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Almost all matter in the universe should have been annihilated shortly after the Big Bang, but looking around, we see galaxies, stars, planets, and, you know... us. So obviously that didn't happen, and the why of it may have something to do with neutrinos.

Hosted by: Hank Green

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Sources:
https://www.nature.com/articles/s41586-020-2177-0
https://arxiv.org/pdf/1204.4186.pdf
https://www.nytimes.com/2020/04/15/science/physics-neutrino-antimatter-ichikawa-t2k.html
http://backreaction.blogspot.com/2020/04/no-physicists-have-not-explained-why.html
https://arxiv.org/abs/1801.10059
https://t2k-experiment.org/t2k/
https://arxiv.org/pdf/1409.1903.pdf
http://www.hyper-k.org/en/neutrino.html
https://www.nature.com/articles/178446a0
https://iopscience.iop.org/article/10.1070/PU1991v034n05ABEH002497
https://arxiv.org/pdf/1306.1246.pdf

Image Sources:
https://svs.gsfc.nasa.gov/12656
https://commons.wikimedia.org/wiki/File:The_entire_view_of_J-PARC.jpg
https://commons.wikimedia.org/wiki/File:Superconducting_Magnets_J-PARC.jpg
https://commons.wikimedia.org/wiki/File:J-PARC_Neutrino_Experiment_Facility_Primary_Beamline_Final_Focusing_Section.jpg
https://home.cern/science/physics/standard-model
https://commons.wikimedia.org/wiki/File:Ilc_9yr_moll4096.png
https://wmap.gsfc.nasa.gov/media/030658/index.html
https://wmap.gsfc.nasa.gov/media/990320/index.html
https://svs.gsfc.nasa.gov/vis/a000000/a000100/a000182/
https://commons.wikimedia.org/wiki/File:Andrei_Sakharov_1989_(cropped).jpg
https://en.wikipedia.org/wiki/File:Oscillations_electron_long.svg
https://commons.wikimedia.org/wiki/File:CMS_Higgs-event.jpg
[♪ INTRO].

Science has all kinds of puzzles and problems — but some seem a little bigger than others. Like, one of the biggest mysteries in physics is that almost all matter in the universe should have been annihilated shortly after the Big Bang.

But looking around, we see, you know, galaxies, stars, planets, and, us. So the challenge is working out how we got here. We're pretty sure it involves the relationship between matter and antimatter, and we've talked about some potential explanations before.

But now, there are new results. On April 15th, 2020, a team working on a physics experiment in Japan called Tokai to Kamioka, or T2K, published evidence suggesting that the relationship between matter and antimatter could be more unbalanced than we thought. The experiment looks at particles called neutrinos and antineutrinos, and it found something weird: The two kinds of particles seemed to be acting differently.

It might not seem that dramatic, but this finding could help shed light on the fundamental laws that govern matter in the universe. And understanding that difference might even tell us something about how all that matter got here in the first place. To understand the significance of this result and the role antimatter plays in it, it helps to look at ordinary matter first.

Everything you can see is made of matter, from galaxies to the pieces of lint on your shirt right now. And when you get down to it, all that matter is made of just a few key building blocks: subatomic particles like electrons and quarks. Those particles have certain properties, like electric charge, that tells us how to distinguish them from one another and how they behave.

For instance, electrons have a negative electric charge, which determines how they interact with an electric field. They also have a tiny magnetic field called a moment, which points in a certain direction, like how bar magnets have a north pole. Pretty straightforward.

Well now, imagine taking all those properties and reversing them. In other words, imagine giving your electron a positive charge, and making its moment point in the opposite direction. Congrats: You have now, in your mind experiment, created antimatter.

Because that's all antimatter is:. It's a particle with the opposite charge and orientation from normal. Or, if you're feeling technical, it's a particle that's undergone a Charge-Parity, or CP inversion.

Anti-particles aren't just hypothetical, though. They really exist and — for the most part — satisfy the laws of physics in pretty similar ways to ordinary matter. I mean, when you put the same kind of matter and antimatter together, they do tend to annihilate one another and convert into photons of light.

But overall, antimatter isn't as weird as some books and movies make it out to be. In fact, for decades, physicists assumed that matter and antimatter were basically perfect mirror images, because swapping them for one another in the equations we knew at the time didn't affect any of the math. In other words, they had what physicists call CP symmetry.

Except... it turns out that some laws of nature do treat matter and antimatter differently, which completely blew physicists away when they figured it out. One of the clearest examples comes from looking at B mesons, particles made of a quark and an antiquark, made at the Large Hadron Collider in Geneva. The data show that positively-charged B mesons undergo a type of nuclear decay about five percent more often than their antimatter counterparts do.

In other words, those quark interactions do not obey CP symmetry. That might not sound like a big deal, because, like, what's five percent among friends? But even the tiniest differences between matter and antimatter could have big implications for understanding the history of pretty much everything.

One of the most unbalanced things about the whole universe is that right now, there's a lot of matter, but there is not much antimatter. We don't see huge amounts of it floating around in space, and the tiny amount generated in certain radioactive decays doesn't stick around to do much. But… if nature treats these things just about the same way, you would expect the scales to be more balanced.

There are two common ways to think about this mystery. One is that we can't solve the problem. That maybe, this involves some initial condition of the universe that naturally tips the scales in favor of matter — some condition we will never know.

But most cosmologists don't think the roots of the problem go back that far. Instead, they believe matter and antimatter did start out pretty much equal in the early universe. See, there's a form of radiation out there called the cosmic microwave background, or CMB.

It's kind of warm afterglow of the birth of the universe, and cosmologists think the photons it's made up of are from particles and antiparticles annihilating billions of years ago. If you look at the CMB, you can calculate the amount of matter and antimatter that was around right after the Big Bang. And when you do, you find that both parts should have been equal, up to like one part in ten billion.

That's a tiny difference, and it shows the scales were almost perfectly balanced between them. But if that's the case… the universe basically should have annihilated itself, leaving barely any matter at all. So the existence of galaxies, planets and us are all a mystery.

As far as the physics goes, none of this should be here! That leads cosmologists to think that the universe started out balanced and then something tipped the scales entirely, to the point where there was virtually no antimatter left. Enter Russian physicist Andrei Sakharov.

In 1967, he proposed some conditions needed in the early universe to tip the scales so dramatically. He said that, if matter and antimatter did start out equal, one condition that could change things is if there was a certain amount of CP asymmetry in the laws of physics. In other words, if you swapped matter for antimatter in an experiment, you wouldn't get the same results every time.

There should be interactions between particles that treat matter and antimatter differently. Those interactions — whatever they are — might explain how matter came out on top. CP asymmetry would stop everything turning into photons because some interactions would presumably favor matter's survival in the process.

We already know CP asymmetry exists in quarks — like with B mesons — so it sounds like it's an open and shut case but unfortunately, it's not enough. To meet Sakharov's condition, there needs to be more CP asymmetry than we know of solely from quarks. It's like the weight on one side of the scales isn't heavy enough to tip them over yet; there should be more asymmetry lurking around somewhere in the laws of physics.

And that - THAT- is where neutrinos come in. Because according to the new results from the T2K experiment, these particles might be the next thing to tip those scales. Neutrinos are particles released in certain nuclear reactions, and they are really hard to detect because they rarely interact with matter.

To actually notice one of them, you either need to put a lot of stuff in a neutrino's path for it to bump into, or be around something producing a lot of neutrinos. Then, once you have your setup, one method of detecting these things is by observing the particles they turn into. This is how T2K works.

On the east coast of Japan in Tokai, a particle accelerator generates a beam of neutrinos and anti-neutrinos. That beam is pointed at a tank of water 300 kilometers away at the Kamioka laboratory, filled with 50,000 metric tons of pure water. And when a neutrino collides with a water molecule, there's a decent chance it will turn into an electron, or similar particles called muons and taus — things that are all much easier to spot.

T2K wasn't just looking for neutrinos, though. Instead, the researchers were specifically studying something called neutrino oscillations. See, the particle the neutrino turns into when it collides with matter determines its identity.

So there are electron neutrinos that turn into electrons, muon neutrinos that turn into muons, and tau neutrinos that turn into taus. But what shocked physicists, is that a neutrino's identity isn't a fixed property. Any given neutrino is kind of all three flavors at once.

Let's say you have an electron neutrino. Well, depending on the energy it carries and the distance it travels, it could actually turn into a muon or a tau neutrino. The probability to be one type or the others changes over time like a wave.

That identity-changing behavior is what physicists call neutrino oscillations, and there's a really important detail in the math behind it. The equations that describe oscillations contain a parameter called delta CP. If delta CP is anything other than zero, then neutrinos would oscillate differently than how anti-neutrinos do.

In other words, there would be even more CP asymmetry of the kind needed to satisfy Sakharov's condition. That parameter is one of the things T2K was ultimately designed to measure. And it seems to have done that!

Their data indicate that their muon neutrinos oscillate into electron neutrinos more often than anti-muon neutrinos turn into anti-electron neutrinos! That's a bit of a mouthful, but the key point is that the difference between neutrinos and anti-neutrino oscillations imply delta CP is something other than zero. That means there could be a new contribution to CP asymmetry, tipping the scales further than we thought and providing evidence that nature treats matter even more differently than antimatter than we thought.

So… that's it! There's definitely a bunch of CP asymmetry, and now we can all go home… right? I'm sure this is no surprise, but… no.

This is an impressive achievement for sure, but the amount of data collected by T2K only passes what particles physicists call a “three-sigma” threshold. That means that, if there's not really an effect here, the odds of a statistical fluke recreating their findings are one in a thousand. Those are pretty small odds, but not small enough that particle physicists would be totally convinced.

At this stage, the finding is what they refer to as “evidence”. The threshold normally used to denote a full-blown discovery requires collecting enough data that the chances of a false positive are only one in about 3.5 million, or what's called a five-sigma threshold. That's what researchers crossed back in 2012 when the Large Hadron Collider discovered the Higgs Boson.

For now, T2K will continue to collect more data. And other experiments, like one currently being built in the U. S., could also verify the finding independently.

But if this does turn into a proper discovery, it could pry open a whole new window into our understanding of matter, the universe, and why we are here. It would not solve the entire mystery of why there's more matter than antimatter, but would be a huge step in that direction. All we have to do now is keep studying these super mysterious, unreasonably elusive particles.

But for some physicists, that's just a normal Wednesday. If you want to learn more about the mysteries of neutrinos, you might enjoy our episode about why neutrinos have mass. Because apparently, nobody knows.

As always, though, thanks for watching this episode of SciShow — and thanks to everyone who makes it possible, including our patrons on Patreon and our channel members. There are a few ways to help us make more content like this, but if you want to learn about the Patreon option, you can go to patreon.com/scishow. [♪ OUTRO].