(Intro Music) If I were to ask you, "Hey what do you think are some of the biggest unsolved problems in physics?", you might think of things like "What happened during the big bang?", and "Is there a Theory of Everything?". Because that's what you tend to hear about in documentaries or on random science news sites. But actually, those questions and many others are all connected to a kind of obscure unsolved mystery. Why do neutrinos have mass?  Neutrinos are weird little particles that are notorious for being super hard to detect.

They rarely interact with other particles, so they have a reputation for being kind of "ghostly". And this ghostliness brings with it a lot of mysteries and every time we solve one, another one seems to pop up. But these mysteries are really worth solving, because it turns out they reveal a lot about the fundamental nature of reality.

Now neutrinos aren't rare particles, they're actually more common than every kind of particle that we know of except photons, the particles of light. In fact, billions of them are passing through you every second. But you'd be lucky to see even one hit any atom in your body.

They just really don't like interacting with stuff. So, like you might expect, doing experiments with them is really hard. But we have managed to work out a few things.

Like we know that there are three kinds, or flavors, of neutrino and each is related to one of three other particles: electrons, muons, and tau particles. So there's an electron neutrino, a muon neutrino, and a tau neutrino. We also know that neutrinos have mass and that might seem obvious because most things do, but we didn't know this for a lng time.

In fact it took us until the late 1990's to figure it out. around then, two teams ran a clever experiment that observed neutrinos coming from the sun. They expected to see a certain number of electron neutrinos coming from our star, just based on what they knew about it. But instead, the number of electron neutrinos that arrived was only a third of what they had predicted.

After some follow-up tests, the researchers found that the other 2/3 has spontaneously turned into the other two flavors - into muon and tau neutrinos. Now it might not sound like a huge deal, but this process - now call neutrino oscillations - was super surprising. Like it won the team a Nobel Prize surprising.

See, according to the laws of physics, changes like that can only happen when particles have different masses, but why is besides the point here. But the key thing is for something like and electron neutrino to have a different mass than a tau or muon neutrino, that means some of those flavors need to have mass in the first place. Measuring exactly what those masses are though, is a whole other story.

For one, there's some messy quantum probability stuff involved, and that's because any flavor of neutrino can come in one of three masses. In other words: when you detect, say, an electron neutrino, there's a certain chance you'll see it with mass #1, but also a small chance that it'll have mass #2 or #3. And the odds of seeing each of the three possible masses is different for each of the three flavors of neutrino.

And, as if that weren't enough, we don't know what exactly those masses are. Because they're so ridiculously tiny that we haven't been able to measure them directly. Like, after neutrinos, electrons are the next lightest particle.

And electrons are tens of thousands of times lighter than typical atoms. But from what we can tell, the heaviest neutrino is at least a half million times lighter than the electron. And the fact that they are so much lighter than all the other particles is something that actually bothers a lot of particle physicists.

See, we think we have a good understanding of how particle mass happens at a fundamental level. It's called the "Higgs Mechanism", and it says that the mass of a particle is determined by how strongly it interacts with something called the "Higgs Field". So the question is, what makes neutrinos interact so much less strongly with this field than the other particles.

And here's where the problem shows up. The current best guess is that neutrinos don't interact with the Higgs Field at all, because they just behave too differently from other particles in too many ways, for that to make sense (3:58).

Normally, that would mean they don't have any mass but, well, they clear do. And so now maybe you can see where the big question comes in. After decades of research and countless experiments, we've finally figured out that neutrinos have mass, but we don't know why. After all this searching, we've basically run into a brick wall.

Now, it is possible that the answer will become clear with more research. For instant, some theorists predict that we'll eventually find that two of those mysterious neutrino masses are very close to each other. That would be a very strong hint that neutrinos have some kind of substructure, like how protons & neutrons are actually made of smaller quarks.

And that would imply that we need to look even deeper inside neutrinos to see what makes them tick. But that's not the most popular idea out there. Instead, the most popular explanation has something to do with a property called the "handedness" of neutrinos.

To work out the handedness of a particle, you look at the direction it's traveling, and the axis it's rotating along. I mean, it's not really rotating because it's particle physics and everything is weird, but it's a close enough approximation. If the rotation axis points points the same way as the direction of travel, we say a particle is right-handed.

And if the axis points the opposite way, then it's left-handed. Now, normally we see right-handed and left-handed particles, but for neutrinos the rules seem to be a little different. We only ever see left-handed neutrinos.

So where are the right-handed ones? Well, the proposed idea is whimsically called the "seesaw mechanism". It predicts that the missing right-handed neutrinos, but are way too big for us to detect.

In other words, the extremely light, left-handed neutrinos also have extremely heavy, right-handed counterparts. And the idea is the two groups are connected. Essentially, the lighter neutrinos get their mass from coupling to the heavier one somehow.

And as the mass of the left-handed group goes down, the mass of the right-handed group goes up. Hence, the seesaw. So, just as the left-handed group's mass is tiny, the right-handed group's mass should skyrocket past the mass of every other particle (5:56).

Like, many estimates say there are tens of millions of times the mass of the heaviest particle we know of. And unfortunately, that would make them way to heavy to find with our current technology. You need an accelerator that can reach outrageously high energies to find super heavy particles. So it's unlikely anyone will confirm this idea any time soon.

Still there are big reasons to keep looking. Even beyond the fact that, well, we're super curious and wanna know why neutrinos have mass. For one, understanding neutrinos could help us find new throies for all of physics. Right now the gold standard in particle physics is called the "Standard Model", and like we've said before on this channel, it's...great!

But we also know it can't be the end of the story. I mean, it's meant to model all particle interactions but it doesn't include gravity.  And, well I'm not drifting off into space right now so THAT definitely exists. 

For decades we've been looking for experiments to guide us to theories beyond the Standard Model. And neutrino masses are one of those few windows that we have to that new physics. Afterall to make progress and make new hypotheses, physicists need experimental methods for things that don't fit the Standard Model picture. 

And the Seesaw Mechanism is exactly that. A new idea that goes beyond the Standard Model based on new evidence. But that's not the only reason people are so interested in this either. And as is so often the case in physics, a discussion about the smallest parts of the universe naturally leads to a discussion about the biggest. 

Our understanding about that very early universe is almost entirely based on mathematical models, where everything is mixed together in a hot dense soup of particles. We rely on our knowledge of physics to make those models and neutrinos play a big part in them. That's because even if each one on its own has a miniscule amount of mass, the sheer number of them means that their collective mass effects the way galaxies evolve. 

And if there were any of these super heavy right-handed counterparts in those early days, that would've added even more mass to things. So that's another reason to keep looking into all of this. It could tell us how galaxies changed and as a result how today's galaxies will change too. But even beyond that, we may need to understand neutrino mass to understand why the universe exists at all. 

Right after the universe formed, physics predicts that there should've been equal amounts of matter and antimatter. Antimatter being matter with properties with properties like the opposite charge.  For instance, an electron has a negative, and an antielectron has a positive one. 

We know that matter and antimatter behave identically in virtually all situations. And when they touch, the annihilate each other. So why didn't everything blow itself up in the very beginning? NOBODY KNOWS!! But neutrinos could help explain it, because some people think it's possible that neutrinos and antineutrinos do behave differently.

Specifically, we think that spontaneous shape-shifting we mentioned earlier, the neutrino oscillations, may happen different for neutrinos and antineutrinos. If that's true, that could influence why matter won out over antimatter. 

But to understand exactly how and what that means for the big picture, we need to know more about the neutrino's masses which affect the rates of those oscillations. And we all know how much of a mess that is. SO in the end, it seems every time we learn something about neutrinos, new questions come up. 

LIke we know that neutrinos are the lightest particles that have mass, except they might be made of even smaller particles. And also they might also be the heaviest particles by a factor of 100 million or more. And we don't know exactly how light they are, and we don't know why they are left handed, and we don't know why they have mass AT ALL! Neutrinos are WEIRD!

But we are making progress. In 2020 the T2K Experiment in Japan reveal tentative evidence the neutrinos can behave differently to antineutrinos which would be HUGE if confirmed to be true. And we'll have another episode about that soon.

So as time goes on, these ghostly little particles are coming a little more into view. And one day soon we'll hopefully bust this mystery wide open. 

Thank you for watching this episode of Sci Show! Particle physics, as you might have noticed, is kind of a hard topic. And we wouldn't have been able to spend so much time figuring out the science and putting together the graphics without the support of our patrons on Patreon. So thank you to everyone who makes this all possible. If you wanna support the work we do here and bring more free educational content to the Internet you can go to patreon.com/scishow