The story goes that Wolfgang Pauli, who first proposed the existence of neutrinos, was embarrassed to have done so, as it was considered uncouth to hypothesize new particles that could not be detected. Modern physicists have no such scruples, of course, but more importantly neutrinos turn out to be very detectable, given sufficient resources and experimental technique. I talk with neutrino physicist Ryan Patterson about what current and upcoming experiments teach us about neutrinos themselves, as well as implications for dark matter and why there are more particles than antiparticles in the universe.
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0:00:00.5 Sean Carroll: Hello, everyone. Welcome to the Mindscape Podcast. I'm your host, Sean Carroll. These days, in particle physics, in quantum field theory, in the attempt to understand the universe at its deepest levels. As you know, if you've been listening to this podcast, we have this great theory, the standard model of particle physics. But we also have plenty of reasons to believe that the standard model is not the final answer. It doesn't play well with gravity. It doesn't explain dark matter. It gives us a bunch of sort of tempting clues that somehow there should be grander unification of some sort. So there's all sorts of reasons to go beyond the standard model to invent new particles and new properties. And indeed, there's lots of papers out there in the scientific literature where people propose new particles. I've proposed new particles. I don't know many of my friends in the theoretical physics world who haven't proposed a new particle or two. But back in the day, long ago, the standards were different. You know, when physics 100 years ago was going on, there was so much data that we had that hadn't been explained by theory yet. The people weren't just proposing new particles just for the heck of it.
0:01:08.0 SC: They would wait until they really were forced to do that by some experimental phenomenon. A great example of this, of course, is the neutrino. The idea of the neutrino was proposed around 1930 by Wolfgang Pauli because there was this known phenomenon, namely beta decay. This is the decay of the neutron. Neutron is a little bit heavier than a proton, so it can decay into a proton, which is a positively charged particle, and an electron, which is a negatively charged particle. But there's a couple of things that weren't quite working out about that idea of a neutron going to a proton and an electron. After all, the neutron is spin 1/2, the proton is spin 1/2, and the electron is spin 1/2. Like, where did the extra angular momentum come from? And more careful experiments showed that even energy wasn't really being conserved in these interactions. So there was what, by modern standards was a perfectly obvious thing to do, propose a new invisible particle that carried away a little bit of the spin and a little bit of the energy. But in 1930, that was just not done. So Wolfgang Pauli eventually did it. He proposed there was a little particle, but he was embarrassed to do so.
0:02:19.1 SC: He was like, "Sorry, maybe it's an idea. Don't take it too seriously." Of course he was embarrassed because he didn't think that the particle could be detected in order to fit the data, in order to explain what he needed to explain, it had to be a very, very weakly interacting particle. And indeed, the neutrino, as we now know it is a pretty weakly interacting particle. But we're much better now than we ever were back 100 years ago at detecting these new kinds of particles. So not only are neutrinos known to exist because we can detect them, but there's an extremely active and interesting neutrino experiment, physics program that is flourishing in the modern world because it turns out that there's a lot more to do. Once you discover that neutrinos exist, you realize that there are different kinds of neutrinos, different flavors of neutrinos, as they are called. You realize that the neutrinos have masses, even though the mass could have been zero back in Pauli's day. Now we know because the different kinds of neutrinos oscillate into each other. That's a phenomenon that can only exist if the neutrinos have mass. And they can even violate interesting symmetries that they might otherwise conserve, like what is called CP, charge-parity conjugation.
0:03:31.9 SC: It's roughly speaking, the symmetry between matter and antimatter. And that means that neutrinos might play a hugely important role in the history of the universe, because the universe has an asymmetry between matter and antimatter. If neutrinos give us a way, there might be other ways also, but neutrinos give us one way to violate that symmetry. Maybe studying neutrinos better in these experiments could help us understand the origin of the matter, antimatter asymmetry in the universe. So this is what we're talking about today with our guest, Ryan Patterson, who is a particle physicist experimentalist at Caltech. My former colleague, Ryan, and he's a member of several different neutrino experimental collaborations. So we're going to get an overview of what neutrinos are, what their properties are, why they're special in the zoo of particles in the Standard Model, what we do know about them and what we don't know about them as much as we do know about them. There's more that we don't know about neutrinos than about any of the other known particles that we can measure, just because they are very, very hard to produce and detect. We can do it, but it's not easy.
0:04:38.5 SC: And so we can't do it to quite the precision we would like. That's why we're building bigger and better experiments to be able to do exactly that hopefully that will help us explain both existing puzzles like the matter antimatter asymmetry and maybe even the nature of dark matter. But also it can just lead us into new clues to go beyond the Standard Model to figure out things that we don't really have clues about right now, to be back in the day where you were doing new theoretical physics based on data and experimental results that we couldn't otherwise explain. So that's an exciting prospect. Let's go. Ryan Patterson, welcome to the Mindscape Podcast.
0:05:33.0 Ryan Patterson: Hi, Sean, it's very exciting to be here. Happy to chat.
0:05:35.9 SC: I got to ask, of all the particles of nature that human beings have ever detected and done physics with, the neutrino is the hardest one to detect. So why did you choose that to base your whole life's research around?
0:05:49.2 RP: Yeah, that's a good question. I think we could get very meta quickly with that question and how graduate students decide what they want to get involved with. I was a graduate student at Princeton, and there were several groups that I was interested in the research of... I eventually did my research with Peter Myers, who's a professor there, and he told me what was going on at the time. It was an experiment called MiniBooNE, which was looking to understand a previous unexplained measurement that had been done by an earlier experiment that was causing some head scratching in the field. It was a good time for that experiment. Particle physics experiments take a very long time to do, and sometimes it could be 20 years between the idea and first results. And so the timing for a new graduate student at the time was perfect. The experiment was approved. It was about to be constructed. I could be involved as a student with the construction, with the figuring out how to make this detector work, all the little unexpected features of how the apparatus is behaving. I'm a particle physicist then and now, but I spent maybe a year and a half trying to understand how light propagates through mineral oil.
0:07:12.8 RP: And you never know what you're going to run into. But the nice thing about this project was the timing was going to let me have that full experience all the way through to analysis. And yeah, it was just maybe seven years or so from construction to thesis, and it was a neutrino experiment. And that's how I "decided" to get involved in neutrinos. And I often tell students now, when I advise them, if there are several different projects and different areas of physics that you're interested in, don't stress it. Whatever you get into, you're going to find interesting because all of it is very interesting. Think about the research group and if the day to day activities of that style of research fit what you want to do. But I went into it and it was interesting. Probably if I had gone to a different project, I would be doing some different research today and I would also find it fascinating.
0:08:01.6 SC: I actually love that answer because I think that people don't appreciate the sort of path dependence of it all. Like, what you do as your PhD thesis project is not necessarily the same thing you'll be doing for your whole life, but it has a big influence on what you're doing for your whole life. And you might be making those choices on the basis of very local, immediate kind of considerations, which is perfectly valid.
0:08:26.6 RP: That's right. And people do switch quite abruptly at times because you have that capability. But often you do just get very interested in something and if that's what you end up being very interested in, by all means carry on in that area. And that's what happened with my case.
0:08:41.4 SC: So let's assume that we're talking to some people out there who don't know a lot about neutrinos. They've heard the word neutrino. They're very light particles. They pass right through us all the time. I mean, what's that? What are the basic facts that someone has to know?
0:08:55.3 RP: Yeah. So maybe the place to start is with a more familiar particle, the electron. It's a fundamental particle. It's in all of the atoms around us. It has an electric charge, and that's mostly how we interact with it. The electric force, the electromagnetic force as a unified force dominates all of our experience day to day. And deeper inside of an atom, there are protons and neutrons. They're bound together with what's called the strong force of nature. And then there is a third force called the weak force, not terribly creatively named. It's named that because it's weaker, at least at the energies that we typically experience it. And so the electron is electrically charged. It's also charged under the weak force. So it does experience this other force of nature. But it is so weak that we typically ignore that in chemistry. And in typical treatment of how electrons behave, the nuclear material with the protons and neutrons, they're experiencing the strong force as well as the electrical force as well as the weak force. So they experience all three. So now we get to neutrinos. Neutrinos only experience the weak force. So they only get this very weak version of the interactions we have in the standard model that leads to as you were saying, that they pass right through things, almost never interacting.
0:10:29.6 RP: Their existence was only first detected in the middle of the last century, which I guess is a long time ago now. But the other forces were doing their thing in laboratories well before that, even if we didn't understand the underpinnings of them. And neutrinos are also extremely numerous. They're the second most numerous particle in the universe after the photon. And if you have a coffee cup in front of you, there's 100,000 neutrinos inside that coffee cup at any given moment, and they're just zipping through and you don't even know about them.
0:11:08.3 SC: I'm showing, you can't see it out there in podcast land. I'm showing Brian that I do have a coffee cup. It is slightly larger than average, maybe 200,000 neutrinos. Who knows? Exactly. But it's interesting. Maybe, I'm never sure how non particle physicists think about these things, but the idea that particles feel certain forces and not others maybe isn't perfectly obvious. I guess people are comfortable with the idea that some objects are electrically neutral and therefore don't feel electromagnetism, but it's also like that for other forces, and the neutrino only feels the weakest one. I think it's a very good way to sort of conceptualize it.
0:11:48.3 RP: Yeah, that's right. And it also... You know, we'll probably get to talking about how we experiment with neutrinos, but a feature of many of these experiments is the fact that neutrinos can pass through material rather freely allows us to set up some of the experiments where we need to get neutrinos from point A to point B over a very long distance. And there's stuff in the way. We just shoot them right through the stuff and it doesn't matter.
0:12:13.5 SC: Well, that's the thing you say you're a particle physicist, but let's let the people out there know. Different particle physicists do some very different things in terms of how they do their experiments.
0:12:25.4 RP: Yeah, there's a wide range you can have, probably the most well known is the Large Hadron Collider at CERN. And it's miles and miles of tunnel with thousands of very complicated magnets with eight tesla magnetic fields in them, bending these very high energy protons around and millions and millions of collisions that have to get sifted through with in real time. You can't record all of the data because there's too much of it. So you have to make, you know, microsecond decisions about what you keep and what you throw away. On the other end, there are particle physics experiments that could sit on a kitchen table, and probably they have a room of dedicated equipment around them. But something like a dark matter experiment deep underground might only have a kilogram of dedicated special target material waiting for a dark matter particle to wander through and interact. And those are two very extremes. And then as just to put a third extreme on it we're building... I like to say that it's sort of ridiculous when you try to measure at the frontier of physics. You can imagine people sitting around, you know, having a drink, saying, "Okay, if we're going to do this, what do we need to do? " And then coming up with numbers on the back of the envelope that are just crazy. And then you go and do it anyway.
0:13:52.0 SC: Yeah. And that's why it takes so long, like you said, that's why it takes a long time to build these experiments, to conceptualize them and so forth. I guess the other thing we need to know about the neutrinos. Well, there's two things. One is you mentioned only the weak force, but of course, there is gravity. They all feel gravity. And the cosmologists will care about the neutrinos in the gravitational context.
0:14:11.9 RP: Absolutely. And that is one of the big aspects of neutrino science right now, and how neutrinos are so ubiquitous and how they affect not only the microscopic world. And we want to understand that microscopic behavior more. But they're everywhere in the universe, and they have been from the very beginning. And so they have had a big impact on the evolution of the structure in the universe. And it is in large part due to their gravitational influence that that impact has come come to be. Neutrinos are... Because they're so light, which is itself a curiosity, there are many, many orders of magnitude lighter than all of the other particles we know about. And because of that, they are relativistic particles, which is to say, at any given point in the universe's history, it's hot, and then it's cooling down with time, as it cools down enough, the heavier, more massive particles eventually slow down at that temperature. But if you're very light, even a little bit of temperature, you're still zipping around at very, very high speeds close to the speed of light. And particles that zip around it close to the speed of light spread out the gravitational influence in the early universe compared to things that just kind of Want to sit still and interact in their own little space.
0:15:38.2 RP: And so when cosmologists look out at, the technical term is the large scale structure, when they look out at how galaxies are distributed over very massive ranges, hundreds of thousands of galaxies spread out over clusters and super clusters and lines of those called filaments of galaxies, all of that structure has patterns in it. And those patterns are in large part dictated by the neutrino.
0:16:07.9 SC: And that's important. Maybe this is a skipping ahead to where we will eventually get to, but I'm sure that a lot of the audience members have on their minds, does this mean neutrinos are dark matter? And I take it that the answer is they're dark and they're matter, but they're not the dark matter.
0:16:24.2 RP: That's right. It is a very sensible place to first go because they also have the nice advantage that we know about them.
0:16:34.6 SC: They exist.
0:16:35.3 RP: And so, yes, they definitely exist. Because they're so light, they don't... Then they have this feature that I was mentioning that they're moving around very zippily in the early universe because they're very close to the speed of light in their travels. Dark matter doesn't do that. Or at least if it's this type of weakly interacting dark matter, weakly interacting particles that have some mass, it has to be slower earlier in the universe than neutrinos ever could be. Also the neutrinos number density, I threw out a number before that it's so many in your coffee cup. We have limits on their mass today from experiments that we'll get to. And if you take the number density, how many of them are there out there and their mass, what does each one weigh? You multiply those together, it's nowhere near enough to be the dark matter.
0:17:34.2 SC: Yeah, of course. And I don't want to get into it right now, but again, just for the audience that is thinking about it, there are non ordinary models of neutrinos where you could have different kinds of neutrino like particles that could be the dark matter. So we'll talk about that. But the neutrinos we know and love, I think we can say with pretty high confidence, are not the dark matter, right?
0:17:54.3 RP: That's right.
0:17:54.8 SC: Yeah. Okay. And the other fact I wanted to get on the table was there are different flavors of neutrinos. You can't actually taste them. It's not a literal analogy, but there's three different types of neutrinos. And this goes along with the bigger family structure of the other particles.
0:18:09.8 RP: Yeah, that's right. So maybe to just expand on the family idea there for a minute. We talked about electrons, and then there are the quarks. And some of your listeners are probably familiar with the colorful names of those. There's the up and the down and the charm and the strange and the top and the bottom. And I said those kind of in pairs because that's important to the story. The up and the down are often shown, if you look on a Wikipedia page, they're often shown connected together. And that pairing is because of the weak force they are associated. When an up quark and a down quark interact with each other, have some physical process that happens that has to do with the weak force. Those two types of quarks can interchange with one another. And so the electron also has a partner in the same way. And that partner is the neutrino. So the electron has a so called electron neutrino that goes with it. And just that prefix, that electron neutrino, is just how we tell which neutrino we're talking about that's associated with the electron. So with the quarks and their colorful names, there are also heavier versions of the electron.
0:19:25.9 RP: The electron is the familiar one of those electrically charged particles that we have in our atoms. There's also a muon, and because it's heavier, it is able to decay to its lighter cousin to its electron cousin. And it does that very quickly. And so we don't see them around in our everyday life. You actually are getting bombarded by muons all the time because of cosmic rays hitting the Earth's atmosphere and making a spew of particles that rain muons through you constantly and you don't notice it. We use those for our particle experiments all the time for various purposes. But in any case, those muons don't stick around. They decay away and you just have electrons left. And then there's a heavier one still called the tau particle. All three of those have an associated neutrino, an electron neutrino, a muon neutrino, and a tau neutrino. And it was in the '60s or 1960s or so that that was discovered that they were distinct neutrinos and that in fact it wasn't just one type of neutrino kind of doing all of this heavy lifting.
0:20:34.0 SC: Yeah. So there's this fascinating structure, like, it demands to be explained. Right. As far as I know, we haven't quite explained it yet. We have different theories that would understand, help us understand why there are three such families. But in the meantime we can experimentally ask kind of how they're connected to each other. Like you said, the up and the down quarks are put together in a pair, but they're not completely unrelated to the other quarks. They can actually transform back and forth in the right circumstances.
0:21:05.0 RP: Yeah, that's right. And the one interesting thing about the... Well, let me first say that there's a, I won't say boring, but it might be to draw a contrast. One boring way to interchange particle types is just to have particle and antiparticle annihilate. So we have, the electron has an antipartner called the positron. And if they interact, they can just go poof. And out of that poof can be almost any other pair of things along with their anti partners. And so you can make and make and break particles sort of at will if you have the right conditions. Without that way of doing it, you can also switch between an up and a down using the weak force. But you can also switch between a down and a charm. There's these cross family transitions that can happen and now we start getting into the weeds of it, because it didn't have to be that way. It didn't have to be that the standard model, that nature would allow such a fundamental process, a weak interaction, to switch from one family to the next. If the particles involved have mass, well, I'll start with that.
0:22:24.6 RP: If they have mass, then you get to invoke a rule that I like, which is that if your model allows it, it should happen, and otherwise you need a reason for it not to happen. And so when you write down the standard model in the way that we understand it in terms of quantum field theory, if you give these particles mass and you understand the weak force as we do, then you end up with the particles with certain masses and the particles with the certain colorful names, the flavors not being the same particles. And it's a quantum mechanical mixture superposition that shows up and the math allows it. And so you say, "Well, nature needs to allow it too, because it apparently should happen." And in fact it does happen. And so you can get these cross family transitions. For neutrinos because they're so light, they might have even been massless when they were first discovered and first contemplated. And if they are massless, you can't get this cross family behavior. And so if you have an electron neutrino, you always have an electron neutrino, but we now know that they do have mass because we have seen electron neutrinos switching families, becoming muon neutrinos or becoming tau neutrinos. And that opened up the floodgates because now that they have mass, they can play many more important roles in a lot of aspects of physics.
0:24:01.0 SC: So the quarks all have mass, and we know that they switch back and forth. I mean, there's a lot of intricacy here, much more than we can get into, you know, because I know that it's easier for a quark to change you into a quark of a different charge than for one of the same charge. But okay, you mentioned this provocative fact that neutrinos wouldn't be able to do this if they were massless. So that's not at all obvious to me why that should be true. Is there an intuitive way of thinking about it or is it just go through the math?
0:24:30.9 RP: Yeah, go through the math is not required. Fortunately for our discussion. One intuitive way to think about it is if they had no mass, they would always be traveling at the speed of light. And therefore any evolution that was going to happen if you started with a particle that had certain characteristics and it's moving at the speed of light in one of the features of special relativity, Einstein special relativity, is that something that's moving at the speed of light has no time evolution of its own. That's the nature of whatever is moving. We see its clocks as not evolving in time at all. And so this is this rather, I mean, we're calling it intuitive here. It's rather non intuitive. That's something that's moving very close to the speed of light. We observe its clock slowing down more and more and more the faster it goes. And if it gets all the way to the speed of light, its clock, for our purposes, as far as we as observers see it, are stopped. And that fact alone is enough to say that the neutrino can't possibly change any of its characteristics if it was moving at the speed of light because it wouldn't have any sense of time as we observe it.
0:25:45.5 SC: I like that. I like that explanation. I'm not sure I've ever heard that explanation before. I've heard the do the math explanation. I've gone through it, you know, in our quantum field theory courses, but that's a much more intuitive one. I love it. So when did we discover that neutrinos had mass?
0:26:01.6 RP: There were already hints that there might be something going on looking at neutrinos from the Sun, and this would have been in the '60s, 1960s. So the, the neutrinos in the whole universe, most of those are originating from the early part of the universe, kind of like the cosmic microwave background that has probably come up in past discussions on your podcast, where there are these photons from the relics from the very early universe. There are neutrinos that are also relics from the very early universe. And they make up the vast majority of the neutrinos that are around today. But they're very low in energy and they've never been detected. So their presence is inferred, but people are trying to detect them. It's very hard. But the Sun is blasting neutrinos out, and we're right by it. We happen to be conveniently right next to a star that's blasting neutrinos out. And those neutrinos are created through the fusion reactions in the Sun. And in the '60s, they were first detected. Now the engine inside of the Sun, these fusion reactions were reasonably understood already in the 1960s. And so you could predict how many neutrinos should the Sun be spewing out.
0:27:19.5 RP: And then you put a giant detector. It was a big vat of cleaning fluid, essentially. It was the chlorine in there was a useful isotope for detecting neutrinos with that particular technology. And so they counted very carefully how many atoms were in their detector, how many transitions of those atoms they got after a certain amount of time by filtering out the byproducts of neutrino interactions and counting those byproducts, because the byproducts actually have their own radioactive decay that you can set up a little experiment inside your experiment to detect. And you make sure you dot all your I's and cross all your T's. And there weren't enough neutrinos. It was off by about a factor of three. This was called the solar neutrino problem. And it remained unexplained, at least not definitively explained until 2001. And the idea that this could be caused by massive neutrinos was on the table, because if neutrinos were massive, then they could transition from one type to another. And this experiment was only sensitive to the electron flavor of the neutrino. And so if these neutrinos were transitioning to the muon neutrino or the tau neutrino, they would have gone through unnoticed, and you would get your count off.
0:28:45.5 RP: And so you could sort of work backwards to figure out, okay, what kind of masses would need to be involved, et cetera. But maybe the models of how the Sun worked were incorrect. So this was a big open question for decades. And then it was in the early 2000s that two experiments, snow in Canada and Super-Kamiokande in Japan, both through very different methods, definitively showed that not only were these neutrinos, the electron neutrinos were coming in at too low a rate, but the other types of neutrinos were also there. They had experiments that could see those other ones and see that the missing neutrinos were accounted for and they were just in other flavors.
0:29:27.8 SC: I am old enough to remember the, you know, talking to particle physicists who didn't believe in the solar neutrino problem in the 1980s or '90s. They're like, "Eh, astronomers don't know what's going on in the middle of the Sun." Like it's much easier to assume that than that we know something from these very hard to do experiments.
0:29:44.2 RP: Yeah. One fascinating thing about it is that, so taking a very brief tangent to the dynamics of inside of the Sun, it's not just one nuclear reaction. There's a whole chain of them, proton and proton combine, and then you can have some deuterium around because one of those protons turns into a neutron. And now you have some deuterium and then those can react. And now you have some helium-3 and some helium-4. And then you can stick those together and you start getting carbon eventually. You start getting nitrogen eventually. And most of it is the low level stuff. But it's a complicated chain. And we now today have the ability to detect neutrinos that can be associated with certain pieces of that chain, mostly through their energy. Some of the parts of the fusion reactions give higher energy neutrinos and some of them give lower energy neutrinos. If you can measure those differently, you can get real precise insight onto how different parts of that chain feed into the next and feed into the next. It exponentially grows your sensitivity. So you end up. The point of this brief tangent is that the neutrinos that we're measuring, the rate of them is dependent on the temperature inside of the Sun at a ridiculous power.
0:31:04.4 RP: It's like the 22nd power or something. So if you change the temperature in the Sun by 1%, it's 1.01 to the 22nd power changes certain aspects of the neutrino flux and so neutrinos are our best measure of the inside of the temperature of the Sun right now.
0:31:24.3 SC: But, yeah, so, but the point being at the time in the 1980s or '90s, you might have said, "Well, maybe astronomers are pretty good at getting the Sun right, but even if they're off by a little bit, it would make a dramatic difference to the neutrino flux. Therefore, I'm going to play a conservative and believe that."
0:31:40.6 RP: Exactly.
0:31:41.8 SC: And it also, these experiments are great. I mean, they did win the Nobel Prize and so forth, but they're a paradigm for neutrino experiments. Neutrinos are hard to detect. And so this idea that you're almost taking advantage of that by building a detector deep underground to shield everything else out and then building the biggest detector you can and comb through what happens inside to look for the very, very unlikely but eventually inevitable interaction of some sort.
0:32:11.5 RP: Yeah. And these experiments do come in many different varieties. I think maybe we'll just pick one and maybe the listeners through to get a sense of how you actually would do an experiment in the modern day. I'll pick the DUNE experiment. DUNE is an experiment that's under construction in the United States right now. It's a huge international collaboration. And so because you're using neutrinos that you want to have a lot of control over, there are neutrinos coming from the Sun, neutrinos coming from cosmic rays, neutrinos coming from outer space. You have less control over exactly what you're getting. If you can make your own, then you can make exactly what you need and you know what you're making. And so with DUNE, we will make a beam of muon neutrinos. So we pick the flavor. And so this is chosen by colliding high energy protons and just smashing them into a target. And when they smash into the target, a bunch of stuff spews out a bunch of esoteric particles called pions and kaons. And those decay, and when they decay, they make more esoteric particles. But some of the decay products are neutrinos, and that's the neutrino beam.
0:33:32.8 RP: So start with protons, which we have a huge history of accelerating protons. We're very good at accelerating protons. Smash those into something and let the debris that comes out, including the neutrinos, propagate forward. And then all of the stuff that's not a neutrino just runs into the ground and stops and then the neutrinos keep going on their merry way. In this case, we are.... I should briefly make sure that we've fully introduced the idea of neutrino oscillations. So I mentioned that neutrinos can transition from one flavor to the other, and that happens as they travel. But it takes a little while for that to happen. If you let them go from one end of your lab to the other end of your lab, the probability that they will have transitioned is too small to do anything with. If you let them go for many miles, which is bigger than a lab, then now you can start to ask questions about whether they're transitioning. How long it takes for them to transition depends on their energy. But this is one of the advantages of creating our own. We can smash these protons into our target, which determines eventually the energy of the neutrinos we get, and we can choose the energy of those protons.
0:34:53.9 RP: So in this case, the neutrinos have a certain energy such that it takes like 1000 km before they transition. And so you need to put your detector to see if any of these muon neutrinos have turned into something else. You need to put your detector 1,000 kilometers away. And this goes back to that idea of like, well, that sounds crazy. How are you going to even aim the darn thing? But that's what we do. And so now you need a detector on the other end, because neutrinos don't interact very much. The beam that's going through all these thousands of kilometers of dirt of the crust of the Earth, you lose a few of them, you lose a very tiny fraction of them, and then they pass through your detector very far away, and hopefully you lose a few of them there, too, because when they smack into an atom in your detector, they leave a signature they create in that little miniature particle collision. That one neutrino that happened to hit something in your detector is a little particle collision. And out comes another spew of things that we can detect with our experiment and interpret what type of neutrino must have done the smacking at that time. And if any of them are not muon neutrinos, then we've seen that transition taking place.
0:36:20.6 SC: And the idea of oscillations. I want to, like, geek out a little bit to get this straight, because we already said that, you know, quarks can transform back and forth into each other, but we think of, like, a top quark as simply decaying into multiple other particles. One of which is a lighter quark. This is not that these neutrinos going back and forth, like one kind of neutrino just turns into another kind of neutrino without actually decaying. Like, how does that work with conservation of mass or energy?
0:36:52.0 RP: Yeah. So, now, yeah, we get to dive into a little quantum mechanics aside. What's happening under the hood here is that when we talk about a neutrino flavor, let's pick one. Let's pick the electron neutrino. You can say, "Okay, you have this electron neutrino. You're holding it in your hand. If you measure its flavor, you will get electron as the answer. If you measure its mass, you will get a probabilistic answer." This is the quantum mechanical essence of measurement, that if there are certain quantities of a system that you can't know at the same time, and for neutrinos, you can't simultaneously know their flavor and their mass because those are different, entirely different ways of describing the set of three neutrinos in quantum mechanics. And they're quantum mechanically mixed up with one another. So separately, we could flip it on its head and say, "Well, here's a neutrino and I know its mass. I weighed it on a scale, and the scale told me something." And if we stick with the textbook Copenhagen interpretation, we've chosen the mass of the neutrino by making a mass measurement of it, and so we know it.
0:38:13.0 SC: Right.
0:38:13.8 RP: So now if you take that neutrino that has a certain mass and you say, what's its flavor? Now we flipped it on its head. You don't know its flavor. It could be an electron neutrino, a muon neutrino, a tau neutrino. You have to make a measurement and collapse the wave function in this Copenhagen interpretation. And so when we make our neutrinos, I said that we make muon neutrinos, but that means what we're actually making is a quantum superposition of the lightest, the middlest, and the heaviest neutrino. And we're letting that run on its merry way. But then the masses of the neutrinos are what dictate how they travel through space. Traveling through space doesn't care about flavor. Flavor is a thing that only has to do with the weak interaction. If you just flick a neutrino out into space, it's its mass that determines how it's going to evolve. And since we've just created a neutrino that isn't a specific mass, its evolution as it travels from point A to point B is now rather complicated because those different sub pieces of the quantum mechanical superposition are propagating differently. And by the time it reaches the detector very far away, it's now a different quantum mechanical jumble.
0:39:26.7 RP: And so if we say, "Now what is your flavor? " The answer is, I don't know. We're all mixed up now. And so that's how we can, when we measure it at that time. Not always, the answer isn't always going to be muon neutrino. When our detector sees that neutrino, it might be one of the other ones.
0:39:41.2 SC: I love that, being a big fan of quantum mechanics. But of course, I'm going to have to ask, why doesn't the same story hold for the electron, the muon, and the tau? Those are three particles that are different flavors. But in that case, the masses and the flavors seem to line up.
0:39:58.4 RP: Yeah. So the mixing that happens, the sort of fundamental quantum mechanical mixing that's driving this, is in essence symmetric between the two. The top side and the bottom side. This happens with the quarks as well. So the up quark and the down quark and the charm quark and the strange quark, et cetera, are mixed in this exact same way. All of the words that were just said about the neutrinos could happen with the quarks and does happen with the quarks. By convention, it is convenient to slosh all of this complexity onto one side of the problem. Either the top part of the family or the bottom part of the family. For electrons and neutrinos, the... We should introduce the word lepton. It might be handy for the leptons, which is the electron muon, tau, neutrino, neutrino, neutrino, set of six particles. For those, the charged versions, the electron, muon and tau are very heavy. When you create them, they're usually coming in and out of existence not because of the weak interaction. Their mass very quickly establishes what particle you want to give an everyday name to. And so those have the names electron, muon and tau.
0:41:19.9 RP: Because any normal interaction that you do is going to create the mass state, the one that has definite mass. And so if you're always creating something with a definite mass because the masses are so different, then give those the names. And then those were the names that were adopted for the flavors. And so then the neutrinos are the ones that have to suffer this naming complexity. You could in a world where you could freely create fixed mass neutrinos. You could concoct a crazy experiment where you get this weird behavior on the other side. But in practice it doesn't come up. And so the language has just kind of evolved to dismiss that possibility.
0:42:02.4 SC: So I think I did completely understand what you said, but I'm going to try to repeat it back so that you could tell me whether I completely understood it. We have three charged leptons, the electron, the muon, and the tau. We have three neutrinos, column 1, 2, 3, with three different masses. If we just talk the language of masses, there are three massive charged leptons, three massive uncharged neutrino leptons. And there is a convention. What we call the particle like the electron, the muon, the tau, or the electron neutrino, muon neutrino, the tau neutrino. And we call the charged ones, we label them by their masses, like the lightest charged one is the electron, the medium charged one is the muon, et cetera. And what that means is that we can't also do that for the neutrinos once we've made that choice. Now what we want to call the electron neutrino is going to be a mixture of mass 1, mass 2, mass 3, and likewise for the muon neutrino and the tau neutrino.
0:43:06.0 RP: That exactly right.
0:43:07.4 SC: I got it right. Okay, very good. But it's kind of, you can see why neutrinos are fun. I gotta say that I did talk once to a well respected neutrino astrophysicist who sort of leaned into me and said, "You know, that most particle physicists don't know anything about quantum mechanics, " because to him, all regular particle physicists do, was calculate a wave function and square it to get a probability. But in neutrino physics, you really have to think about quantum mechanics at an interesting level to get these interesting phenomena to come out.
0:43:42.1 RP: That's right. And this is maybe even a good segue for the fact that this complexity exists, that the flavors and the masses can't be simultaneously defined for a given system. That same complexity is what introduces the possibility that neutrinos and antineutrinos have different interaction rates out in nature. And maybe just to introduce the idea then that we know the universe has matter in it, we look out and we see matter. We don't see large amounts of antimatter. We can get into how we, you know, our particular points of evidence of that. But There are many. And so early in the universe, there had to be some way that matter was somehow getting a leg up on the antimatter so that it would persist to today and the antimatter wouldn't, with any residual leftover having to be the matter. And so you need some way for those two sides of the standard model to behave differently. And we know that the standard model allows that through this same quantum mechanical mixing phenomenon that neutrinos undergo and also that quarks undergo. The quark amount of this, I'll give it a name, it's the charge-parity violation or CP violation.
0:45:21.3 RP: Charge and parity are two bits of jargon having to do with how you could take a physical system and let's say parity first, you could look at it in a mirror. Not exactly in a mirror because you need to flip all three axes and not just one. But if you look at it in a mirror, left handed things go to right handed things. The charge operation that you can do to a system is just change all the pluses to minuses. If you have an electron, make it a positive. If you do both of those simultaneously. That's called the CP symmetry. If it would be a symmetry if the laws of nature stayed the same when you did that. We know the laws of nature don't stay the same when you do that. If you have a system and you make those two changes to it, the system might be something that is impossible in the universe, or at least behaves differently in the universe. Most of the familiar stuff that we see day to day respects these symmetries. Electromagnetism respects these symmetries, gravity respects these symmetries. And so if you took a video of something that was moving under the influence of gravity and you ran it backwards in time, or you looked at it in a mirror, or you changed all the pluses to minuses, all of those copies would be perfectly valid systems.
0:46:40.4 RP: But for this CP case, if the weak interaction is involved, which certainly happens when neutrinos are involved, then you can end up with a video that you go, "Ah, that's not actually our universe."
0:46:54.5 SC: So, sorry, why is this special for neutrinos?
0:46:58.8 RP: Well, it's only special for them in that they only experience the weak interaction. And so since this particular violation is showing up in the weak interaction, neutrinos are a particularly interesting place to look to see if this violation is happening. It also happens with quarks and that has been very well measured. In fact, it was the first. The early experiment showing that this violation was happening in quarks is how we knew there had to be three families, because if you only have two families, you don't have enough quantum mechanical complexity to have this happen. You need that third family. And so it was like, wait a minute, we see quarks potentially violating this symmetry. There must be a third family. Let's go find it. And then it was later found.
0:47:50.5 SC: And that's one of those things which, as far as I know, is something where you just have to do the math. The fact that you need three families to allow for CP violation. So CP just says, "I change all the particles to antiparticles. I look at it in a mirror, and somehow if there were only two families of particles in nature, I would be a perfect symmetry. If there's three, there's enough room in there for it not to be a symmetry. And indeed it is not."
0:48:16.0 RP: Yeah, you have to do the math. As maybe one way to brush it away, another is peeling one little layer of onion skin away. If any of your listeners are familiar with the idea of complex numbers. So this is the square root of negative one, sometimes called I, shows up all the time in quantum mechanics. Quantum mechanics is swimming in complex numbers. And if you only have two families, you can get a lot of the complex numbers to go away. When you have three, there's too much complication, and you can't hide the complex numbers anymore. And it's those complex numbers that stick around all the way through your calculation that leads to having this effect.
0:48:59.5 SC: Yeah, I think that counts as doing the math. I'm going to count that as doing the math.
0:49:02.7 RP: Fair enough. Fair enough. Certainly not necessarily an intuitive explanation.
0:49:07.1 SC: Okay, so. But we're beginning to see why the study of neutrinos is more than just the study of neutrinos. It connects to other, bigger questions. Obviously, there's questions about mass and dark matter, and we'll try to get back to that one too. But this fact that there can be CP violation in the neutrino sector and that CP violation distinguishes matter from antimatter, is at least connectable to the fact that there's more matter than antimatter in the universe. So is that in fact the connection you want to draw?
0:49:40.8 RP: It is, yes. Connectable is a very good word to use there. If you look at the conditions of the early universe and you say, "I want to somehow give matter a leg up in just the right amount to give us the universe we observe today, " then it's a very small amount that you need. One part in 10 billion is one way to measure it out of different ways. But the dynamics in the early universe are messy. Things are out of equilibrium, which is a fancy way of saying it's not just sitting there living its life, it's changing, it's expanding, it's cooling. Certain physical processes are allowed, and then suddenly they're not allowed anymore because things got too cool for them to proceed. And all of that evolution needs to have mixed in with it some process that can make matter and antimatter behave a little bit differently while all that is going on and then have that excess of matter stick around. And we do not today know of a process that does that. There are many theories on how you could have enough of this matter to antimatter difference in the early universe, the stuff that was measured with quarks and is now very precisely measured isn't enough. It's a very small amount of the CP violation. It's measurable because we have very precise experiments these days. Neutrinos now can come to the plate.
0:51:25.1 RP: And the neutrinos that we've been talking about so far are these very light neutrinos that can be made pretty freely in today's universe. And as we've established pretty clearly, they're very hard to work with, very hard to measure. So they could be violating this CP symmetry maximally, which is to say the knob could be cranked all the way to the max with how much CP violation they're doing. We just haven't measured them well enough to even know yet. Could be that they don't do it at all. And so one of the things we're trying to do with our experimental programs is determine if they do violate the CP symmetry. Those neutrinos aren't necessarily the dominant neutrinos in the early universe. So wait, Ryan, you're introducing new neutrinos all of a sudden? Why are you doing that? Well, neutrinos are funny in other ways. I mentioned earlier that they're so super duper light, and so you kind of need a reason for them to be so light, and they hand you a reason on a platter. They are unique among all the Standard Model particles that we've talked about so far in that they are neutral.
0:52:45.4 SC: Yeah.
0:52:45.9 RP: And so because they are neutral, they...
0:52:48.0 SC: Electrically neutral.
0:52:49.1 RP: Electrically neutral. Yes. Electrically neutral. They can get their mass a different way from everything else. I won't go down the tangent. But some of your listeners probably have heard that the Higgs particle is related to the Higgs field, which has a mechanism associated with it that can lead to massive particles in the Standard Model. And the neutrinos could do that too, with some caveats, but they have a completely different way that they could get their mass. And if that's the case then it's... I'll say that there is a.... Don't want to do the math on the podcast, but it's very elegant and convincingly so that a lot of people are sort of happy to spend their time looking for the phenomena associated with this idea. So basically the idea is that there would be very heavy partners to the very light neutrinos and how light the neutrinos are, the lighter they are, the heavier these other neutrinos would be. And it's those other neutrinos, the heavy partners to them, that would be driving the behavior in the early universe. So then what we're measuring, the CP violation we're trying to find with the light neutrinos, then needs to be connected to the CP violation we would have with the heavy neutrinos.
0:54:10.8 RP: And you can make that connection. And so in fact, in certain rather simple, well motivated theories, you can even find that the CP violation we would measure with the light neutrinos is exactly the same CP violation that would be dominating in the early universe. You can also make models where it isn't. But those are more of the original when this idea was coming about that this light to heavy partner relationship might be driving the behavior in the early universe. The first ideas of this in the 1980s, I guess were very simple, single family, just make all the masses the same type of thing to show that it works in principle, when you actually put some meat on the bones, you start seeing the connection between the light and the heavy ones come into the floor.
0:55:01.3 SC: So this is the famous seesaw mechanism.
0:55:04.7 RP: Yes, the seesaw mechanism, so named because of the light to heavy behavior I was just talking about.
0:55:12.1 SC: And it's a weird thing because like you said, because the neutrinos are neutral, there is an extra way they can get mass. But rather than simply all becoming heavier, some of them become much heavier and some of them actually become light.
0:55:25.7 RP: Yeah. And there too, I don't know if anything that isn't do the math on it, but when you write down terms in your model, they need to be well behaved. They need to not break calculations you want to do in your model. And so if you write down the most general version of that for these neutral, electrically neutral particles, you can write down the most general version of that and then say, what masses will I actually measure if I go to measure these things? And it spits out in these two directions.
0:56:03.6 SC: Yeah, and you're going to measure the lighter ones first. They're easier to find, easier to make, easier to get out there. Speaking of which, you've been very, very patient with us, Ryan, and talking about this whole picture of neutrinos, but we want to get to what you're actually doing on a day to day basis. So how are we on the ground? I want to say on the ground, but you're underground. How are we measuring these parameters of CP violation in the neutrino sector?
0:56:29.9 RP: Yeah, my research is largely with accelerator based neutrinos these days. And so as I was describing earlier with the DUNE experiment, where we make a beam of neutrinos and shine them a very long distance and then they go in to our detector far away and we try to tell what type of neutrino interacted with. DUNE is what we call, would call a next generation experiment. It's going to be bigger and better and brighter, but we're doing it now as well with the current generation of experiments. And so one is called NOvA, and that's an experiment that I've been very heavily involved in for a while. All of these experiments are large international teams. NOvA's 200 people, DUNE is over 1,000. And so, we focused on CP violation a little bit here. There's one other piece of the story that I think I want to bring in, which is that the neutrinos have mass. We know that we don't know what their mass is because neutrinos can do this transition if they have mass. But measurements of those transitions don't tell you what the mass is. They tell you aspects of the mass.
0:57:57.8 RP: They tell you how much heavier this one is to that one, but they don't tell you... I'm being a little bit loose with my language, but they don't tell you the actual overall mass of them. So this one is heavier than that and that one's heavier than this. So we haven't mapped all of that out. There are experiments that we surely won't get to here that are trying to measure the mass directly through, not through this oscillation phenomenon, but Just looking at neutrino relevant processes like nuclear beta decay, because beta decay spits out electrons, but it also spits out neutrinos. You don't see the neutrinos in these experiments, but you measure the electron very precisely and try to infer something about the neutrino mass as one example of a way to get at it. But not only do we not know their absolute mass, but we don't know the ordering of the masses. So we know that one of the neutrinos has to be the lightest one, but we don't know which one is the lightest one. And that seems like, okay, well, fine, you'll figure that out eventually. What's the big deal? That ends up having a lot of implications tying back to this quantum mechanical mixing thing, because some of the neutrinos that have definite mass also have a lot of electronness mixed into them.
0:59:20.8 RP: And some of them don't. Some of them have almost no electronness mixed into them. And since the universe is full of electrons and most of the processes involve electrons, then all this neutrino stuff that feeds into how the early universe is going, how supernovas evolve as they explode, and neutrinos are a big driver of supernova explosions. Whether the electron full massive neutrino is the heaviest one or the lightest one matters for all of that. And it also matters for this very question of how neutrinos get their mass. This thing we hinted at earlier about how neutrinos can get their mass in the center model a different way requires them to have a particular property, which is that they are their own antiparticle. That may be confusing because we've been talking about how particles and antiparticles maybe behave differently. And all of that, well, all that can be resolved in time. But we don't know if this feature of neutrinos is true or not. We don't know if they get their mass in this other way. And if we could figure out if they were this special type of particle, it's called a majorana particle as a keyword, if you want to look into it.
1:00:36.3 RP: If they're majorana particles, then we know that this new, the seesaw mechanism, for example, could work. And there are experiments trying to determine that. But those experiments need to know the ordering of the masses to interpret their data. So all of that was to introduce this idea that the ordering of the masses is something we need to know. If we shoot our neutrinos through hundreds or thousands of miles of Earth's crust. Those neutrinos experience the presence of all of that stuff that they're traveling through. They're not directly interacting with it, they're not scattering off of it and pointing off in a different direction. But the very presence of all of that stuff, in particular all the electrons on their route to the detector, modifies the oscillations that we're going to try to measure. And the higher the energy the neutrino, the farther we're going to be shooting it and the more all this matter matters. And it is through that phenomenon that we can try to determine the ordering of the neutrino masses. So CP violation is what we often talk about first. When we talk about the physics we're doing with these experiments.
1:01:55.5 RP: It's the... I don't want to say the easiest because there's a lot to get through to even motivate why CP violation is interesting. But there are many other things that we're trying to extract with these experiments. The mass ordering is another one for the reasons I mentioned that it has implications and how masses are generated in the first place. Implications to supernova and other astrophysical and cosmological things. We're also, when you build these big detectors, and when I say big, these are many tens of thousands of tons of material, very pure and carefully instrumented material. When you build these giant things, you can detect supernova neutrinos. So neutrinos that would come from a potential supernova in the galaxy. You can look for rare decays, you can look for proton decay, which is a process that, for their well motivated theoretical reasons, why you might expect proton decay to happen at a very slow rate. And so you can look for that and other physics that we can do with these. Maybe I'll let you direct from here. I've been rambling for a minute here.
1:03:02.0 SC: No, yeah, there's a lot of good stuff here. I do want to remind people, in 1987 we had a supernova, right? You know, in the, was it the Large Magellanic Cloud? I forget the large or small, but.
1:03:12.2 RP: It was, yeah, one of the satellite galaxies.
1:03:13.8 SC: One of those. And we detected the neutrinos from it, which is just amazing. And we haven't, as far as I know, we've not detected neutrinos from any subsequent supernova, but we're ready like there's a whole network that is waiting and listening in for the neutrinos to appear from the next supernova. Is that right?
1:03:30.8 RP: That's exactly right. Yeah. The supernova early warning system or snooze is operating today. And yes, just to set a scale for it, if a supernova were to go off somewhere, say that around the middle of the galaxy Dune, when it's operating in a few years, and some years would see thousands of neutrinos in comparison to the 24 neutrinos that were discovered that were detected from the one in 1987.
1:04:01.1 SC: So this is a very good motivation for having an experiment that is always turned on.
1:04:06.0 RP: Absolutely. Yeah. Having lots of different physics that you can extract, it's a requirement anyway these days. There aren't enough resources in physics, you know, monetary resources in physics to be inefficient with the experiments you do. So you want your experiments to have many things that you can accomplish if you're going to go through the trouble of making them.
1:04:36.9 SC: And let me ask one just sort of tying up the bow question about the masses. I guess it would seem to me a reasonable expectation if you say that the electron is lighter than the muon and the muon is lighter than the tau to just guess that whatever was the lightest neutrino would be the one that was mostly electron and the middle one would be mostly muon and the heaviest one would be mostly tau. Is there a reason to doubt that or is it just, we're trying to be careful in making sure that everything works out in the most obvious way?
1:05:12.3 RP: That's a very fun question. So you could guess that you actually run into a problem pretty quickly, which is this... For the quarks, we've established that this mass to flavor connection is jumbled for the quarks as well as for the leptons for the neutrinos and such. In the quark case, it's an ever so slight jumbling. If you say, "Here's a flavor, what is its mass? " You can pretty much know what the mass is. Occasionally you'll measure it to be one of the other masses, but it's a low probability. For the neutrinos, it's quite a different picture. They are heavily jumbled. And it's one of the big open questions. Why would quarks be just ever so slightly... You can think of it as a rotation. If you have your map in front of you and you have north and east pointing in your two directions, you know, north south is one way and east west is another. And you can just very slightly tilt your map and now you have a new upward direction that's not north anymore, it's like mostly north, but it's a little bit of west sticking in there. That's the court case.
1:06:28.0 RP: The neutrinos, it's like someone just spun a roulette wheel and all of the numbers just came out completely mixed up. So for the electron case, it is true that one of the, we know that one of the known mass neutrinos has a lot of the electronness, but for the other ones it's less clear. In fact, one of the things we're measuring with our current experiments, NOvA and others, is trying to figure out if what we call NU3, the one with mass 3, even though we don't know what the mass is, whether it has more muon-ness or more tau-ness, we don't actually know which one is more because they're very close to the same amount. So even if you wanted to make this assignment, it's not immediately obvious how you would make the assignment.
1:07:14.4 SC: All right, good, very good to know that that is very helpful. And then I just want to make sure we're doing fair to all the different experiments here. There's also the IceCube experiment, which we haven't even mentioned yet, where you're literally using Antarctica as your detection device.
1:07:29.2 RP: Yeah. Maybe this is. We can do a quick, kind of, almost surely overlooking something, but a few name drops just because. Yeah, I want to make sure that all my colleagues that I work directly with and indirectly with out there are getting their mention. So. Yes, IceCube is a detector with instruments dug deep into the Antarctic ice. It's an array of detectors, a kilometer cubed of detector. And with such a massive size, you can be very sensitive to very high energy and rare events. So they're looking at neutrinos from astrophysical sources both in the galaxy and outside of the galaxy and trying to understand the very most violent engines of particle creation that are out in the universe. There are experiments that are in water, KM3NeT ORCA that's doing a similar thing in Mediterranean Ocean. I mentioned Nova and we have our partner experiment T2K partner in the sense that we've recently put out a paper where we combined our data sets to get a little extra sensitivity. It's one of the reasons I was taking a tangent to this ordering of the masses question, because the experiment that I'm directly involved with, NOvA, we send our neutrinos over in that experiment, 810 km, T2K sensor neutrinos over 295 km.
1:09:07.1 RP: And those are different distances, which means we have to use different energies, which means we have different sensitivity to this ordering question, which helps us pick apart what's going on in the data. Because if you measure... Two different experiments can measure a phenomenon with different levels of sensitivity, in the presence of all this other stuff that's going on, you can pick it apart better. Yeah, so we were doing some name dropping. In addition to DUNE, there is a follow up to the T2K experiment called Hyper-Kamiokande that is also under construction and will be the largest detector that's constructed. I say, IceCube is bigger, but the ice was already there, so I don't know if that means it was constructed or not. That's semantics, I suppose. There's also the JUNO experiment which is using neutrinos from nuclear reactors to do their measurements.
1:10:03.2 SC: And when you do something like detect a neutrino in the ice or underwater, you're clearly not sifting through all the decay products looking for some heavier nucleus or whatever. So you must be able to see it in real time, see the collision or whatever.
1:10:20.8 RP: Yeah, I think that DUNE. All of these detectors operate under their own specific principles. Some of them have similarities. But I'm going to pick the DUNE one because I think it's a fun engineering tale. So the DUNE detector is a, think of a room filled with liquid argon. That's already quite a challenge because liquid argon is very cold and you need cryogenic equipment to keep the argon in liquid state and you need to fill a room with it. It's not just a room, it's a carefully constructed container inside of a cryostat, which is a fancy word for a thermos really. And when a neutrino enters and hopefully interacts in the detector, it'll smack into an argon nucleus. And we never really said this before, how do we know that we detected a muon neutrino or an electron neutrino? Well, if it's a muon neutrino, when it smacks into the nucleus, it will spit out a muon. When it's an electron neutrino and it smacks into a nucleus, it will spit out an electron. And we see that muon or that electron and we say, "Ah, that tells us the flavor of the neutrino that just came in and smacked into this nucleus."
1:11:39.7 RP: It can also smack into the nucleus in ways that doesn't Produce those flavor specific particles. And for most of the physics we do, those are just events that we see. And we say, "Yeah, that one wasn't useful, " and we throw it out. But for the case where it does produce their partner particle, then we can tell what happened. So then, okay, neutrino comes in, it makes a muon. Muon travels through the detector. Now can we see the muon traveling through the detector? This is something I think also that often is non intuitive to those outside the field. Particles just slow down and stop. You think of them as zipping around all the time. Particles are just whizzing through stuff. But in the same way that the atoms in your body are just sitting there, that's true for muons and other stuff. They can just be sitting there. So when we make these muons, they have enough energy to punch their way through this vat of liquid for some meters of distance, and then they just run out of steam. And we see that track, that trail that they leave behind of ripped apart argon atoms.
1:12:54.8 RP: So as they travel, they're knocking electrons off of atoms. And all of a sudden, instead of a bunch of neutral atoms, we have a bunch of ionized atoms. And it's that process that they're doing of ionizing all those atoms that leads to the detection capability which we can come back to in a second. So, yeah, maybe, actually, this is a fine time. So when you do ionize an atom, a lot of stuff can happen. One is that it could just recapture its lost electron. That's what I mean by ionize. I mean ripping an electron out of an atom. It could recapture it. When it does, it can emit photons. And those photons could be detected. We do that in DUNE, even though it's not the kind of principal method, it's an aspect of our measurement. What we actually do in DUNE is we apply a very large electric field across this room full of liquid argon. Just think of it as a big voltage across it. And it's not 10 volts or 100 volts, it's hundreds of thousands of volts. And so the electric field that's present across this big vat will, as soon as an atom gets ionized, it just has to wave goodbye to its electron.
1:14:15.6 RP: Because this electric field is pushing that electron away, drifting it farther and farther away from its partner that is now a positive ion sitting there, wishing it still had its electron. And so the products that come out of this neutrino interaction, the muons and other things that spew through the detector and travel for many meters, make lines of free electrons ripped off of atoms. And then this electric field pushes them all the way to the side of the detector. And then on the side of the detector we have delicate detection wires or pixels. There are a few different technologies we're developing that can pick up those electrons. And it's like taking a CCD camera picture of that pattern, and you just see the pattern of the particle tracks that had taken place in the bulk of the material.
1:15:08.7 SC: So in particular, you're not detecting, or maybe you're also detecting the photons coming from the ionization or recapture, you're detecting the actual electrons that are ripped out.
1:15:20.4 RP: In DUNE, we're actually detecting both. But it is the ionization electrons that provide our very fine resolution, spatially fine resolution, pictures of what took place. There are other experiments that rely entirely on the light. There are experiments that rely on yet a third aspect of particles traveling through media, which is called Cherenkov radiation, where the T2K experiment and the future Hyper-Kamiokande are the most classic examples of this, where it's a big vat of water surrounded by thousands of light detectors. And when a fast charged particle, like a muon travels through, if it's moving fast enough, faster than the speed of light in water, it emits a cone of light that comes out, a sort of sonic boom. But like a light boom of light that comes out, and then that light can hit the side of the detector where all the light sensors are.
1:16:17.3 SC: Has anyone thought to build the experiment that shoots a beam of neutrinos at the South Pole or at Antarctica, that to be detected by IceCube or a similar one for underwater experiments.
1:16:30.6 RP: Yeah. A tricky thing with neutrino beams, if you're going to make a beam, is they're not very steerable. You can't direct your neutrinos once you create them.
1:16:43.2 SC: Right.
1:16:43.7 RP: So you need to point your entire accelerator apparatus, or at least the very tail end of it, that's going to make the neutrinos at the direction you want the things to go. And that last little dog end of the accelerator complex that makes the neutrinos is about a kilometer long. And so you would have to point that you dig a tunnel straight down, isn't terribly practical. And so in fact, for DUNE, the distance between Fermilab, which is where the neutrinos will be created, and the detectors which will be in South Dakota. That distance is long enough that the Earth's curvature is big enough that you have to point the beam downward enough that you can't actually get that dog end of the accelerator to point into the ground because you have to dig too deep. So the beginning of that part of the accelerator complex is going to be lifted up on a hill that's artificially created so that you don't have to dig so deep into the ground.
1:17:42.8 SC: I did not know, that's pretty awesome. And of course, both Fermilab and the South Dakota detector location, these are pre existing facilities. So we're trying to do the best with the equipment we have a little bit.
1:17:55.9 RP: That's right.
1:17:57.2 SC: And when is DUNE supposed to come online?
1:18:00.1 RP: So the construction is well underway. The far detector caverns, deep underground. As you mentioned before, going deep underground shields you from a lot of things you don't want to be seeing in your detector. The caverns are excavated. It's ultimately going to be four detectors with 17,000 tons of liquid argon. So these are very big caverns. A lot of rock had to get excavated out. Very large civil construction project. And so the next step is to get the cryostats installed. That'll start in the early part of next year and then we can get the detectors installed after that.
1:18:39.3 SC: It is true mad scientist stuff, right? Like many tons of argon, huge electric fields stretching across it just to observe the lightest particles we know about in nature.
1:18:49.6 RP: Absolutely.
1:18:51.6 SC: And I guess, okay, we're nearing the end of the podcast. There's just a couple of like fun side topics I did want to get on the table a little bit. Let's revisit this dark matter thing. You hinted that they're the light neutrinos that we know and love, but maybe there could be heavier neutrinos that we haven't actually seen. The partners that we get from the seesaw mechanism, are those candidates for dark matter? Or are neutrino dark matter candidates, things we just have to like, add completely new neutrinos to our mix?
1:19:24.4 RP: As is the want of theory, you can come up with ways to make any of those options work. And it is our jobs as experimentalists to figure out, to sift through, to figure out really what is going on. The most basic seesaw mechanism tends to lead to neutrinos that are... The partner neutrinos that are extremely heavy and do all their business in the early universe and would not be particularly good candidates for dark matter. However, you can squeeze that seesaw range down to have heavier partners, maybe the lightest version of those, let's say there are three heavier partners with three light neutrinos. Maybe there are three partner ones that could go along with it. There are natural reasons to choose that. And so maybe the lightest one is light enough and sticks around with enough number density, enough of them left over today to be a candidate for dark matter.
1:20:25.1 SC: I think it's interesting because maybe people don't quite appreciate, unless they spend a lot of time thinking about it, that to be dark matter, it is not enough to be dark and to be matter, you also need to have all the properties to get the right abundance, the right temperature, things like that.
1:20:40.8 RP: That's right. And having... A lot of times the popular science description of dark matter goes back to the original evidence, which is these galaxies and how quickly things are spiraling around the center. And that is an important piece of the story. But there are many, many, now today, much more powerful pieces of evidence, many of which are independent and are measuring different aspects of dark matter. And so if you come up with a piece of dark matter, it needs to satisfy all of those different observations clustering at the galaxy level, at the supercluster level, the cosmic microwave background, the large scale structure, all of this stuff, it has to work with all of it, because the data show that you can make all of that work together.
1:21:31.8 SC: Yeah. And the final thing which is related is the cosmic neutrino background. I mean, you said provocatively, neutrinos are the second most popular particles in the universe by number. And we kind of, like you say, have evidence for that indirectly from cosmology as well as it's the prediction of what we understand, but we haven't detected them yet. Why aren't we trying harder to do that?
1:21:56.9 RP: Yeah. So maybe as a... To give a little bit of firm footing, the cosmic microwave background. So talking about the photons for a second, those were created when the universe cooled enough to form neutral atoms. So when it was super hot, if you had mostly hydrogen would have been what we're talking about. So you'd have protons and you have electrons and it would be just in a hot plasma. And so a photon couldn't go around the block before it ran into some charged particle and got scattered. Eventually things cooled enough that the photons could start passing through the now neutral universe. And those were relatively high energy photons. But it was a long time ago. And so now, after billions of years of evolution and the universe has expanded and those photons have been redshifted through the expansion of the universe. Their wavelengths have been effectively stretched out that today they have microwave energies. And so we call it the microwave background. But they weren't created as microwaves.
1:23:02.5 SC: Right.
1:23:03.2 RP: So for neutrinos, the same story happens. It's not a neutral atoms that determine the cutoff point. It's a little earlier in the universe when neutrinos were no longer caught in a neutrino, proton, neutron, plasma type of deal. It was eventually cool enough that neutrinos could stream freely through the universe. And they were very high energy, MEV. Well, I shouldn't use that right now. We haven't introduced the concept of MEV units, But anyway, very, very high energy neutrinos, but they also got redshifted over the subsequent billions of years. So today they are very low in energy as well. Their temperature, maybe a more appropriate unit would be to talk about their temperature rather than their energy. And it's about 2 Kelvin, so a little bit colder than the cosmic microwave background. A problem is that neutrino interaction rates scale with energy. So the lower and lower and lower energy neutrinos Become less and less and less likely to interact at all. And even the ones we make in the lab, which have tons of energy relative to these, are very hard to detect. So that's the biggest experimental hurdle. There's a lot of these neutrinos, but they just simply don't want to interact.
1:24:25.8 RP: So you need to make experiments that one have a detection mechanism to see such a tiny amount of energy being deposited in the detector. And how that is accomplished is actually pretty cool. But you also need to be shielding yourself from all of the backgrounds, et cetera.
1:24:44.2 SC: Are there any prospects for being able to do it?
1:24:46.7 RP: Yeah, there are experiments trying to do it. They're related to how neutrino mass experiments are operating and trying to detect the actual mass of the neutrino. And so there's one called Ptolemy, an experiment called Ptolemy that's trying to eventually get to the cosmic neutrino background measurement. And essentially what you will want to do is have tritium, which decays on its own after a certain amount of time. But in the presence of these neutrinos, the neutrinos can actually catalyze that decay, rather than being a spontaneous decay, you can actually induce the decay and the sort of particles that come out of it in that case have a very specific energy that you can look for as you look for electrons with a very distinct energy signature that tells you that it was induced by a little neutrino sitting around in the area.
1:25:43.0 SC: I think that you made the right decision when you were a graduate student and decided what to work on. There's a lot of very rich stuff going on in the neutrino sector, and you definitely educated us about that. So, Ryan Patterson, thanks so much for being on the Mindscape Podcast.
1:25:55.0 RP: It was a pleasure, Sean.
Great episode. You asked Dr Patterson about aiming a neutrino beam at the IceCube detector. In the 1980’s a steerable neutrino beam generator was proposed by Rujula, Glashow, Wilson and Charpak.
https://www.sciencedirect.com/science/article/abs/pii/0370157383901084
https://www.science.org/doi/10.1126/science.220.4602.1142
The idea was to place an accelerator in the ocean so it could be arbitrarily oriented.
I have a question.
To give the background of that question I first go over the ways in which neutrino’s are different from other particles.
The electron, muon, and tau are in two ways in a hierarchical relation to each other:
There is decay mode: tau and muon undergo decay. Tau decay produces either a muon or an electron, with the decay mode that produces an electron being the most probable. (And of course the decay additionally produces neutrino’s in accordance with conservation constraints.) The electron doesn’t have a decay mode available.
For all three, electron, muon and tau: the amount of inertial mass of each has been determined by way of experiment.
To my understanding: neutrino detection finds three neutrino flavors, in such a way that it can be inferred that for the three electron flavors there are corresponding neutrino flavors.
But then: for the neutrinos the flavor state and the mass state are not correlated.
It would appear that none of the three flavors of neutrino has a decay mode.
So that is very different from electron, muon, and tau.
Lower limit
A lower limit for neutrino mass is not known. There is the Karlsruhe Tritium Neutrino Experiment (KATRIN), and for years now the KATRIN results have been pushing down the lower limit of neutrino mass.
Neutrino mass
There is a prominent candidate for the origin of neutrino mass, but as of now other candidates cannot be excluded.
In all the impression that I get is that description of the properties of neutrino’s is very much tentative.
What puzzles me:
How can particle physicists be so confident that in order to be capable of cycling through flavor states neutrino’s must have inertial mass? Given all the unknows, how does it come about that for that particular correlation particle physicists are certain of it?
Cleon, the simplest answer I found to your question is the following:
o Neutrinos come in three “flavors”: electron, muon, and tau.
o When they’re created (say in the Sun or in a particle accelerator, they start out in one flavor.
o But here’s the twist: the “flavor states” aren’t the same as the “mass states”.
Think of it like this:
o ‘Flavor states’ = how neutrinos interact (what detector sees).
o ‘mass states’ = how neutrino actually travel through space.
Easy Analogy
Imagine three musical notes being played together
o The detector hears a “flavor chord” (electron, muon, or tau).
o But as the notes travel, each one drifts slightly out of syn because they have slightly different pitches (masses).
o Over time, the chord changes – sometimes sounding more like one note, sometimes another.
That’s exactly what happens with neutrinos: the mismatch between flavor and mass makes them “oscillate” back and forth.
Why It Matters
o If neutrinos had no mass, they wouldn’t oscillate – they’d stay fixed in one flavor.
o Oscillation is proof that neutrinos ‘do have mass’, which was a huge discovery because the original standard model of physics assumed they were massless.
o This tiny effect helps explain big mysteries like why fewer electron neutrinos arrive from the Sun than expected.
Ref: Microsoft Copilot
Hope that helps!