S3 E7: DUNE: The Neutrinos Must Flow

May 7, 2019

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Cover art for Direct Current podcast season 3, episode 7, "DUNE: The Neutrinos Must Flow" featuring an illustrated sci-fi landscape depicting the DUNE neutrino experiment.
U.S. Department of Energy

Here's a question. Why does anything exist?

The key to this age-old mystery could lie with neutrinos, elusive particles that flow all around us and (through us) by the trillions. Join Direct Current on a subatomic sojourn into the Deep Underground Neutrino Experiment (DUNE), a massive international research project aiming to unlock the secrets of the neutrino with help from more than 175 institutions in over 30 countries.

We'll take you from Fermilab to the home of the Large Hadron Collider in Switzerland to the bottom of a former gold mine a mile beneath the hills of South Dakota as we explore one of the most ambitious particle physics projects of our lifetime.


Small Particles, Big Science: The International LBNF/DUNE Project

DUNE is a project of superlatives. It will use the world’s most intense neutrino beam and largest neutrino detector to study the weirdest and most abundant matter particles in the universe. Watch the video, then check out five fascinating facts about DUNE from Symmetry Magazine.

Digging for Neutrinos

Pre-excavation work for DUNE begins at the Sanford Underground Research Facility in South Dakota.

Pre-excavation work at the Sanford Underground Research Facility in Lead, South Dakota, began in early 2019. Check out photos and diagrams of the massive effort to remove 800,000 tons of rock from a mile below the surface to make room for the DUNE detectors.

More Power to Fermilab

PIP-II: the new heart of Fermilab

Fermilab's Proton Improvement Plan-II (PIP-II) isn't just about generating the beam of neutrinos for DUNE — although that's a big part of it. There's so much more in store for the lab's brand-new particle accelerator powered by superconducting radiofrequency technology!

All Eyes on ProtoDUNE

Inside the ProtoDUNE neutrino detector at CERN prior to the start of its operation.

ProtoDUNE may be just 1/20th the size of the planned DUNE detectors, but it's already the largest liquid argon neutrino detector in the world. It began recording particle interactions at CERN in September 2018 and will provide invaluable data in the lead-up to the full-scale experiment. Check out these amazing photos of ProtoDUNE under construction.

Rebel with a Physics Degree

Art depicting a motorcycle rider with saddlebags containing particle physics text books.

For Mark Hanhardt, experiment support scientist at Sanford Underground Research Facility, teenage rebellion meant steering clear of motorcycles and mines and pursuing a career in astrophysics. (via Symmetry Magazine)




MAN'S VOICE: “Deep in the human unconscious is a pervasive need for a logical universe that makes sense. But the real universe is always one step beyond logic.” — Dune, by Frank Herbert. 

CORT KREER: As human beings, we tend to think of ourselves as pretty solid. 

MATT DOZIER: Ditto for the world around us. This chair I'm sitting on (CHAIR CREAKS), the desk in front of me (KNOCKING ON DESK), the microphone I'm speaking into (TAPPING ON MICROPHONE), they all seem pretty substantial, right?  

KREER: Even the air around us has substance. You can feel the breeze on your face, or subtle air currents when you wave your hand. 

DOZIER: All of that is what we call "matter." It's the stuff that makes up the parts of the universe we can see, and touch, and generally interact with. 

KREER: It's what makes us us. 

DOZIER: But take this desk, for instance. What's it made of?  


DOZIER: Right, but what's the wood made of?  

KREER: Molecules.  

DOZIER: And what about those molecules?  

KREER: Atoms, carbon mostly? 

DOZIER: But... what about the atoms?  


KREER: OK. So as you keep zooming in, all the way down to the subatomic level, you get to the smallest stuff we know of, tiny particles with names like "quarks" and "muons."  
DOZIER: These particles that make you... *you*... act really strange. They spin, they wobble, they flip, they change flavors. 

KREER: Mmmm, flavors. (LAUGHS) 

DOZIER: Not like that. And they do all kinds of things that we're still trying to understand. 

KREER: Down there in the quantum world, the concept of "solid" pretty much loses any meaning. It's all just particles. 

DOZIER: In fact, from the standpoint of some of the most abundant particles in the universe, you don't seem very substantial at all.  


KREER: They're called neutrinos, and they're what we're talking about today. A few basic facts about neutrinos right off the bat: they're incredibly small — some of the smallest particles we know of — they have no electric charge, and they are absolutely everywhere. 


DOZIER: But — and here's one of the most interesting things about them — they hardly ever interact with matter. All those atoms and molecules that make up you and your chair and this microphone — neutrinos just pass right through them like they're not even there. 
KREER: To neutrinos, you might as well not exist.  
DOZIER: And since they're so abundant, we are essentially swimming in a neutrino soup at all times. Hold out your hand for three seconds. Go ahead, do it. Three trillion neutrinos just passed through it. Trillion.
KREER: Trillion. So, why are we telling you all this? 
DOZIER: Well, neutrinos are at the center of this massive, and I mean massive, research effort that spans more than 175 institutions, in over 30 countries, the Department of Energy, our National Labs, from the home of the Large Hadron Collider in Switzerland to the bottom of a former gold mine a mile beneath the hills of South Dakota.  
KREER: The answers they find there — and I feel like you hear this a lot in particle physics, but it's true! — could help us unravel the mysteries of the universe. 
DOZIER: This is a story about scientists asking the question, "Why does any of this exist?" 


BONNIE FLEMING: You know, you break the world down to its tiniest components, and you come up with 12 building blocks of matter, and 3 of them are neutrinos. The electron neutrino, the muon neutrino and the tau neutrino. 
KREER: That's Bonnie Fleming. 
FLEMING: I'm a professor of physics at Yale university, and I also hold a part-time position here at Fermilab as the deputy chief research officer for neutrinos.  
DOZIER: Fermilab, for those of you who aren't familiar with it, is the Energy Department's flagship particle physics lab just outside of Chicago. They're all about probing the mysteries of the subatomic world.  
KREER: Neutrinos are the stars of the lab's current research efforts. Bonnie is one of more than a thousand scientists from around the world involved in this "megascience" project called the Deep Underground Neutrino Experiment, or "DUNE." There's no connection to the Frank Herbert sci-fi classic we quoted at the start of the show.
FLEMING: Studying things at the most fundamental level, meaning those building blocks, is incredibly fascinating, and neutrinos in particular are rather challenging to study because they don't interact much. That's the neutral part of them. They're very small, that's the "ino" part of them. Yet they're second in nature in abundance only to photons. So they permeate us, and they can tell us both about how those building blocks make up the ordinary matter that we see, and they can tell us about the rest of the universe.  
DOZIER: OK, so, the stuff that we can see in the universe. What is that?  
FLEMING: It's everything that makes us up: the stars, heavy elements, the earth. That's the ordinary matter that we see. The truth is that we don't understand much of what's out there. 25-ish percent of the universe is dark matter, 75 percent of the universe is dark energy. We call them "dark" because we don't know what they are. That's slang, one would say, in particle physics for "I don't know." So it's really only a tiny fraction that actually comprises what we're made up of in the universe. But of that that we know of, the neutrinos are a big player.  
DOZIER: We've dipped a toe in this pool before... and it's a deep one. Check out Season 2, Episode 6, "A Shot in the Dark" for our investigation into dark matter — another elusive ingredient of the cosmos. 
KREER: We're steering clear of dark matter this time around. There's plenty to cover with neutrinos. As Bonnie said, neutrinos are everywhere, and they are connected to the stuff you are probably familiar with: things like protons, neutrons, and electrons.  
FLEMING: Exactly. So of the 12 building blocks of matter, the quarks make up things like protons and neutrons. The leptons, charged leptons are electrons that make up electricity, that kind of thing. And then the neutrinos associated with those charged leptons are the other three particles.  


DOZIER: OK, so now that we've established WHAT neutrinos are, let's try to tackle the WHY... Why devote such a far-reaching international effort (and DUNE really is enormous — more on that in a bit) to study these unassuming particles that don't really seem to... do much? 
FLEMING: There's so many things I'm excited about. The first thing I'm excited about is the general question that we're addressing, which is basically, are neutrinos the reason we exist? On the most fundamental scale. Are there differences between matter and antimatter? And can we find those differences and explain why we live in a matter-dominated universe?  
KREER: If you've heard of antimatter, it's probably only in science fiction, right? But it's a real thing. We've known about its existence since around 1930.  
KREER: Pop quiz: what happens when matter and antimatter meet? (EXPLOSION NOISE) Close enough. 
FLEMING: So let me back up a step and say that in the laboratory we can take energy and create equal amounts of matter and antimatter. And those then annihilate back into a puff of energy. And what is confusing is, if that had been the same process that had happened in the early universe, we should have all annihilated away just to a bath of energy in the universe. Instead, there's some excess amount of matter left over, which is a relief -- that's what we're made of.
DOZIER: And that's one of the trickiest puzzles in all of physics. After the Big Bang, if there was an equal amount of matter and antimatter flung out into the universe — as scientists have predicted — wouldn't it all have eventually collided and destroyed itself? 

KREER: But of course, that didn't happen. Somehow, there was enough matter left over to form stars, and planets, and asteroids, and desks, and microphones, and you and me.  
DOZIER: The secret to that could lie with something really odd about the way neutrinos behave.  
KREER: Neutrinos come in three flavors, which basically means three different varieties of the same particle. Think ice cream.


KREER: You can have strawberry, vanilla, or chocolate, but it's all still ice cream. Same with neutrinos, except the flavors are electron, muon, and tau. 
DOZIER: Here's where it gets weird. UNLIKE ice cream, neutrinos don't just stay one flavor. Imagine going to the store and buying a pint of mint chocolate chip, and by the time you get home, it's rocky road. 


DOZIER: That's what happens with neutrinos — they actually *change flavor* as they zip around the universe. 
FLEMING: It sounds crazy. It's quantum mechanics (LAUGHTER). The quarks do it too. So in fact, this oscillation process tells us that neutrinos have mass, which is a very fundamental property of particles in general.   
KREER: That flavor flip, called an "oscillation," happens as neutrinos travel large distances. It just so happens that the distance between Fermilab and Lead, South Dakota — about 1,300 kilometers — is ideal to study neutrino oscillations. 
DOZIER: Time to zoom back out — way out — to the full scope of the DUNE experiment.  
FLEMING: So what we're doing at Fermilab is we create intense beams of neutrinos and antineutrinos, we let them travel over 1,300 kilometers between Fermilab and the SURF facility in South Dakota. And we start with a beam of pure muon neutrinos, for the most part — mostly pure — and we see if any of them have morphed into electron neutrinos by the time they've made it to South Dakota, and see if there's some difference in the oscillation rates between those neutrinos and those antineutrinos that could give us a clue to this matter-antimatter asymmetry in the universe.  
KREER: OK, time out! Earlier, we called DUNE a "megascience" experiment. And we're not going to lie, it's really difficult to convey just how ginormous this project actually is. 

DOZIER: To illustrate: What if I told you that I had this plan... A plan to build a brand new particle accelerator to generate the world's most powerful beam of neutrinos... and shoot it straight through the crust of the earth at a detector more than 800 miles away... AND the detector is made up of four giant refrigerators filled with thousands of tons of supercooled liquid argon... 

KREER: Uh huh?

DOZIER: AND those refrigerators need to be assembled piece by piece at the bottom of a former gold mine a mile below ground? 
KREER: And what if I told you all the different pieces of this experiment are being built simultaneously by thousands of engineers and scientists all over the world? 


KREER: And that in France, there's a 1/20th scale model of the detector running to make ABSOLUTELY SURE it will work once the real thing is installed at the bottom of the mine shaft?  
DOZIER: You'd think we're crazy, right? But — that's DUNE. This entire project is on a scale that, frankly, boggles the mind.
KREER: The only thing more intimidating than the project itself (at least for us) is explaining it. So, we're going to break this down piece by piece, and we're going to meet some of the people who are responsible for pulling off one of the most ambitious physics experiments of our lifetime. 
DOZIER: Let's start with the "Deep Underground" part of "Deep Underground Neutrino Experiment." 

MOSSEY: Lead is in western South Dakota, it's right in the Black Hills. It's a very pretty area. It's a town of about 3,000 people, just up the hill from Deadwood, SD. It's a former gold mining town because of the Homestake Gold Mine that was there for over 130 years, and when it closed in the early 2000s, the leadership in South Dakota saw an opportunity to convert this facility into an underground research facility.  
KREER: Chris Mossey is the deputy director for the Long Baseline Neutrino Facility at Fermilab. He's the guy responsible for delivering the facility that's going to make DUNE possible.  
DOZIER: The science laboratory he was talking about at the bottom of the former gold mine is called the Sanford Underground Research Facility, also known as Sanford Lab. At roughly 5,000 feet below the surface, it's one of the deepest research facilities of its kind. And it has a really important role to play in DUNE's supersized physics investigation. 
MOSSEY: Because of the characteristics of this experiment, the scientists need a very quiet place — one that's free of cosmic particles that might occur if you put these very large detectors on the surface.  
KREER: OK, so, imagine you're at a party. 


KREER: It's crowded, it's noisy, there's lots of people there, and you're trying to have a conversation on your cell phone. 
DOZIER: I don't know why you'd do this — it's very rude. 
KREER: Very rude. 
DOZIER: But humor us. Say the person on the other end of the line is speaking really softly, and you can't quite make out what they're saying. There's just too much noise around you to hear them clearly. 
KREER: If you want to have a conversation, you need to go in another room and shut the door. Which, honestly, you probably should have done in the first place. 
DOZIER: Probably. Now, think of our galaxy as a big party... a particle party? There's particles milling about everywhere, electrons having loud conversations with protons, quarks coming and going, always in pairs. In physics terms, it's really noisy. 
KREER: Neutrinos are what scientists are listening for on the other end of that phone call. It's hard to hear them over the cosmic din, so we need to find somewhere more private. Somewhere we can shut out the rest of the chatter. 

DOZIER: You know one thing that's really great at blocking unwanted particles? How about a mile of solid rock? Remember, neutrinos can pass right through just about anything — including the Earth. So by "taking our phone call," so to speak, at the bottom of a mine shaft, we can listen for them without all the background noise. 
MOSSEY: Neutrinos are very unique in that they go through matter as if it isn't there, and they only very, very rarely interact. Because of that characteristic they're very hard to observe, and that's why we have to make these detectors underground where they're shielded from everything else, and they're very sensitive and very large, so that every once in a great while when a neutrino interacts it can be observed and better understood.
DOZIER: So it's quiet, but not in the way we think, right? You're not talking about sound? 
MOSSEY: 07:18 Exactly. No, it's "quiet" in the sense that the mass of rock -- this will be about a mile underground -- shields those particles so that when you see something you can be certain, relatively certain that it is from the neutrino beam that we will be sending towards the detectors.  
KREER: The detectors are the heart of the DUNE experiment. They're the king-sized refrigerators we referenced earlier — scientists call them "cryostats." 
MOSSEY: If you can picture something that's about four stories tall, about 20 meters wide, and about 3/4 of a football field deep, that's the size of each one of these cryostats. We plan to build four of them, eventually, and they'll all work together to form a very large detector for the Deep Underground Neutrino Experiment, the DUNE experiment.  
DOZIER: Which begs the question, how does Chris plan to fit these building-sized cryostats down into their new subterranean home? 
MOSSEY: There will be three caverns that we will build underground, and each one will be somewhere between 500 and 600 feet long, so they're very large caverns. Two of the caverns will hold the DUNE detectors, and then one will be what we call the central utility cavern, which will be the space that supports all the cryogenic systems that have to refrigerate the argon that's inside each of the DUNE detectors.  
KREER: To hollow out those caverns, miners need to blast and remove more than 800,000 tons of rock from a mile underground. That's the equivalent of two Empire State Buildings, and then some. 
MOSSEY: Besides the excavation of large amounts of rock, we will have to build the systems that move that rock from the location that the cavern is being built to the base of a shaft that was used back during the mining days — up that shaft after we've finished renovating it, about a mile to the surface, and then we've got to move that rock about a kilometer to where we're going to deposit it in an old open cut, which was another type of mining activity where they excavated from the surface. And so we'll move the rock through this path from the cavern, up the shaft, then out to an open cut that's right, in fact, in downtown Lead, South Dakota. 

DOZIER: Yes, all this action is happening right in the heart of Lead. Look up a satellite view of the area and you'll see the "cut" Chris is talking about, a cavernous hole smack dab in the middle of town. The Sanford Lab visitor center is perched on the rim of the crater. 
KREER: And while Lead's mining days may be over, the Sanford Underground Research Facility offering a nice, quiet spot to listen for neutrinos, things aren't always so peaceful above ground. 
MOSSEY: Yeah, it's a nice town. It does have this big motorcycle rally that happens every August for a couple weeks, and I would say it's not quite as quiet then. Lots of Harleys riding up and down the road, and it's hard to get a hotel room, but it's a very nice place and it's a great community.  
DOZIER: The scientists will have to keep that in mind for future travel out there.  
MOSSEY: Well, you know, scientists like to go to motorcycle rallies too.  

DOZIER: OK, so, after the digging is complete, it'll be nice and roomy a mile below the bikers rumbling along Lead's main street. But there's still the challenge of getting everything assembled. After all, you can't very well fit a four-story-tall cryostat down a 13 x 5-foot mine shaft in one piece. 
MOSSEY: That's right. And that's really one of the real challenges of the project. You've got to move everything down this shaft. And so you really have to have a good understanding of how all these pieces fit together, what the maximum size of the pieces can be. Once you get them down the shaft, a mile underground, you're going to have to move them through what are called drifts — which are basically tunnels that start and end underground — and you've got to make sure that you can fit them around the corner. It's kind of like a super-sized moving a couch down the stairway into your basement. Another analogy might be, it's akin to building a ship in a bottle. 
KREER: There isn't much room for error. 
MOSSEY: No, exactly. You've got to get it right the first time. It's a lengthy construction project. It'll take many years to do this, and you don't want to have to take something out and then ship it back up the shaft, and then understand what didn't work right. Once these cryostats are filled with argon and the detectors start operating, it would be extremely difficult to go back and open them and change something if you didn't have something right. Each cryostat holds over 17,000 tons of argon, and so it will take about 9 months just to fill one cryostat with the argon and refrigerate it to the proper temperatures, they're so large. 
KREER: OK, so what's the deal with the liquid argon, anyway? And why do they need so much of it? 
DOZIER: The basic idea is this. You take your intense beam of trillions upon trillions of neutrinos and aim it at the biggest, heaviest detector you can get your hands on. The vast majority of those neutrinos will sail right on through the detector, out the other side, through the earth and out into space. 
KREER: What you're hoping for is that a handful of neutrinos — out of trillions — will actually collide with another particle within the confines of the detector, so you can take snapshot of what that par-tic-ul-ar meeting looked like. 
DOZIER: Liquid argon makes a great target, because it's really cold — around 90 Kelvin, just a little above absolute zero — so it’s really dense. Slippery as neutrinos may be, they can't all sneak through without bumping the nucleus of an argon atom. 
FLEMING: Once they hit an argon atom they turn into something. If it's an electron neutrino, they turn into an electron. If it's a muon neutrino, it turns into a muon. And those charged particles you can see in the liquid argon because as they travel through the liquid argon they knock off electrons as they go and create trails of particles — like a perfect photographic image of the event as it goes through the detector.  
KREER: Scientists then "drift" that image through the clear, purified liquid argon to the edge of the detector, where panels made up of thousands of individual wires record the particle tracks.  
FLEMING: The drifting sounds slow, because in particle physics it is. It's milliseconds, as opposed to say microseconds. Still it's, you know, almost nothing in time. But nonetheless, you essentially have this perfect 3D image of the particle interacting in the detector, and you drift that image and essentially take a picture of it with the electronics of the detector. A 3D picture. It's really beautiful.    
DOZIER: I think it's time now for us to retrace our steps and make a bee line from South Dakota back through the Earth's crust to Chicago, where our story began. 
KREER: As impressive as the project taking shape in the Sanford Lab is, it's only one piece of the "Long Baseline Neutrino Facility," or LBNF, which Chris Mossey helps oversee.  
DOZIER: The huge detector in South Dakota is actually known as the "Far Detector" of DUNE since it's at the far end of the neutrino beam. There's also another, smaller detector called (what else?) the "Near Detector" that will measure the stream of neutrinos right as they're leaving Fermilab.  
KREER: And it's actually just as important! Remember how we talked about neutrinos changing flavors being the key to this whole cosmic mystery?  Well, the only way you can test that is if you know what flavor they started out as — and then what they flipped to.  
DOZIER: We've talked a lot about neutrinos so far, and how scientists are trying to study them with their fancy detectors, but let's not forget that this neutrino beam ain't going to generate itself. That's all Fermilab. 
KREER: At the heart of Fermilab's accelerator complex, construction is under way on a snazzy new upgrade.  
DOZIER: This new addition is called the Proton Improvement Plan-II (or PIP-II), a state-of-the-art particle accelerator set to launch sometime around 2026. In addition to sending a blast of neutrinos to the South Dakota DUNE site, this accelerator promises to be an instrumental tool in particle science for decades to come.  
KREER: But let's back up a little bit. What IS a particle accelerator? 


LIA MERMINGA: A particle accelerator is a complicated and high-tech device which starts with a source of particles, in our case they are protons, in other cases there are electrons or other types of particles. And then it propels them through electromagnetic waves to very high energies that near the speed of light at the end.  
KREER: That's Lia Merminga, project director of PIP-II. Her team is responsible for bringing to life this crucial piece of the DUNE puzzle.  
DOZIER: Lia says that, despite the sci-fi sounding name, particle accelerators are fairly common and have been around since the '30s. In fact, there about 30,000 of them around the world, and they're used to support all sorts of science, from intense nuclear physics experiments to everyday x-rays.  
KREER: Now if you're anything like me, the movie Ghostbusters probably gave you your first (wildly inaccurate) idea what a particle accelerator is, in the form of the proton pack. But real accelerators come in different sizes: from tabletop equipment to a couple city blocks to several miles long.  
DOZIER: Basically, accelerators work like this: in order to really see how particles behave, we need to make them go really fast and then smash 'em into something. A particle accelerator does this by first generating the particles — in this case, protons — by peeling off the proton within a contained electric field. The protons are gathered at the front part of the accelerator, sort of like runners at the starting line of a race.  
KREER: Starting gun fires, and the protons are propelled through the accelerator, often in controlled bunches. They pick up speed as they go, helped on by electromagnets, which steer them, and cavities with electric fields that give them a boost.  
DOZIER: Once they're at the right energy, they're smashed against a target. Then scientists can look at what came out of that collision and see what is normally unseeable: particle interaction.   
KREER: Accelerators can come in different shapes and sizes, like linear or ring-shaped. Up until a few years ago, Fermilab had the most powerful particle accelerator in the world — that is, until the Large Hadron Collider entered the picture in its search for the Higgs boson particle.  
DOZIER: Number 2's not bad! 
KREER: Not at all. And you can still see Fermilab's ring-type accelerators in aerial pictures of the campus. There are two of them, and they're several miles in circumference. Now the laboratory uses its accelerator complex to provide the best neutrino beams in the world. 
DOZIER: Okay, so how does PIP-II come into this? What's the "Improvement" part of the Proton Improvement Plan? Here's Lia again: 
MERMINGA: So PIP II has a three-fold mission. The first element is to provide the world's most intense beam of neutrinos to DUNE on day 1 of its operation. The second element is to provide multiple beams for a broad physics research program at Fermilab in particle physics. And the third is to upgrade the Fermilab accelerator complex, which is nearing 50 years old. 
KREER: So, item 1: the most intense beam of neutrinos. No biggie, right? But how's it going to deliver that? 
MERMINGA: So we start with an ion or proton source of particles, then we send those into metallic structures in which we set up electromagnetic waves. Think about your microwave, for example, except they are a lot more powerful. So the protons then ride on the electric wave that we set up in the structures, and as they ride on the electric wave they pick up energy along the way.  
KREER: These metallic structures she's talking about look kind of weird. Like one of the Michelin Man's arms. 
DOZIER: Like a stack of really big donuts.  
KREER: Yes. Shiny metal donuts. But this metal donut stack is state-of-the-art tech. And it's not just any metal, either. 
MERMINGA: The structures are made of niobium. When it is cooled to very, very low temperatures, specifically in our case 2 degrees above absolute zero (2 Kelvin), niobium becomes a superconductor. And that means that the electrical resistance vanishes completely.  
DOZIER: This is crucial, because, if you're going to generate a ludicrous amount of power — enough to shoot a beam of particles from Chicago to South Dakota, for instance — you want to hold onto as much of that energy as you can.  
KREER: Now take several of those superconducting structures (scientists call them "accelerating cavities"), and chill them down in a sort of refrigeration tank called a cryomodule. "Cryo" for the intense cold.  
MERMINGA: So, you can think of this string of accelerating cavities as a long necklace, and say each one of these accelerating structures is a bead on this necklace. In the case of PIP-II, we have 5 different types of the accelerating structures. Some are round, and some are elliptical, and some are more elongated. That aspect makes the PIP-II project, from a physics and engineering point of view, very complex and very challenging, but very interesting as well.  
DOZIER: The whole necklace-like setup (the cavities, the cryomodules), all of that forms the new linear accelerator — linear, as in a straight line. And it's about two football fields long. 
MERMINGA: For the PIP-II accelerator we have 25 of these cryomodules. And so, as the protons go through these 25 cryomodules, they get accelerated up to 800 million electron volts of energy. 


KREER: Just like we described earlier, you shoot the beam of protons through this accelerator.  


DOZIER: From there, they'll jump over to a booster ring to gain even more speed...  

KREER: ...then whip over to a second larger Main Ring.  

DOZIER: Like the miles-wide rings you can see from the air? 

KREER: Yes. Then finally? Aim this thing at the target. In this case, that target is a cold graphite rod about 6.5 feet long, and it's going to hit this thing end-first.  

DOZIER: By this point, the sheer intensity of the proton beam is so great that it creates neutrinos from the target by the billions. More magnets help direct this torrent of neutrinos — first through the Near Detector at Fermilab, then onward into the crust of the earth toward the Far Detector waiting in South Dakota.  
KREER: And they won't have to wait long for this delivery. The neutrinos are traveling near the speed of light -- meaning they'll cross the 800 miles to the Sanford Lab in roughly 4 thousandths of a second.  


DOZIER: Right now, you might be picturing this beam as the Marshmallow Man-melting blast of energy from a proton pack.  

KREER: It... doesn't look like that. This isn't Ghostbusters, remember? 
KREER: (LAUGHS) Boo. But it DOES pack a real punch. 

MERMINGA: The protons that we're accelerating -- we're accelerating them at very high energy (800 million electron volts) and we're accelerating a lot of them, so the intensity is very high as well. Think about the lightbulb in your house, which is just 60 watts for example. Now the beam of protons will have power that is starting at 1.2 million watts. 
DOZIER: PIP II, once it's finished with this first round of upgrades, expects to deliver a 1 megawatt beam, which is 60% higher than what it's got now. And future upgrades promise to triple that. 
KREER: This power is vital though, not just to generate the particles themselves, but to make sure that DUNE's neutrino beam has the right energy and can get to where it's going — South Dakota and beyond — with its bits intact. 
DOZIER: And that's just the power going out of Fermilab in the form of the beam. What about the power going in to fuel this beast? 
MERMINGA: I ran into the CEO of the local electrical company in a community outreach event, and she said, "We can tell when you guys don't run the accelerators, we can see the dip in our distribution." So about 40 megawatts is the Fermilab power during peak load, and PIP II is going to add roughly another 20 megawatts when it goes into full operation.  
KREER: For context, a 60-megawatts power load is the equivalent of over 6 year's worth of energy used by the average American home. But the electric bills are the least of Lia's worries. It goes without saying that a project like PIP II comes with enormous challenges and a ton of delicate moving parts. Both in the literal sense and in the personnel sense.  
MERMINGA: To build an accelerator as complex and as modern and state-of-the-art as PIP II is going to be, it requires a broad range of physics and engineering expertise. For example, it requires experts who can build these exquisite accelerating structures, cool them down to 2 degrees Kelvin, establish the electromagnetic fields, and then make sure the fields are controlled to a very high precision in order for the beam of protons to be accelerated exactly as specified by the design. 
DOZIER: PIP-II is unique in that it's the first particle accelerator in the U.S. built with significant contributions from international partners. Teams in India, France, Italy and the UK are all working on this project. So not only do you need to juggle a team of highly-trained engineering masterminds, but do it across 11.5 time zones.  


KREER: You know the line: "With great power comes great responsibility." And heat. And radiation. Which brings — you guessed it — even more challenges. 
MERMINGA: Around the target area where the proton beam of the million watts of power hits the target of graphite, you can imagine the huge amount of heat as well as radiation, and so one might think, "How does one repair any equipment in this environment after several years of operation  at these high-power levels?" And so now during the design of this environment we have to think ahead and make the design so that the repair or replacement of these components can be done remotely and non-invasively. 
DOZIER: If the stakes sound high, that's because they are. But the potential rewards are nothing short of astronomical. 
KREER: The project that Lia is helping direct right here in the U.S. has the potential to be as much a pioneer in particle physics — and to our understanding of the universe — as CERN's Large Hadron Collider in Switzerland. Except in the case of Fermilab, the new ground being broken — literally and figuratively — centers around the neutrino.

MERMINGA: CERN is exploring is the energy frontier of particle physics, going to higher and higher energies. In an analogous way, LBNF and DUNE are going to explore neutrino physics. So scientists from around the globe who are interested in neutrino physics, they will come here to Fermilab to study neutrinos. 


DOZIER: For Lia, whose career started at Fermilab, there's an added sense of pride at being a part of the lab's historic next chapter. 
MERMINGA: I'm originally from Greece, I was born and raised there. I came to the US to go to graduate school and as a matter of fact, I did my Ph.D thesis work here in the late '80s. So I'm back again now, helping to upgrade the very same Fermilab accelerator complex where I started my career. It is awesome, I would say. It's an awe-inspiring experience. I just feel I'm so grateful for this opportunity. To be able to contribute back to this outstanding facility and laboratory, one of the best in the world, that gave so much to me in my early days in this field of accelerator science.  
DOZIER: Don't think for a second that the people involved in this project aren't aware of the size of the challenge they face. Every part of the process has been shaped by the vast resources, technical hurdles, and complex international coordination involved. 
CHRISTOS TOURAMANIS: This is a project which already has 1,100 participants, mostly physicists but quite a few engineers also. And all of these people come from a large number of institutes, from more than 30 countries now. That gives it this big coverage of the globe; that is another "mega" aspect of the project. So everything about this project is big.   
KREER: Christos Touramanis is a professor of particle physics at the University of Liverpool. He's also one of the leaders of a project that's putting DUNE's bold ideas to the test — before the full-scale experiment gets rolling in earnest. 
DOZIER: It's called "ProtoDUNE," fittingly, and it's underway right now at CERN, the world-renowned physics laboratory near Geneva, Switzerland, that's home to the famous Large Hadron Collider.  
TOURAMANIS: CERN is the largest particle physics laboratory at this time in the world. It is very international endeavor. So what CERN has which is unique is that it is a very big laboratory with a lot of technical depth, expertise, and some of the best people in accelerator physics, in frontier electronics, in radiation and particle detection techniques, and all of that.   


KREER: CERN is a crucial partner on DUNE, in more ways than we even have time to count. It has the technology, the space, and the expertise to support a project of this magnitude — so what better place to build a miniature version of the DUNE detectors? 
DOZIER: By miniature, we're talking 1/20th the size of the full-scale units. Which, of course, is still really big. Even shrunk down, the prototype Christos' group has been working on is the largest liquid argon neutrino detector ever constructed — larger than ICARUS, the former reigning champion. 
KREER: And not only that, they worked incredibly quickly, building warehouse-size spaces to hold the detectors, assembling and installing them in less time than it takes to build your average office building.  
TOURAMANIS: Well actually, that is another thing which demonstrates the ability of CERN to really do unique projects very efficiently and very fast. Two years ago, there were big machines shifting earth to dig down to make the "trench" as we call it, which is a huge volume: 12 meters deep, where the detectors are sitting. In addition to this, the previous detector built of this technology, ICARUS, which is actually now at Fermilab, had taken a decade to construct. We designed and constructed ours in 2 years. So basically we are pushing the times down by factors between 3 and 5. So that has been done very fast.  

DOZIER: The first ProtoDUNE detector came online in September 2018, and to everyone's surprise, started recording crystal-clear images of neutrino interactions right away... That basically never happens. Flavio Cavanna, a scientist at Fermilab and, along with Christos, one of the co-coordinators of ProtoDUNE, said “The entire technology operated as we wanted it to, which is beyond what one can dream." 
KREER: If you haven't seen pictures of the detectors under construction at CERN, we'll have some on our website — they are really impressive. Like their much larger DUNE siblings, the prototypes actually use technology created for tanker ships that transport natural gas to contain the super-cold liquid argon. 
DOZIER: On the outside, there's a red steel frame the size of a three-story house — and on the inside, the walls shimmer with golden light, like ornate temple of science. Except, that's actually an optical illusion. 
TOURAMANIS: Oh, that is a big joke between colleagues and my Facebook friends when I post photos. Actually, there's nothing golden in there. The detector wall, the inside wall, is made of stainless steel of very high quality, because that is what separates our argon from the outside world. But because the detectors we are putting inside have some chemicals which are sensitive to ultraviolet, we use yellow light, so it appears to be golden. So yeah, no, that's an illusion. We didn't do it on purpose, honestly.  

KREER: So this isn't gold-plated physics?  
TOURAMANIS: No, I mean our physics, will be "golden" physics, metaphorically. But we are not using gold, no. And actually it wouldn't be the right material to use for that, even if someone gave us billions more to do it with.  
KREER: ProtoDUNE has already given scientists and engineers at CERN the chance to address a bunch of big question marks. Could they really assemble parts fabricated and shipped from all around the world into a working unit? Does the liquid argon cooling system function properly at such large scales? Are the super-sensitive panels inside the detectors recording neutrino interactions? 
DOZIER: So far, the answer to all of those has been a resounding "yes." It's a key step toward making sure DUNE will work as expected.  
KREER: Especially because the team running it — they're the ones who will assemble and operate the real deal beneath the hills of South Dakota in a few years. 

TOURAMANIS: A large number of physicists, both from the U.S. and European institutions, we have gathered at CERN where we have formed the team that will actually be doing the detector integration, testing, installation, and then operation. So essentially we are building the nucleus of the international team which will be required to deliver the full DUNE project in the U.S., and we will build a community of experts that will do all the science over the next 20-25 years.  
DOZIER: To pull off this level of grand, global scientific effort, Fermilab and CERN need more than just technical know-how. For all the pieces to fit together perfectly, the people have to be in sync, as well. 
TOURAMANIS: Right. So, it is a unique challenge to bring so many different people from so many different countries. We all share the same love for nature and understanding its workings. We all like research. However, there are deep cultural differences which go way beyond the scientific aspects. But by having a large number of people who have different ways of thinking, approaching questions, collaborating. You can bring out and utilize the best elements from essentially all of the cultures and the disciplines that mankind has developed over the centuries. 

KREER: Obviously much remains to be done before DUNE even begins to take its final shape. Pre-excavation work — so, like, the preparations before the real digging happens — started at the Sanford Lab site in January 2019. PIP-II broke ground on its accelerator upgrade in March, with a goal of delivering its "first beam" in the mid-2020s. ProtoDUNE is humming away at CERN.  
DOZIER: If all goes well, the DUNE experiment will be fully operational by around the 2026 timeframe. The plan is for it to run continuously for a decade or more after that, steadily gathering evidence for what scientists hope will be the crowning achievement of neutrino science. 

FLEMING: So, you know, after about years of data taking, we'll collect thousands of events. That's small compared to the number of neutrinos that are going to wash through the earth and through the detector. But that's the point of having a very big detector and a very intense neutrino beam is that some of them will interact, on the scale of "events per day," essentially, to be able to look for these differences between neutrino interactions and antineutrino interactions.  
KREER: And that's not all! You didn't think that they're going through the trouble to build this megascience project just to study one thing, did you? 
FLEMING: Absolutely. You know, this is going to be a huge facility that will do lots of different physics in addition to accelerator neutrino physics. Looking for neutrinos from supernova, from neutron stars, those that are produced in the atmosphere — there's neutrinos all over the place. In addition to that, it will look for proton decay, which is something that the particle physics community has been looking for for many years. So it'll be live for lots of different things in addition to just the accelerator neutrino physics, which is hard to say “just.” It's really exciting.  


DOZIER: Of all the challenges the folks involved with DUNE have to overcome, waiting may be one of the hardest. But by building up to the main event gradually, they will have plenty of time to tweak, and test, and refine their approach, and just... do science!  
DOZIER: How hard is it to wait for all this to be completed so your experiments can begin?  
FLEMING: No kidding. But we have projects along the way. We have these smaller-scale, school bus-sized detectors that are running on-site at Fermilab now, and they're addressing a host of really interesting physics questions, too. My group in particular combines data analysis on these smaller-scale experiments, but looking for new particles, new neutrinos, en route to building the biggest — this DUNE-class experiment. You know, there are so many interesting questions in science. So we do a little bit near-term and a little bit far-term. 
KREER: All of these components — Fermilab's accelerator, the ProtoDUNE detectors, even the construction project at the Sanford Underground Research Facility — offer new opportunities to explore other scientific questions along the way. Even if they're not the big question DUNE's seeking to answer, they're still incredibly valuable.   
DOZIER: The science goes on.... and so could we. But instead, we'll leave you with another line from Frank Herbert, creator of “Dune” the novel. Which, now that I think about it, has as story about as sweeping and mind-bending as what DUNE *the experiment* is trying to achieve. The neutrinos... must flow. 
MAN’S VOICE: “There is no real ending. It’s just the place where you stop the story.”  
KREER: Thank you to our guests, Bonnie Fleming, Chris Mossey, Lia Merminga, and Christos Touramanis, for guiding us through the thorny world of particle physics. 
DOZIER: And a huge thanks to Lauren Biron, the Fermilab multimedia team, and all the rest of the folks at the lab who helped arrange interviews and make sure we got our story straight. 
DOZIER: We have so much more good stuff on our website for this episode. Photos of the detectors, the-gold mine-turned-research-lab, maps and diagrams of how DUNE works, and even a story of a father and son, rebellion, and motorcycles. Find all that at energy.gov/podcast.  
KREER: And as always, if you've got a question or want to leave us some feedback, email us at directcurrent@hq.doe.gov, or tweet @energy. And if you're enjoying the show, share it with a friend and leave us a review on iTunes. We read them, and we listen. 
KREER: Direct Current is produced by Matt Dozier, Paul Lester, and me, Cort Kreer. I also create original artwork for every episode, and you can find that on our website.  
DOZIER: Additional support from Ernie Ambrose, Gigi Frias, and Atiq Warraich. We’re a production of the U.S. Department of Energy and published from our nation’s capital in Washington, D.C. 
KREER: Thanks for listening!