WINSLOW: Yeah. We argue about that a lot.

POCAR: And we argue how to use it too.


POCAR: Maybe the same one, but used in different ways.

WOLCHOVER: So what’s the material that each of you-

WINSLOW: So Andrea and I both like xenon.


WINSLOW: I use it in a warm tank of liquid and with … This liquid makes light when charged particles move through. That’s how we will see those electrons. And then you detect that light with photo detectors. And then you like to use your xenon …

POCAR: Cold and liquid. Yeah. In a tank that’s made cold and pure. We don’t mix it with anything.

WINSLOW: So what temperature is…

POCAR: It’s refrigeration really. So -100 C or 170 Kelvin, roughly.

WINSLOW: So it’s not really the liquid nitrogen temperature, but I bet it makes great ice cream anyway would be my guess.

POCAR: Yeah, yeah, yeah. It’s enough.

WINSLOW: Very expensive, great ice cream.

POCAR: But you like really cold as well.

WINSLOW: Right, right, right. So then some days I like xenon, and other days I like tellurium. And so my other experiment is … well, the tagline for it’s the coldest cubic meter in the known universe. And we will give up the known universe tagline if we discover aliens doing dual beta decay research.

POCAR: With antimatter.

WINSLOW: With anti-

WOLCHOVER: How cold are we talking?

WINSLOW: 10 millikelvin. So the universe at large is around 3 Kelvin. So that’s outer space. That’s not very cold compared to us. We are 100,000 times smaller in temperature. So this is the world’s most powerful … It’s called a dilution refrigerator. It’s a very expensive version of the refrigerator you have at home. So they’re 5 by 5 by 5 centimeter crystals of tellurium dioxide. So they are just a little bit cloudy. They’re mounted in copper, and the copper is connected to the refrigerator. And so the copper gets cooled down, and then it cools down all of those crystals you see there. That’s 19 towers, 988 crystals.

CONRAD: So something that you have to be very careful with when you’re building an experiment like this is to have everything be very, very, very clean. Because even the tiniest amount of dirt actually has something in it that’s likely to have a radioactive decay, and then that will fool your experiment.


CONRAD: And so it’s not only one of the coldest places in the universe, it’s one of the cleanest places.

WOLCHOVER:When did we first start looking for this decay and what were those early attempts like? How did we get to the point we’re at now?

CONRAD: I would say first you have to discover neutrinos to decide if you’re going to look for no neutrinos. So I think that the first thing you had to do is discover the neutrinos, which actually we did through looking for them coming out of the beta decay process that happens inside of reactors. So the first discover of the family of particles we call neutrinos was actually an antineutrino, and that was discovered in the 1950s.

WOLCHOVER: So how do we know it’s an antineutrino if we think they might be the same?

CONRAD: Okay. Because when these particles interact they are going to produce. If it is an antiparticle coming in, it’s going to produce an antiparticle coming out. And so in comes my antielectron neutrino and out comes a positron, which is the antielectron. So you don’t actually know exactly what’s coming in. You can’t see it because you can’t see neutral particles in your detectors. You only see charged particles. But out suddenly out of nowhere pops this positron, and you say ah ha, I must’ve had an antielectron neutrino coming in.

CONRAD: Now, the thing is that that’s how we’ve built the theory. We build the theory this way because with all of the other charged particles that we see it behaves this way. If you have a particle coming in, you have a particle coming out, antiparticle coming in, antiparticle comes out of these interactions. But we don’t know that for sure. But that was the assumption was that neutrinos and antineutrinos are distinct and that they create these … their partner.

CONRAD: So we actually can’t tell in these interactions. And the only way I think we’ll be able to tell is through this neutrinoless double beta decay instead. But we see many cases of both neutrinos and antineutrinos now. In fact, the largest source of neutrinos coming at you are neutrinos coming from the sun.

CONRAD: If you could see neutrinos, you could look at the sun. And this is what the sun would look like if you were looking at it in neutrinos. That’s actually what the sun looks like in one of our very large neutrino detectors called the Super-K Detector. But in fact, you can see here that these are pixels, right? You can see each little square. And the actual size of the sun is the size of the little square that’s in the middle. So your resolution, your ability to actually resolve something is very poor if you’re trying to see things in neutrinos. So it’s not really a very good sense to go ahead and develop if you were evolving, so there isn’t a lot of need for it. Plus, you also need to become very, very massive in order to be able to see them.

WOLCHOVER: They don’t interact very much. There’s billions of them going through each of us every second.

POCAR: Ten billions per square centimeter or the size of a thumb.

WINSLOW: Yeah, the size of your thumb. Yeah.

POCAR: Per second.

WOLCHOVER: Constantly going through us. And they just don’t touch anything? They don’t …

CONRAD: Right. They don’t interact very often. They’re a very independent particle. So we call it the weak interaction because of the ones that are in our standard model, it is the least likely to actually have an interaction. And so this makes them wonderful particles to study because they can come from a very long distance to you. And then if you get lucky, they’ll interact in your detector. But the interactions are so rare that we have to build very, very large detectors.

WOLCHOVER: Would you describe it? It’s such an amazing thing that humans have built the IceCube detector.

CONRAD: I love this experiment. It’s really fun. It’s actually at the South Pole. It’s right at the South Pole. We use the Antarctic ice as the interaction mechanism for the neutrino. So we’re looking for neutrinos that are produced in the universe to come through and interact in the ice, and then we’ll see the particles that come out of those interactions. So to be able to see the particles that are coming out, we need something that will detect what the particles emit. It turns out that they will emit photons, the particles that are coming out. So we can use these detectors, which are called phototubes, which are absolutely beautiful, spherical objects that are gold in color. The golden metal is what’s responding to the light. So we need to drill a hole because we wanna put this deep below the Antarctic ice about a kilometer down. And so how do you drill a hole in ice? Anybody know?

AUDIENCE: Hot water.

CONRAD: Hot water. That’s exactly right. We just take hot water, and we melt our way all the way down in the hole. We put in the detector, and we allow it to refreeze around the detector. And it is literally a kilometer cubed in size.

WOLCHOVER: So there’s one of these in many places.

CONRAD: So there’s … Yeah. There’s about 5000 of these light collection modules over this kilometer.

WOLCHOVER: And they form this kind of cubic array.

CONRAD: Mm-hmm- And we look for neutrinos to come in from outer space and interact in this and produce a big burst of light which we read out, and from that we can understand all kinds of interesting information about them. But the one thing that we can’t tell is if they’re a neutrino or an antineutrino. We can tell you what kind of neutrino they are. Neutrinos come in three different types within our standard model, and we define what the type of neutrino is based on what it produces in the interaction. So we can tell you if a neutrino came in and produced an electron. We can tell you if it comes in and produces a muon. We’re looking for the case where a neutrino comes in and it produces a tau. But we cannot tell you if that was an antielectron neutrino or a … because we don’t have a magnetic field. It would be very, very hard to build a magnet that could cover a kilometer cubed. Yeah.

WINSLOW: We have other uses for that magnet.

CONRAD: Right.

WINSLOW: But what’s neat though is that you can get the handle of is how important neutrinos are. Because they run the gamut from these very, very high-energy neutrinos that are more energetic than anything we’ve ever been able to make on Earth down to the thermal neutrinos that were made in the Big Bang. And therefore, if you tweak the properties of neutrino just a little bit … and the biggest one is this Majorana neutrino antineutrino difference … you can really change how the universe formed. And I think that’s really what drives all of us here on stage is … That’s why we love this particle. It’s not…

WOLCHOVER: And there might be other … Besides just the Majorana question, there could be also other neutrinos that we haven’t discovered yet?

CONRAD: Alright, that’s my favorite question. So the thing about neutrinos is that it’s the one part of the standard model where we really see deviations from what we actually expected from what the theorists were telling us we ought to see. It is a place where nature is really talking to us instead of us maybe telling nature what to do with our theories, right? I really like exploring there. And we have seen some hints out of nature that there might be additional neutrinos beyond the three that we know of and love so well. But it’s very complicated, the picture of what we’re seeing in all of our experiments.

CONRAD: So we’ve been working slowly toward definitive evidence that something is really going on. And one of my experiments, MiniBooNE, just took a really big leap this week. We just put out a new paper that moved us closer. But Natalie had asked me do I see this as my eureka moment, and the answer’s not quite yet. It takes scientists a long time to decide this is really something completely different.

WOLCHOVER: Basically you saw evidence that maybe there’s a sterile neutrino, right? Which is …

CONRAD: Right.

WOLCHOVER: So if neutrinos are ghostly particles … People often describe them as … just with that nickname … sterile neutrino is a shadow of a ghost.

CONRAD: Right. Absolutely. I think ghost is a great description for neutrinos. Because how do you know you have a ghost in your house? You know because you look around and there’s this debris. The ghost came in and made a mess.

POCAR: Oh, that’s why.

CONRAD: Yeah. You thought those were the neutrinos, didn’t you? So the same thing happens in our detector. The new neutrino comes in, and it makes a mess. We don’t see the neutrino come in itself, but we see the mess that it makes. So I think ghost is a really good description of it. But the sterile neutrino actually will interact even less often than the standard model neutrinos that we have. And so what happens with them is we have to see them when they play a game with the other neutrinos of neutrino oscillations causing those neutrinos to disappear and come back again. So that’s where the sterile neutrino comes in.

CONRAD: But all of this tells you how rich the field of neutrino physics is, that we have all of these different clues like they might be Majorana coming from theory or there might be sterile neutrinos coming from experiment. And so it makes it a really rich place to work. And we actually think that if you put these ideas together you might be able to get an overall theory that can explain all of these different aspects.

35:13 WOLCHOVER: Yeah, that’s something that has always struck me is that we kind of look to neutrinos to solve many of the mysteries that we have, questions about

CONRAD:For being a particle that you are not all that aware of probably, they’re actually a pretty important particle in your life. Because for example, the sun wouldn’t shine if we didn’t have neutrinos. The very first process that ignites the sun is actually one that involves neutrinos in it.

WINSLOW: But more importantly, neutrinos are what blow up stars and supernova and make all the elements.

CONRAD: I want the sun to light, not blow up.

POCAR: And I measured that.

CONRAD: That’s right.

POCAR: The neutrinos from the sun.

WINSLOW: Oh, okay.

POCAR: This particular process in fact was actually published in 2014 for the first time.

WOLCHOVER: Oh, tell us about that. What did you find?

POCAR: Borexino is the name of the experiment at the same lab where is in central Italy. And it’s a big sphere of an organic liquid that has a property of producing some light when interactions happen in it, like a neutrino comes in and hits an electron for example, and that’s how we detect these neutrinos from the sun. And it’s arguably the one largest, radio cleanest volume in the universe except vacuum.

POCAR: And yeah. So this experiment was able to measure the neutrinos from the sun at low energy for the first time on a event by event, and that allowed us to identify that these belong to this process as opposed to another process in the sun.

WOLCHOVER: And then you did say…

POCAR: It’s a whole chain of processes that emit them.

WOLCHOVER: So you can say based on this, okay, now we know how the sun shines.

POCAR: We know how the sun shines.

CONRAD: You can say the sun is shining, right? It takes a really long time for the photons that are produced in the center of the sun to make their way out, go down lower in energy and lower in energy until they’re visible, and then they finally come to us, the eight minutes across to come to us. But it takes 10000 years.

POCAR: Between 10 and 100000 depending on who you talk to.

CONRAD: Right, so you never know. Maybe the sun turned off.

WINSLOW: And we have 10000 years

CONRAD: You can tell us that it didn’t…

POCAR: But we will know only in 100000 years, so I think we’re fine. At least this room is fine.

CONRAD: But you can tell us that it didn’t. It’s fine, right? Yeah, the neutrinos are there?

POCAR: Right. So we know that we’re safe for another 100000 years.

CONRAD: Yeah. Right, right, right.

WOLCHOVER:Yeah. To, I guess, getting back a little bit to these experiments that are directly looking for neutrinoless double beta decay. I’m still a bit curious how we figured out even how to do an experiment like this. I mean, what … You said you prefer xenon. How do we figure out, okay, if we get a bunch of xenon together maybe we can see this?

WINSLOW: Well, I was telling you a little bit about the type of nuclei that can do this process. And so you can actually then go through … We have tables of isotopes for a variety of reasons, and you can pick out. And there’s about 50 candidate isotopes.

POCAR: Mass. Yeah.

WINSLOW: Yeah. And then in order to detect something, the higher energy it is, the easier it is. And so we’ve sort of taken the 10 highest energy ones, and those are the ones that are easy to … would be easy to see. And then you try to figure out how to build a detector with them, and that’s really … Actually all three of us on stage are experimental physicists. Our job is to build detectors and answer questions. And that’s why actually this field for me is so fun is that it’s this game of, okay, I have xenon. What can I do with xenon? And xenon’s fun because you can actually do actually every technique with xenon.

CONRAD: I think one of the interesting things about it though is that the ideas behind how to do this, how to look for neutrinoless double beta decay, we’re actually identified by one of the really great female physicists, Maria Goeppert-Mayer. I have an award that’s named for her, and she’s just an … was an amazing person. I know she’s Lindley’s-

WINSLOW: She’s my hero.

CONRAD: Yeah. Great. So maybe you wanna tell a little bit about her.

WINSLOW: So right. So as Andrea was saying, sort of the 30s was sort of this time of great jump forward with our understanding of nuclear physics. And she had a preliminary model for how the nucleus worked, and she did the first calculations of this rate to kind of give us the goal post. And if you ask sort of why it took so long, well first we didn’t know if neutrinos really existed. We had to measure them in the 50s. And then we had this sort of detour where we didn’t … We wanted to see the sun shine. And so in the 60s, we started to try to look for these solar neutrinos, and that turned into a debacle that took 30 years. 40 years? 40 years.

POCAR: Oh, a great debacle I would say. I mean, it’s-

WINSLOW: It was a great debacle.

POCAR: It pays for our jobs I guess.

WINSLOW: It did.

WINSLOW: But that simple question of will this detect the neutrinos from the sun turned out to be really hard. And that’s easy compared…

CONRAD: And complicated.

WINSLOW: And yeah. And that’s easy compared to double beta decay. That’s where we are now is now it’s okay, now we know kind of what to do. Can we do it?

POCAR: Yeah. And we’ve been doing it … We I mean as a community. We’ve been doing it for about a half a century almost now. I think the first experiments were in the either late 60s or early 70s with detectors of the size of a gram or so. And then now we’re thinking about tons.

WOLCHOVER:So I guess there was a range of … that Mayer calculated. It could be this likely or it could be this likely.

POCAR: Yeah. Well, actually there’s a history to that too. Originally, the first calculation seemed to say that the neutrinoless decay should’ve been faster than the regular two-neutrino decay. And then we’re looking at those calculations, it turned out not to be so. But there was uncertainty into how to calculate these things because it’s new physics. And so any time there’s a new process, you put in numbers, which are reasonable or minimal extensions of what you know, but you fundamentally don’t know. And so you have to at some point look at that. And then you do the biggest, most sensitive experiment you can do in a reasonable timeframe, a few years or something, and you look for the process. And if the experiment is too small, you won’t see it. And then you go to the next stage because you’ve learned something. You learned how to do it better. And that’s how the field has progressed.

WOLCHOVER: Mm-hmm. So there is some range, and we’ve kind of cut through part of the range. We’ve excluded some range, and now we know that this process is rarer than a certain length of time. Is that kind of how it works?


WOLCHOVER: So how far are we along the scale of …

WINSLOW: So I would say we’ve just reached a very exciting point. Because of this information coming from sort of other types of experiments, we now kind of know where the goal posts are. So the best limits now are 10 to the 26 years. The next set of experiments is aiming for 10 to the 27 years. And then if we can build an experiment that’s sensitive about to 10 to the 28 years and we don’t see something, then we know that the neutrinos are not Majorana particles because there’s just not any … not much theoretical space left for them to be. And so we’re really-

WOLCHOVER: Okay, so we have to do 100 times better right now?

WINSLOW: We gotta do 100 times better. So that’s really kind of a neat place to be. Of course, if Janet’s sterile neutrinos exist, we could have a even more fun thing in that for us. It moves things around as to where this decay would … where we’re looking in that.

CONRAD: It would be much more fun.

WINSLOW: It would be so much fun.

POCAR: Also, the goal posts you talk about are based on this minimal diagram that has been shown, which is a very reasonable place to go. But on the other hand, neutrinos have surprised us. And so we might actually see the double beta decay with a half-life that doesn’t quite match this expectation because the process might actually be more complicated than what was shown on the screen. We’re talking about a fundamental process of nature, if it exists. And maybe nature is more complicated than the minimal complication we’re trying to add to explain things.

WOLCHOVER: Mm-hmm- And so what would it be like if you did discover this? How would it all play out?

POCAR: Champagne.

WOLCHOVER: Well, I actually … I wrote a story about one of these experiments a few years ago that had finished. GERDA. Yeah. And they talked about how they blinded the data, and then they had an unblinding. So maybe … I mean, I don’t know if people are aware that that’s how it’s done, but physicists are so careful that you don’t even know that you’re gonna make … you’re not biased while you’re doing the analysis, right? You just do it without even looking at the numbers, and then everyone gets together and then unblinds it? Is that how it happens?


POCAR: I guess in most cases that’s how it happens.

CONRAD: Yeah, that’s-

POCAR: It’s a little experiment specific exactly how that is done.


POCAR: But yeah. And the pressure’s very high on this measurement in particular now. There’s a lot of competition, a lot of people trying to do it. And any claim… In fact, there have been in the past a positive claim of having found this decay that turned out to be wrong. And so even more so I think there’s the burden of proof on us if we think we found something new.

WOLCHOVER: Which experiment was that?

POCAR: It was the GERDA predecessor.

WOLCHOVER: Oh, okay.

POCAR: But it was a much smaller collaboration.


POCAR: But kind of using this similar technique.

CONRAD: But I think it shows you can go wrong, and his discussion shows you can go wrong in both ways. So for example, there’s been experiments that set limits on the two-neutrino double beta decay that were just not correct. And it’s very important to go and explore even those regions that are ruled out because it turned out that they had made a mistake and missed the signal. That’s a really crummy thing to have happen. And it can go the other way also. You can have some kind of an effect in your experiment that is looking a lot like the signal, and it’s really important for somebody else to come along and do a different experiment in order to make sure that what you are seeing really is the signal.


WINSLOW: So I think this goes to sort of why I work on two different experiments and why on the stage you see three different experiments is that in order to really know that we saw the signal, we probably wanna see it in two different isotopes. So tellurium and xenon. We can share that. And two different detector techniques because it could be a detector artifact, and that has happened in the past that we’ve detected things that turned out to be something we didn’t understand about the detector.

CONRAD: They’re only talking about five events if they get lucky, and so it’s a very tiny number of events. And so it could be that something’s gone wrong.


CONRAD: People are a little bit hard on scientists in the sense that when something … when they see something that looks like a signal, scientists can say, “Oh well, we have observed this to a certain level,” and people are like have you discovered something or not? And it’s really hard for us to say yes for a very long time until there’s many, many cross checks on these things. Because it’s so easy to go wrong. Experimental physics is a real art.

WOLCHOVER: Mm-hmm. So when … If or when you discover this-

WINSLOW: When. I’m an optimist.

WOLCHOVER: How big of a deal would it be? I mean, what is this?

WINSLOW: This is the last great question of the standard model.

CONRAD: I think it’s really huge because right now we have no idea what the larger theory is. We have reason to think that there is a larger theory because we can put together this thing called the standard model that has many particles in it, and we can start arranging them, just the same way as you would arrange a periodic table. And we have a lot of history with putting together tables of things and then discovering that there was an underlying theory behind it. Plus there’s stuff that we don’t understand like dark matter, right? But we have no idea what the larger theory is. For many, many years we pursued super symmetry, and that just has not turned out to be the right direction, even though it was theoretically very, very promising.


CONRAD: So we need something that’ll direct us toward what kind of larger theory there is. And neutrinoless double beta decay connects to a very specific class of theories and would allow us to take all those ideas that are out there and really narrow them down.

WOLCHOVER: Mm-hmm. Yeah, so maybe some … I know you’re all three experimentalists and there’s kind of a wall between you and your theorists colleagues, but maybe you could talk about just if this decay is observed what larger theories that might point to. It would mean that we would have this mechanism for understanding. Yeah. Is there a name for it? It’s not string theory or … Yeah.

POCAR: I guess … Let me just make a little intro to this. If neutrinos behave like this in this funny way of being their own antiparticles, in a way, that naturally opens the doors to these objects to exist. I mean, there might be particles out there that also have this feature, no charge and behave, but which are heavy enough that have never been seen. And in fact, I would say that a majority of theorists … Now, that doesn’t mean that it’s the right way to look. Sometimes as a consensus, that doesn’t …

CONRAD: Yeah, the vote of everybody doesn’t necessarily mean it’s right. Yeah.

POCAR: Doesn’t mean that it’s more probable necessarily, but …


WINSLOW: The only vote.

CONRAD: Super symmetry being the good example of that.

POCAR: Super symmetry being a good example of that. But there’s a lot of thinking about whether dark matter, for example, is made of particles which are also of this kind and are maybe linked to processes in physics which are mediated by particles which are too heavy for the LHC, for example, have discovered. And the other thing is our current theory lacks to tell us why these neutrinos don’t exist. I mean, suppose they don’t exist and neutrinos are just the standard ones that we know. A good theory to me is a theory that explains, predicts, but also tells us whether any of the possible solutions that doesn’t violate any fundamental postulate of a theory isn’t seen. And neutrinos, based on what we know, should actually behave this way because you can write terms in the theory that behave exactly like a neutrino that turns into a antiparticle without really violating any of the fundamental pillars of the theory. And so a theory that has solutions that you kind of say, oh, I just throw these out because they’re unimportant, is still an incomplete theory to me.

CONRAD: Yeah. We’ve made a lot of progress by arguing if it can happen, it will happen. Something has to stop things from happening for us to not see it, and so that’s sort of what’s behind that particular idea.


CONRAD: But one of the things that we think is that at very high-energy scales there is a grand unified theory, a theory that is very simple and then as you go to lower and lower energies becomes more and more complicated.

WOLCHOVER: So whatever existed right after the Big Bang, the theory was very simple. And then as the universe cooled and energies decreased, symmetries broke and things got more complicated.

CONRAD: Things became very complicated. So people like to describe it as you make a pot of soup and it’s all very homogenous, and then you let it cool and you get globs of stuff in it. And kind of that’s what’s happened, we believe, with our particles. And so there are these grand unified theories that have these Majorana heavy partners in them. And to try to probe at those, we need to look for the light Majorana particles.


POCAR: And it’s also very hard to get rid of them, to write a theory that doesn’t have these pop out in many ways. I mean …

CONRAD: Right.

POCAR: In that sense, it’s very compelling. And the neutrino is the only particle that we know exists that we can directly probe.


POCAR: And so it’s a natural place to do it.

CONRAD: So this is always a problem for theorists because there’s a whole set of things out there that it’s very hard to get rid of in your theory. One of them is these Majorana particles. Sterile neutrinos are an example of this extra neutrino that I’m looking for. Proton decay is another big one. The fact that we haven’t seen protons decay, which is quite good for us because it would be bad if our protons were decaying, but many, many theories have died on the point that they are predicting proton decay and it hasn’t happened.


CONRAD: So yeah. So we’re really at a point where I think we also need to start thinking a little bit more about the way we approach our theories and whether this if it can happen, it will happen is the right way to think about it. I think that at this point, particle physics is really at a turning point, and I think it’s a turning point that’s gonna be really driven by experiment. So sometimes theory drives experiment, and sometimes experiment drives theory. The healthiest view of the field is when it’s going back and forth, rotating back and forth, and I think we’re seeing a rotation right now.

WOLCHOVER: So to that point of just that even though it can happen, this … the particle going … and neutrinoless decay, it actually might not happen. So could either of you who are actually searching for this decay, could you talk about what it would mean if this decay doesn’t exist and the neutrino is not its own antiparticle? And that’s called a neutrino, right? As opposed to Majorana neutrinos. So yeah-

WINSLOW: Well, in that case the neutrino is just like all the other particles of the standard model, which would be disappointing for us, but we still answered a really important question. And then going back to what Janet said is working on this theory where if it can happen it does, then we’d have to find a reason why it’s not happening. And so that would be then pushing back on our theory friends. Okay, explain to us then, what exactly is preventing this from being there?

WOLCHOVER: Mm-hmm. So if a neutrino and an antineutrino are different particles, then … But they don’t have charge, so it seems like they should be the same one. So maybe they have some other property that-

WINSLOW: There’s some other property. We know particles carry these sort of intrinsic properties. Charge is the easiest one to discuss.

CONRAD: They’re sort of like … It’s sort of like the DNA of the particle, and charge is one little bit of its DNA and it can have lots of other aspects of its DNA also.

WINSLOW: So then neutrinos would have to have an extra little chromosome that’s preventing them from being Majorana, and you have to explain.

CONRAD: But if it’s there, we don’t know why and we have no explanation for that.

POCAR: Experimentalists are also looking for any magnetic tiny behavior of the neutrino. So far there’s only limits. We haven’t found any. But that would be again, if found, that would be a strong indication of Dirac behavior. Because now you have electromagnet properties of this neutrino, so it’s not completely chargeless in the sense of that we think it is so far.

CONRAD: That’ll be a really hard experiment to do.

POCAR: Yeah.

CONRAD: That’s a really hard one. Yeah.

POCAR: Yeah.

WINSLOW: So that’s the thing though about experiment. If you do the easy experiments first … And then what’s left gets harder and harder. But yeah.

POCAR: Harder and harder or brand new?

WINSLOW: Brand new. You then have no idea what’s gonna…

POCAR: But it’s only by trying that you’ll hit the brand new. I mean, it’s not by being idle and not doing anything that you’ll hit something.

CONRAD: The brand new is an important point though. Right now, at least in my area of neutrino physics, one of the things that worries me is I see people proposing larger and larger versions of the detectors that we’ve worked on for many, many years. And I worry that at some point it’s just not gonna be sustainable to build these detectors bigger and bigger. That we have to actually completely rethink our technology and our approaches, and we really need to put some investment into that.


POCAR: Well, I guess accelerator science has also been going that route, right? I mean, you have bigger and bigger accelerators, higher and higher energies, but more and more the low energy effects of physics at high energy is being pursued also because practically building bigger machines gets harder.

CONRAD: I’m actually working on something related to that. I’m actually working on how to take tiny accelerators, which are called cyclotrons, make them even more powerful than they have been in the past, and then you can bring the accelerator to the experiment instead of having to build the experiment next to the accelerator. And so you can take these existing, very large detectors and put an accelerator next to them. The nice thing about this is that actually this particular accelerator that I’m working on will also be I think a really valuable source for medical isotopes too at the same time. So you can feel like you can do more than just the basic science with it.

WOLCHOVER: Yeah. I was thinking about this earlier actually when you talk about that right now the limit is on 10 to the 26 years that this … What is that by the way? That’s a trillion trillion… a hundred trillion trillion years.

WINSLOW: I would do a piece of paper to check that.

WOLCHOVER: Yeah. So it doesn’t decay in a hundred trillion trillion years.

POCAR: It’s a hundred trillion trillion.


WOLCHOVER: But then now you’re trying to look to see if it decays in a thousand trillion trillion years, so you need 10 times more material to do that, right? And then to go one more order of magnitude you need 100 times more material where you’re studying it … monitoring it for the same amount of time hoping that one particle in there will undergo this decay. So is it possible to get 100 tons of xenon? I mean, aren’t we already kind of at the limit of what we can do?

POCAR: Possible is very possible. Certainly technology doesn’t scale that easily. And when you scale up an experiment there is a phase where you gain quickly, but then there’s a second phase where the complexity of the scale up itself, the engineering complexity of the scale up, kicks back. And so it’s unclear whether, as Janet said, you can just brute force scale up only. I think you have to get smarter as well. And so mitigate the scale up with smart tools, smart techniques that you can implement. And I think we’re all trying to think about these possibilities. There are some ideas there that are being developed still in the protophase.


POCAR: Yeah.

WINSLOW: So sort of building on that … So you guys saw that pretty picture of CUORE with all those crystals. The next thing we need to do with CUORE is actually we’re gonna take crystals that not only are cold, but they also give off light. And so that’s actually what that red crystal was about because it glows if a charged particle goes through it in addition to this heating up. And so that’s sort of the things that we’re looking at is how to be smarter about the detectors that we’re already building.


POCAR: Yeah.

WOLCHOVER: Are these experiments running right now? Both of your experiments?

POCAR: Yeah. I mean, as far as I’m concerned, EXO 200 is running in a salt mine in New Mexico.

WOLCHOVER: Why are they always in these mines and…

WINSLOW: Because we like…No.

POCAR: Because we don’t like easy things. No. The reason is, our experiments all have to run underground to shield them from cosmic rays in the atmosphere. And so we use the earth as a shield, and we have to go roughly a kilometer or so underground. And for EXO 200, at that time in the United States that was an available hole in the ground that we could go to. In this particular case, unlike the CUORE example, this is not a laboratory. This is a salt mine where they dispose nuclear contaminated materials from the laboratories I have enriched for the bombs.

CONRAD: Which sounds like a really bad plan for an experiment that needs to be very clean.

POCAR: It does. But the mine is very large, and it’s kind of a proof that is actually done fairly well.

CONRAD: Yeah, yeah.

POCAR: In a way. Because we could run one of the cleanest experiments in the world a kilometer away from a storage of barrels of plutonium contaminated stuff. We’re underground, but the actual detector itself is a xenon liquid container like a bucket inside a cryostat, which is an instrument that makes it cold. And then it has layers of shielding from radiation that comes from the periphery of the detector. It’s been running since 20 … late 2010 I would say. We’ve already published data three times. It’s scheduled to end in 2018, and we’re already into the design phase of a five-ton follow up, which is still on paper or silicon, called nEXO. And that’s gonna be a scale up of what we’ve learned with X 200 with a number of, we think, clever additions or changes that make the scale up better.

POCAR: But there’s gonna be, as far as EXO’s concerned, the EXO program, a gap of taking data for a while. Our technology in its … the strength and the risk of the technology is that it goes in steps. There’s one detector, you build one detector, you run it, and then if you want a bigger one you have to build a bigger detector. CUORE for example is made of crystals, and so there’s technologies like that that could be scaled up more in phases in principle. Maybe not CUORE itself specifically, but others. And that really depends on the choice of the technology. In a way, going for the big jump-

WOLCHOVER: What are we looking at here by the way?

POCAR: Well, this is a container of the EXO 200 experiment. It’s made of copper. This is commercial copper, but its commercially selected copper. Every screw that went into this detector that now is … in the picture is empty, but it’s instrumented inside and then filled with a liquid, has been screened for radioactivity. So we have to go an excruciating program of monitoring and every material, every component, ever cable, every screw that goes in there because one hot spot, one screw that wasn’t cleaned appropriately, there’s a fingerprint on it, will swamp the rate of the detector completely. And so that’s a big risk obviously. And sometimes you know only when you put it together and run it. And opening it up to fix it is months of work that you don’t want to do.

POCAR: So X 200 was put underground. It was actually welded in this container never to be opened again thankfully, but it was designed to be possibly opened if needed. And it was in that sense built very much like a satellite is built. You test it, and then you seal it and you just hope it runs. You hope. You’re pretty confident it does, but you know, you’re never really sure. The turning on of the detector was an interesting phase.

POCAR: So nEX is gonna come next hopefully. I mean, it’s gonna be a much more expensive experiment. It’s a bigger collaboration. We’ve expanded the collaboration as well. And then on the side we’re thinking also about what after nEX, so in terms of being more clever. And some of our collaborators are developing brand new techniques to identify the appearance of barium atoms in a xenon five-ton container. That would be the telltale sign that a double beta decay has occurred. So they’re developing imaging techniques to measure not just a single atom in a matrix, but that one that corresponds to a certain amount of release of energy in the detector and so on and so forth. That’s kind of beyond the nEXO project, but it’s possibly ways of being smarter.

WOLCHOVER: So before we go to questions from the audience, I just wanted to ask each of you just to make a prediction I guess of when you think this decay is going to be seen. First of all, if you think it’s gonna be seen and then kind of what your hunch is about the prospects.

POCAR: You know, I have a mild optimism it will be seen. I have to be careful. It goes back to blinding. You look for something because you really think it could be there, and that’s for sure. You have to stay honest with the other answer being possible as well, otherwise I think that goes down a bad spiral in general. Whether … When … If yes, then when is beyond me, but I hope in my lifetime. I really can’t make a prediction on that.

WINSLOW: So I think I’m going to make a harder prediction. I think we’re gonna see it in the … at the end of the next generation of experiments, so 10 years, and it’s going to be in a part of the parameter space that no one was expecting to see it.

CONRAD: You just stole mine.

WINSLOW: Oh, I did?

CONRAD: Yeah. That was my answer too.

WINSLOW: But I don’t-


WINSLOW: I’m gonna let you have the sterile neutrinos, and I’m gonna say that it’s some combination of sterile neutrinos and a weird mechanism.

CONRAD: Oh, okay.

WINSLOW: Okay. Now you can go.

CONRAD: Yeah. I have the same view. One of the things that happens is that you do these blind analyses and you open the box and you expect one thing, say nothing, or a signal, a specific kind of signal, you open the box, and you discover it’s not what you expected at all. And I think that’s what’s gonna happen to them, and I think that that will be really fantastic for particle physics. And I also was gonna guess 10 years.

POCAR: And that will guarantee a lot of jobs too.

CONRAD: A lot of fun.

WOLCHOVER: Alright, well I’m sure everybody has some questions they’ve been racking up.

WOLCHOVER: Yeah. Back there.

AUDIENCE: Trying to understand this, but is there any evidence of the annihilation of the majority of matter? You know, they measured the cosmic background radiation for the Big Bang. Is that related to the annihilation of matter and antimatter? Is there any empirical or real evidence of that occurrence?

WOLCHOVER: When everything annihilated except all the matter that’s left. Yeah. Do we have any evidence?

CONRAD: I’m afraid that that happened so early in our history of our universe that we actually can’t look back to that. But I think one of the things that is really interesting about neutrinos is that they actually allow you to look further back in the history of the universe than we can with the photons. So what happens is that you have a universe that’s just full of energy photons, and they’re just sort of swimming around. And then finally the universe gets to be big enough where the photons get far enough away from each other that they’re not interacting, and then they sort of free stream outwards. And that’s the point. That’s the last … You can look back to that point, and you can’t look any further into what happened in the early universe for these fingerprints.

CONRAD: Neutrinos, that happens with very, very early in the universe. And so if we could see the cosmological neutrinos, we could learn an enormous amount. The problem is those neutrinos really don’t have very much energy. They’re all hanging around right now. There’s about … What is it? A billion of them in every cubic meter of space. But they’re not doing very much because they’re not very energetic, and so trying to figure out how to find them, that is one of the holy grails of neutrino physics. And there’s some ideas out there, but it’s… that’s…

WINSLOW: If you thought double beta decay was hard

CONRAD: Try to do… But it’s a good question. And if we could get there, we would get there. We would go. Yeah.

WOLCHOVER: Alright. Well, that’s a great place to finish. Let’s thank our speakers.

WOLCHOVER: Alright. Thanks so much.

Big Ideas
Neutrinos, Matter, and Antimatter: The Yin Yang of the Big Bang

What happened to all of the universe's antimatter? Can a particle be its own anti-particle? And how do you build an experiment to find out? In this program, particle physicists reveal their hunt for a neutrino event so rare, it happens to a single atom at most once every 10,000,000,000,000,000,000,000,000 years: far longer than the current age of the universe. If they find it, it could explain no less than the existence of our matter-filled universe. This program is part of the Big Ideas Series, made possible with support from the John Templeton Foundation.Learn More

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