How To Capture Ghost Particles ft. Don Lincoln (script) (Patreon)

If it looks like we’re at a sci-fi headquarters for a league of superheroes – it’s because we are. This is Fermilab, For over half a century this has been the premier particle accelerator facility of the United States. These days many of the super-powered geniuses of Fermilab are tackling the most feeble particle in the universe – the neutrino. Why? Because this elusive particle may hold powerful secrets: from the unification of the forces of nature to the biggest question of all: why is there something rather than nothing?

I want you to meet Dr. Don Lincoln. So Don is a particle physics particle physics researcher here at Fermilab, but he also writes and hosts many of the awesome videos on Fermilab’s YouTube Channel.  Don, thank you for having us. 

You know, we’re glad you could visit. 

I’m kind of obsessed with neutrinos cause it seems like they could open the window to the next frontier of physics.  

We have a very broad research program, but our core research program is centered around neutrinos. And the big one, the future one is the DUNE experiment. Its core, the central most important thing its looking for is to study whether or not matter and antimatter act differently. As you know from E=MC2, matter and antimatter should be the same and they’re not. Dune may tell us why. 

Well it sounds like we need to learn a little bit more about neutrinos. 

Neutrinos are elementary particles of a type called leptons. That’s the same family as the familiar electron and its heavier cousins - the muon and tau particle. Neutrinos also come in 3 flavours - one for each of the charged leptons so we have the electron neutrino, muon neutrino, and tau neutrino. But all neutrinos have the bizarre quality that they oscillate between these types over time. Measuring neutrino oscillations is a big part of how we unlock their secrets. But it's not easy! Neutrinos are among the most elusive elementary particles in nature, only interacting by the weak nuclear force and gravity. In fact it would take a wall of lead 5 light years thick to have a 50-50 chance at stopping a single neutrino from the sun. Figuring out ways to detect them is tricky to say the least.

The weakly interacting nature of the neutrino makes them insanely difficult to detect – and we’ll be seeing exactly how that’s done soon. But in order to have a chance at spotting even a single neutrino, a truly enormous number needs to reach the detector. In order to do that, neutrinos need to be focused into a beam. But if neutrinos are so weakly interacting, how do we channel them into a beam in the first place? The answer is … we don’t. At least not directly.

The main Fermilab beamline accelerates protons around a 2 and a half mile circumference ring using giant electromagnets to around 99.997% the speed of light. That’s possible because, unlike neutrinos, protons have an electric charge. Those protons are then smashed into a graphite barrier, and as they collide with nuclei they produce all sorts of particles. More magnetic fields are used to sort the positively charged pion particles from the debris and focus them into a beam. Those pions quickly decay into muons and muon neutrinos. Put a simple concrete block in the way and the muons are blocked, but the neutrinos pass straight through into an almost pure beam of muon neutrinos, ready to be sent to our detector.

Down here we have the ICARUS neutrino detector. Don, as cool as ICARUS looks – I’m pretty sure this is not a 5-light-year thick wall of lead. How is this supposed to stop a neutrino. 

Well, it’s entirely probabilistic. Each neutrino has only a very tiny possibility of hitting an argon atom. So what we do, is we shoot 10 trillion neutrinos per second through this detector and only a handful of them actually interact. And so by doing that, we’re able to understand what’s going on. 

I think we should go down there. 

Let me try to give you a sense of why neutrinos are so elusive. As we mentioned, they only experience the weak nuclear force and gravity – and the latter is so weak we can ignore it altogether.

In order to interact with other types of matter, a neutrino needs to exchange one of the carriers of the weak force – a W or Z boson. It’s really a virtual boson – it exists only for an instant – just long enough to exchange energy between the neutrino and, say, the nucleus of an atom. It borrows the energy it needs to exist from, well, nowhere really. It sort of cheats energy conservation by taking advantage of the Heisenberg uncertainty principle, which tells us that there’s a fundamental uncertainty between certain pairs of properties – in this case, energy and time. So the briefer the lifetime of this virtual boson, the more energy it’s allowed to borrow.

Now the thing about the weak force bosons is that they are massive, unlike the massless photon or gluon, which carry the electromagnetic and strong nuclear forces. In order to exist they need to borrow a lot of energy to cover their rest mass. That greatly limits the amount of time they can exist, and so limits the distance they can travel. In order for a neutrino to interact with an atomic nucleus it needs to pass so close that it’s essentially inside the nucleus. In other words, we need a direct hit.

And if a neutrino interacts in our detector, an argon nucleus is broken apart and charged particles are released - in particular pions and muons. Those particles then travel through the liquid argon knocking electrons free from atoms. We charge the sides of the detector so a giant electric field fills the entire tank. That draws these free electrons to the walls of the tank, which lets us trace out the path of the particles. From those paths we can learn all about the neutrino collision. And that includes the flavour of the neutrino that caused it - be it an electron, muon, or tau neutrino.

But remember, our neutrino source produces only muon neutrinos, so if we detect tau or electron neutrinos then we’ve seen neutrino oscillation. And we can use our measurements to determine how much oscillation has occurred. When we add that we know how far the neutrinos traveled at the speed of light, we know how fast they oscillate. 

ICARUS is much smaller than the upcoming DUNE experiment. Let’s see some of the new accelerator that’s being built to feed DUNE.

We’re standing in front of the injection test for PIP-II that’s the the proton improvement plan two – which will massively enhance the number of neutrinos that Fermilab can produce for the DUNE experiment. Don, I think you need to tell us a bit about how DUNE is actually going to work. 

Sure, well what we’re going to do is send a super intense beam of neutrinos through the Earth 1,300 km from Chicago to South Dakota, where a huge detector weighing 70,000 tons consisting of liquid argon and located a mile underground will catch the beam and will tell us what happened to the neutrinos as they passed from here to there.

Which will hopefully tell us, something fundamental about the difference between matter and antimatter which also hopefully will tell us why we live in a universe made of matter in the first place.  

Our best understanding of particle physics tells us that matter and antimatter should have been created in equal quantities in the early universe – and so should have perfectly annihilated each other, leaving a universe of only photons. The fact that we see a universe full of matter means there must have been a tiny imbalance between matter and antimatter – enough to leave a bit of leftover stuff to produce the stars and galaxies and particle physicists that we see around us today.

In order for that to happen, there must have been a tiny asymmetry between the behavior of matter and antimatter. The full details of this are described by leptogenesis – a hypothetical physical process that may have occurred right after the big bang – and Don has already made an excellent video on the subject which you should check out.

But the short version is that according to leptogenesis, neutrinos in the early universe may have decayed into other matter particles, with matter neutrinos producing antimatter particles and anti-matter neutrinos – or antineutrinos producing matter particles.  If there was an imbalance in the number of neutrinos vs antineutrinos that could lead to an ultimate imbalance in the amount of matter vs antimatter. And that same initial imbalance should also be reflected in the way neutrinos oscillate, with antineutrinos oscillating between the three different types more slowly than matter neutrinos. And THAT is what the DUNE experiment seeks to detect.

Don, when can we expect to know the answer?

Well It sounds like you all have a lot of work ahead of you, so we’re going to get out of your hair, but thank you so much for taking us literally behind the scenes.

You know you’re welcome to come back any time. We love Space Time. And we loved Fermilab.

And that is how you study the most elusive particle in the universe. Check out the Fermilab channel for a much deeper dive into neutrinos and lots, lots more. It’s honestly one of my go-to resources when I’m researching a new episode of Space Time.