In the last few episodes, we’ve been talking about the standard model of physics, explaining what everything is made up of. But the reality is that we probably don’t know a fraction of how everything is put together. This week we’re going to talk about baryons, the particles made up of quarks. The most famous ones are the proton and the neutron, but that’s just the tip of the baryonic iceberg. And then we’re going to talk about where the standard model ends, and what’s next in particle physics.
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Female Speaker: This episode of Astronomy Cast is brought to you by Swinburne Astronomy Online, the world’s longest-running online astronomy degree program. Visit astronomy.swin.edu.au for more information.
Fraser: Astronomy Cast Episode 395, baryons and beyond the standard model. Welcome to Astronomy Cast, our weekly facts-based journey through the cosmos, where we help you understand not only what we know, but how we know what we know. My name is Fraser Cane. I’m the publisher the universe today. With me is Dr. Pamela Gay, a professor at Southern Illinois University Edwardsville, and the Director of Cosmo Quest. Hey Pamela. How you doing?
Pamela: I’m doing well. How are you doing Fraser?
Fraser: Great. I’ve got nothing. No announcements. Nothing to tell people or ask people. Do you have anything? Astro Gear?
Pamela: I would remind people, especially since it is Cyber Monday, if you’re watching live, to go to astrogear.spreadshirt.com and check out our cool shirts, hoodies, mugs, all of these random things that you can use to inflict sciency things upon people. My favorite is our Ceres, the other former planet shirt. Because I’m Team Ceres.
Fraser: And you have worn that many times, even on the show.
Pamela: I have. I have. We also have a Pluto shirt. Explore. Classic Pluto.
Fraser: Classic Pluto. Not that newfangled Pluto.
Pamela: Well it’s Pluto planet classic, sort of like Coke Classic.
Pamela: Just Pluto planet classic.
Fraser: All right. Show now?
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Fraser: In the last few episodes, we’ve been talking about the standard model of physics, explaining what everything is made up of. But the reality is, we probably don’t know a fraction of how everything is put together. This week we’re gonna talk about baryons, the particles made up of quarks. Famous ones are the proton and the neutron, but that’s just the tip of the baryonic iceberg. And we’re gonna talk about where the standard model ends, and what’s next in particle physics. So we kinda glossed over baryons, and I think we realized we went straight to leptons and quarks, but, in fact, we didn’t spend a lot of time on baryons. And I think, originally, in my very small understanding, we have protons and we had neutrons. Those are your baryons. But apparently, now looking at the list, there’s a lot!
Pamela: There are a lot. And just to review things for those trying to follow along at home, because this is a crazy alphabet soup. And I love your phrase just the tip of the baryonic iceberg. So we have leptons, which are fundamental particles that cannot be broken up any further. And these are our electron, muon, and tau particles and their partner neutrinos, the electron neutrino, the tau neutrino, and the muon neutrino. Then we have the quarks, also fundamental particles, up, down, charmed, strange, top, bottom. And these combined are called fermions. And fermions are things that have a multiple of spin ½. So they have a spin of a half, three halves, five halves, spin half is the key. And because of that, they follow the Pauli exclusion principle, and you can’t have two things with the same spin in the same energy level. So for instance, in an atom, you will have a spin up, and a spin down sharing an energy level. Okay. So that’s the fermions.
Then we have the bosons. These are the things that carry force, give us mass. So we have the photon, which is tied to the electromagnetic force. We have the W-, the W+, and the Z boson, also called the Z zero boson. And these are what handle the weak force.
Pamela: We also have the gluon, which is the strong force, and the Higgs boson, which gives us mass. Okay. So those are all the fundamental particles. Everything else is a non-fundamental particle. So everything else has to get built out of these fundamental particles.
Fraser: And one of the things that get built is baryon.
Pamela: Exactly. So things that are made of quarks are called hadrons. And hadrons are generally grouped into how many quarks make up the thing in the hadron. So we have mesons, which are combinations of two quarks. We don’t deal with those in real life, generally. And then we have the baryons. Baryons are what we deal with all the time. You and I are pretty much baryonic matter.
Fraser: Right. We are made of baryons.
Pamela: We are made of baryons. This is an awesome thing. We are fermions and we are baryons. And those baryons that make up us are made out of three quarks. In fact, they are made up of the ever so boring up and down quark, and these are what are called the protons and the neutrons.
Fraser: And so what is sort of the major difference between the proton and the neutron? I mean, we know that it’s the – neutrons have a neutral charge, while protons have a positive charge. Are there any other differences?
Pamela: So the proton is an up, up, down. The neutron is an up, down, down. And, yeah. They’re basically very close to exact same mass. The biggest difference is as far as we know, protons are annoyingly stable. Now I say annoyingly stable because a lot of theoretical physics would like these suckers to decay. It just makes the physics actually work. But as near as we can tell, they are utterly rock solid, not changing their identity, stable. The neutrons, on the other hand, when they’re not bound up inside the nucleus of an atom, they like to decay rather quickly, in about 100 seconds or so.
Fraser: What do they decay into?
Pamela: They actually decay into a proton, a electron, and an electron neutrino.
Fraser: Oh, okay, okay. And I guess that’s – when we talk about neutron stars, which are this degenerate matter, right? This is where you’ve artificially taken all the protons and all the electrons, and you’ve mashed them together into neutrons, right?
Fraser: And I guess it’s just the opposite way.
Pamela: And this is where you end up with protons and neutrons being close to the same mass, but not the exact same mass. So that difference in mass is how you get both a proton and an electron out when you let the neutron sitting on your shelf decay.
Fraser: Right, right. And as you said, they do adhere to the Pauli exclusion principle. Right? So you can’t put two protons, neutrons, et cetera into the same –
Pamela: No, no, no, no, no, no, no. They don’t. It’s the electrons that are spin ½ –
Pamela: and are Pauli exclusion principle. The way these work, because quarks, they’re also spin ½, but they combine, and it just works out that you end up with spin zero, if I’m reading the chart I’m looking at correctly. Anyways, they don’t obey the Pauli exclusion principle. They’re good.
Fraser: Now we’ve got a lot of building blocks. We’ve got the six different kinds of quarks that we know about. And yet the two major kinds of baryonic matter in the universe, right, with the proton with the up, up, down, and with the neutron with the up, down, down. But with all of those other combinations, there must be a lot of other baryonic matter out there. So what else is out there?
Pamela: Well, they go on forever, it feels like. So if you think about it, we have these six different quarks. And then we’re looking to combine them in combinations of three. And so that is one heck of a recombinant works problem. And what’s kind of awesome and disturbing at the same time is people have gone through and named pretty much all of the combinations. And this means that we end up with really funky names. So if you combine a down quark, a charm quark, and a bottom quark, because you can, you end up with what they call a charmed bottom.
Pamela: And that –
Fraser: Do we have to put an explicit language on this episode now? Well anyway.
Pamela: That’s it’s actual scientific name.
Fraser: Yeah. Yeah, yeah.
Pamela: We also have the double charmed omega particle, which is a strange charm charm. It’s just sort of like at a certain point; they just decided to start amusing themselves while naming these things. But the thing is all of these things decay, and some of them actually haven’t even been observed. So, for instance, we’ve named the charmed bottom, but it’s never been observed. We don’t what its decay route is. We don’t know if it does exist, how long it exists for. But that’s what we’re looking at.
Fraser: Right. How many times, how many different ways can you take strange, charm, down, up, down, charm – right? You just keep putting those combinations together, and you’re gonna come up with new particles. And you can work out the math, I guess. You can work out what the spin is gonna need to be. What the charge is gonna be. But it’s not; you don’t know whether this can even exist.
Pamela: Exactly. And what’s cool, though, is we can work out so much about it. So for instance, we refer to baryons as general as falling into two different families. There’s the angular momentum half baryons – this gets into crazy quantum mechanics stuff that you really need two years of quantum mechanics to understand. I’m not going to torture you that much. Suffice to say angular momentum half, and angular momentum three half baryons. And all of the three half ones, all of them, are unstable. So we have these two different families of particles. But we’ve detected things in both families.
So, for instance, a delta particle, which is made up of three different ups, because why not combine three things going up. That sounds kind of positive on a Monday afternoon. We know it has a rest mass of 1232 mega electron volts. We know it decays into a proton and a pion, and we know how long it lasts, which is way less than a second.
But what’s cool is we have by slamming things together in particle accelerators to generate vast amounts of energy in a very small place, we’ve been able to create circumstances that allow that energy to rain out into all of these different unstable particles that we can then study. So we have the ability on the math side to go through and say, “Okay. We’ve got six different things that we can combine together in all of these crazy different awesome ways.” There’s a triple charmed omega particle, for instance, that is a kind of awesome idea. There’s also a charmed double bottom omega particle, just if you wanna get weird.
Fraser: But with all of these combinations, though, right, these don’t occur naturally in the universe.
Pamela: So there’s a difference between not occurring naturally and not being stable. All of these things probably exist in the universe for fractions of a second. When I say fraction –
Fraser: In a supernova.
Pamela: In a supernova, in a collision between two cosmic rays. So you can imagine a situation where you have a high energy event that isn’t a supernova. So perhaps a coronal mass ejection lets off a cosmic ray, and it’s zipping its way through the universe. And it just happens to collide with another particle zipping its way through the universe. And they combine for that fraction of a moment, to, for ten to the 23rd, 24th of a second, create a sigma particle, a charmed sigma particle. Whatever it might be that falls out of that energy.
Fraser: Right, and with way more energy that a large hadron collider could ever produce.
Pamela: And that’s the wild thing, is we’re working to catch up with the universe. The universe just does this because it has really big naturally-occurring magnetic fields. We have to do really bad thing to the power grid to create these really big magnetic fields.
Fraser: And so this is one of the great things about the large hadron collider. Everyone’s afraid that the LHC is gonna destroy the planet.
Fraser: But the reality is that it generates energies that are vastly, at many orders of magnitude less than the universe does all the time.
Fraser: So we talk about okay, so maybe these particles are being generated; in fact, vast amounts, and they are all decaying. And so the trick is that nothing else is stable in the way that protons and neutrons arguably are.
Pamela: Yes. And so this is where we live in a reality where yeah, we occasionally do experience muons in detectors, because they rain out of the upper atmosphere. But most of our day-to-day interactions are protons, neutrons, electrons, and that’s about it.
Pamela: Up and down. That’s all we’ve got. Up and down quarks.
Fraser: Right. Okay. So sort of the second part of this show is we wanna talk a bit about where are the gaps, where are the holes, and what comes next? So let’s do this in two ways. One is what are some of the things that we know about that we don’t know how they fit into the model?
Pamela: The biggest one, which is kind of ridiculous to admit to, the biggest one is actually gravity. We talk about there being a graviton boson, maybe, sort of, we don’t know. But as near as anyone can calculate, gravity just doesn’t fit in at the quantum mechanics level. So there’s that big problem.
Pamela: Looking beyond that big problem, we don’t know how to explain dark matter and dark energy. And the one thing we do know about dark matter is it’s some sort of non-baryonic particle. So it’s something that must be stable, or at least is constantly regenerating, that doesn’t interact via the electromagnetic force in a way that we know how to deal with.
Fraser: So could one possibility be, then, that maybe there is some other stable form of, some other stable baryon that was maybe generated during the big bang, or is still being generated by all of these cosmic rays bumping into each other, and this is somehow producing some particle that – at a higher level of energy than we create in a lab – that has these properties?
Pamela: Well, the issue seems to be that quarks are quite happy to interact via the electromagnetic force. So since we know that dark matter is not interacting via the electromagnetic force – this is where it doesn’t deal with light, as light passes through the universe, except to bend it, it doesn’t interact in any of the normal ways that other stuff does. That means that since baryons are made up of quarks, and they interact via the electromagnetic force, we’re looking for something that isn’t a baryon. And so we’re looking for something outside of all of these awesomely-named quark-made particles.
Fraser: So, right now then, where do particle physicists think that dark matter exists compared to the standard model? Which group would it probably be in?
Pamela: It’s not. That’s the thing.
Fraser: No group. It’s not a lepton.
Pamela: It’s outside.
Fraser: It’s not a hadron.
Pamela: It’s outside.
Fraser: It’s outside.
Pamela: Yeah. It’s a non-baryonic particle.
Fraser: But is the thought, okay, so then what, I mean I understand that it’s outside, but particle physicists have done plenty of math and done plenty of predictions about what dark matter particles can be, and they’re going to try and create them with things like the LHC. So what kind of characteristics are they looking for? I mean, give us something.
Pamela: So we’re currently at a stage where it’s hard to say that folks are doing much more than looking at the universe, putting constraints based on what we’re observing on what we expect to find, and kinda hoping that things that fall out of theories like super symmetry, which we’ll get to, explain what we’re observing. So we know that there is a very small cross-sectional radius for collisions between dark matter particles. We see this from looking at how clouds of dark matter pass through one another when super clusters interact. We know that it doesn’t interact with the electromagnetic force. We know it does interact with gravity.
That’s kind of where we’re at, and different people are going down different rabbit holes trying to find a rabbit to pull out and call dark matter. So we have some folks that are going down loopy gravity. Some people that are going down string theory. Some people that are going down super symmetry, and string theory, actually, can go down wherever it wants and call itself super symmetry, or call itself string theory, rather.
Pamela: So we don’t know. We have a word, non-baryonic dark matter, and that’s a good word. And that’s as firm as our observational understanding really lets us stand right now.
Fraser: So if you were able to generate it in the large hadron collider, how would you know that you found it? How would you know that those are the characteristics of dark matter?
Pamela: Well, what we’re looking for as we crank up the large hadron collider, and prepare to crank up the cyclotron at fermi lab, what we’re looking for are significantly more massive particles than we’ve found so far. And different kinds of things in super symmetry are starting to get predicted that have these higher masses and don’t interact. May be? But honestly, we’re sort of grasping at straws saying we need a weakly interacting mass of particle. Let’s see if we can get a massive particle that weakly interacts to fall out of the collisions.
Fraser: Okay. So that’s one, which is dark matter. And now dark energy, I guess, is another one. Maybe particles appearing, but maybe it’s just space itself, right?
Pamela: Right. So dark energy has no explanation. We don’t know where the explanation’s gonna fall out of. What we do know is attempts to explain dark energy using vacuum energy, which is that flux of particles coming in and out of reality because there’s just this background energy that they can use to temporarily exist. When we start looking for leftover energy from things popping in and out of existence, all attempts to use that leftover to explain dark energy is off by like 120 orders of magnitude. So that’s just wrong.
Pamela: And then there’s other things that leave us perplexed, like standard model physics doesn’t require the neutrino to have mass. And the problem is we know from a variety of different experiments that neutrinos that are formed with one identity, formed as an electron neutrino, for instance, can transmute into a different type of neutrino, like a tau neutrino, or a muon neutrino. And this ability to flip identities requires the neutrino to have mass that can go into that identity flip. So that’s not explained. And then there’s just the whole fact that why do we have so much matter and so little anti-matter in the universe? That doesn’t fall naturally out of anything.
Fraser: Okay. And you’ve made a couple of references now to super symmetry. So we actually did an episode on super symmetry, quite a few hundred episodes ago. But can you give, sort of, us the short version of what super symmetry is?
Pamela: So the short version is that there is a entire mirror set of generations of particles. So when we talk about the particles that we normally deal with falling into three generations – the generation that corresponds to the electron proton up and down, and so on through, going from the lightest and most stable to the heaviest.
Fraser: Right. I remember that. That was a couple of episodes ago. We talked about those three generations.
Pamela: So take those exact same three generations, flip them over, and call them the super symmetrical particles. And so these are essentially super partner particles. So we have – for the photon, we have the photino. For the gluon, the gluino. For the leptons, we have the sleptons. For the neutrinos, we have the S-neutrinos. I don’t know how you say S and N.
Fraser: Right. You put an S in front of everything.
Fraser: Right. I know. So instead of the top quark, you get the stop quark.
Pamela: And so the idea is that all of these partner protons and neutrons or I guess photons and bosons, all of these partner things, they’re there to basically balance everything out, and they have different masses. And these different masses start to crop up as explaining what dark matter is, and helping to allow to fix the problems with vacuum energy. Maybe. We don’t know. And that’s where it all gets highly, highly confusing.
Fraser: And so if true, these particles are gonna have incredibly high masses compared to the regular masses of the partner particles, right? So the –
Fraser: And so there’s just, there are limits to what even the most powerful collider possible, the large hadron collider so far, on what kinds of particles it can actually generate. And so these particles might just be beyond what it can create.
Pamela: Exactly. And it’s hoped that the top quarks’ sister particle –
Fraser: The stop quarks, yeah.
Pamela: is actually the lowest mass of these, and is something that’s potentially within reach. So we’re trying to find that. So, we’re not sure –
Fraser: And if I understand, the recently upgraded large hadron collider can just reach the stop quark.
Pamela: And that’s the hope.
Pamela: So we’re looking. We’re looking. And it’s tricky. So they do have differences. So for instance, the S-fermion, there’s been zero particles, so they’re gonna behave without that Pauli exclusion principle. There just gonna act differently, and we’re hoping that the way that they act differently allows them to explain dark matter, and we’re hoping that the predicted energies, at least for the stop quark, is in reality as low as its theorized to be, and is thus observable. Now, if it’s not, it doesn’t mean that anything’s wrong with the universe. It just means we get to keep looking, and particle physicists still have a job, and there’s no Nobel Prize on the imminent future, but hey, maybe there’s funding for bigger and bigger accelerators.
Fraser: Does super symmetry, I mean, one of its things is it will help unify gravity into the standard model, right?
Pamela: It tries to. I wouldn’t say it succeeds, but it works towards that. We actually don’t have any theories that are fully successful at unifying quantum mechanics and relativity.
Fraser: Wow. It’s kind of amazing. I mean it’s pretty – you think about the future people, and they’ll be able to look back – hopefully, if this ever gets figured out, right? – and they’ll be like, look at those people. Back then, they didn’t know that here’s how you unify gravity and the standard model.
Pamela: But that’s really an internal prejudice. It’s the human desire for there to be one underlying cause that explains everything. One of the things that people find extremely dissatisfying about the standard model is I think it has 19 finely-tuned constants in order to work. And people don’t want a universe that’s filled with constants. They don’t want a fine-tuned universe. They want a plop down these laws of physics, and it just goes.
Pamela: And it could be gravity doesn’t unify.
Fraser: There’s a great game coming out that I wanna get called “No Man’s Sky.” I don’t know if you’ve heard about this?
Fraser: But in the game, the whole universe is procedurally generated. And so no matter where you go, every part of the universe has been figured out, sort of, just in advance, by the math. And it’s the same thing. They picked these very specific – they picked a seed that then produced everything. And if they pick a different seed in their math, then it creates something different. And this is sort of this idea that our universe exists the way that it does just because all of these variables ended up the way they are. All these constants, right? And then if they’d ended up a different way, then gravity wouldn’t work properly, and so you can have all kinds of malformed universes from our perspective, but this is the one that was just barely suitable for life.
Pamela: And that gets back to the ideas of multiverses, which I think we did an episode on.
Pamela: And so there’s really nothing wrong with gravity not unifying. It’s just uncomfortable. And sometimes we just need to be a little bit uncomfortable.
Fraser: I need my whole universe to fit on a t-shirt. It’s gotta happen. Very small font is fine, but I still need to have the theory of everything on my t-shirt. Okay, cool. Well thanks a lot Pamela.
Pamela: Oh, for sure.
Fraser: We’re gonna go in a different direction next week.
Pamela: We really are. Next week we’re gonna talk about happy books and all of the nerd stuff that you can get in this season of giving.
Fraser: We will recommend our favorite things to get people for the holidays.
Pamela: Whatever holiday you might be celebrating.
Fraser: Yeah, whatever holiday you celebrate. Yeah, exactly. Festivus.
Pamela: Birthdays! My birthday is in December.
Fraser: That’s right. Well thanks again Pamela.
Pamela: Sounds great.
Fraser: Thanks for listening to Astronomy Cast, a non-profit resource provided by Astro Sphere New Media Association, Fraser Cain, and Dr. Pamela Gay. You can find show notes and transcripts for every episode at Astonomycast.com. You can email us at firstname.lastname@example.org. Tweet us @astronomycast. Like us on Facebook, or circle us on Google Plus. We record our show live on Google Plus every Monday at 12:00 pm Pacific, 3:00 pm Eastern, or 2000 Greenwich Mean Time. If you miss the live event, you can always catch up over at cosmoquest.org.
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