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Physicists are getting a handle on the structure of the Universe, how everything is made of something else. Molecules are made of atoms, atoms are made of protons, neutrons and electrons, etc. Even smaller than that are the quarks and the leptons, which seem to be the basic building blocks of all matter.
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This episode is sponsored by: 8th Light
Show Notes
The Particle Adventure
Leptons and Quarks
What Are Leptons?
Transcript
Transcription services provided by: GMR Transcription
Pamela Gay: 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.
Frasier Cain: Astronomy Cast episode 393, the standard model, leptons and quarks. Welcome to Astronomy Cast, our weekly fact-based journey through the cosmos. We help you understand not only what we know, but how we know what we know. My name is Frasier Cain, I’m the publisher of Universe Today, and with me is Dr. Pamela Gay, professor at Southern Illinois University, Everettsville, and the director of Cosmo Quest. Hey Pamela, how are you doing?
Pamela Gay: I’m doing well. I’m coming to you from my office on campus this week, so if folks are noticing a slightly different audio quality, I’m so sorry. I’m using a professional mic, but this particular one is about a decade old. It was one of the first ones that we got. It actually is the first one that we got when we started recording.
Frasier Cain: Wow, vintage Astronomy Cast microphone. Well that’s special.
Pamela Gay: This episode of Astronomy Cast is brought to you by 8th Light Inc. 8th Light is an agile software development company. They craft beautiful applications that are durable and reliable. 8th Light provides disciplined software leadership on demand and shares its expertise to make your project better. For more information, visit them online at www.8thlight.com. Just remember that’s www.8thlight.com. Drop them a note. 8th Light, software is their craft. Astronomy Cast is proudly sponsored by cleancoders.com. Training videos with personality for software professionals.
Frasier Cain: So physicists are getting a handle on the structure of the universe, how everything is made of something else. Molecules are made of atoms, atoms are made of protons, neutrons, and electrons, etc. But even smaller than that are the quarks and the leptons, which seem to be the basic building blocks of all matter. So this is part two of our journey through the standard model. Last week we sort of talked about some of the larger scale structures and some of the big forces that communicate that, what is it, mediate the objects together, but we’re going to talk about sort of how small you can go. Turns out it’s not turtles all the way down.
Pamela Gay: I want it to be turtles all the way down. I like turtles.
Frasier Cain: Yep, but no, so let’s sort of dig down. So we take protons, tear them apart, what’s inside?
Pamela Gay: So, inside of a proton you will find up and down quarks. In fact, all normal matter that we deal with is basically made up of four different things. Up and down quarks combine in various ways to make protons and neutrons, and then also electrons and electron neutrinos. That’s all stable matter, all of it tied up with just those four particles.
Frasier Cain: Whoa, okay, so hold on. Up down, so we’ve got quarks, and neutrinos, and even more basic than that, right?
Pamela Gay: And only one flavor of neutrino, the electron neutrino is the stable one that is the one that we end up with when we have reactions involving electrons or positrons.
Frasier Cain: Okay, I think we’re going to be a little more general than this. So then what are quarks?
Pamela Gay: Quarks are itty bitty little tiny but not so tiny that they’re easy to make, particles that have an electric charge that is either positive or negative 2/3s or 1/3, and they bunch up in different ways to form different kinds of particles. So if you combine two quarks, you get something that’s called a mason. Masons don’t usually hang out. They like to decay into being other things. If you combine them in groups of three, you end up. If you have two downs and an up you end up with a neutron, if you have two ups and a down you end up with a proton, the neutrons are charged zero, protons are charged one, and so that’s our initial standard set of particles, and to get to things more complicated than that, you need to add some energy, and with that extra energy comes a little bit more instability.
Frasier Cain: Okay, so, now you say that it has a 1/3 charge, which is super weird, because as we understand, like when I think about some of my chemistry or physics and stuff, we would consider something to have a positive charge or a negative charge, or maybe you’ll have a positive two charge so that it can find a negative two charge and create some kind of molecule together. So what is a 1/3 charge mean?
Pamela Gay: So when our good old friend Benjamin Franklin was playing with electricity back in the day, he first of all didn’t understand the electrons and protons were, he didn’t know what they were, and so in defining what’s positive charge and what’s negative charge, he just kind of made stuff up. Eventually when J.J. Thompson was doing his work to discover the electron, when he hit that single charged something, he gave it a charge of one, and the electron with a charge of one electron charge sort of set the standard, and then of course protons conveniently have the exact same but opposite sense charge of the electrons, so if the electron is charge one, proton is also charge one, except you can break up a proton. And when you break up the proton you’re also breaking up its charge. So it turns out that that one that we thought was the minimum unit of charge is not the minimum unit of charge. It just comes from when we set up numbers before we have the fullest picture.
Frasier Cain: So if we had originally set, say, the charge of a proton to be three, that would have made a quark not sound so weird.
Pamela Gay: Exactly, but at the end of the day it just turns out that we’re dealing with increments of 1/3 instead of increments of one, and yeah, it sounds weird, but it’s not that bad, it’s just 1/3.
Frasier Cain: But it’s weird for something in the natural world to have that three, right? For it to be a multiple of three like that. But anyway, so in other words, if you take, so I can imagine you can have a plus 1/3 quark, a negative 1/3 quark, and so if I add two together, if I get two positives and a negative, what is it two ups and a down is what you said?
Pamela Gay: So a neutron is two downs and an up and a proton is two ups and a down, so you have 2/3 plus 2/3 minus 1/3 for the proton, and then you have minus 1/3 minus 1/3 plus 2/3 for the neutron.
Frasier Cain: Okay I understand, I understand. But those aren’t the only kinds of quarks that you can get, right?
Pamela Gay: So it turns out that all of particle physics that we know of so far falls into three different, we call them generations. The generation, the first generation, which is what we’ve talked about so far is the electron electron neutrino and that up and down quark, but if you start looking at things that are a little bit more energetic, you start ending up with this particle called a muon that is intrinsically unstable, it wants to be something else. They are formed in cosmic rays in the top of our atmosphere. There’s a muon neutrino associated with the muon particle, so along with that muon we also have two associated quarks, the charm and the strange quark, and these crop up in various unstable particles, masons for instance, that are made up of charm and strange are something that exist, although it’s usually a charm and an anti-charm and a strange and an anti-strange that get added together to make these particles.
Frasier Cain: Now when you say an anti-charm and anti-strange, is that like an anti-matter or is it just a different kind of quark?
Pamela Gay: It’s anti-matter. So the thing that is hard for most people to follow is really the only difference between a particle and an anti-particle is the anti-particle has a negative charge, so in an electron and a positron are absolutely identical except in what charge they carry.
Frasier Cain: Well it also has a tiny goatee.
Pamela Gay: Well yes, yes.
Frasier Cain: So okay; so we talked about you’ve got the up and you’ve got the down, we’ve got the charm and the strange, and if I was to sort of look at let’s say the charge of a charm, comparing that to the up and the down, what differentiates them from the ones that make up the protons and the neutrons?
Pamela Gay: It’s literally a difference in what their mass is, what the intrinsic total amount of energy that E=MC squared that goes into making these. So the up, and the reason I tied it back to that E=MC squared is when we start talking about this kind of subatomic particle, we stop using normal mass terms and we start talking about the giga electron volts divided by the speed of light squared that the particles have. It’s just a more realistic unit, and when we’re creating most of these things in high energy collisions, it just makes more sense to talk in terms of their energy.
Frasier Cain: Okay, so we’ve talked about four, but the point is that I couldn’t just take a bunch of strange and charm quarks, mash them together, and make a proton out of it. Like the math just isn’t going to hold.
Pamela Gay: You would have to wait for them to decay into something else that hopefully you could combine to create that proton. A charm and a strange, as they are, they weigh too much. The up and down are 0.002 and 0.005, in other words little tiny amounts of giga electron volts. The charm and the strange are 10,000 times more massive and 1,000 times more massive at 1.3 giga electron volts and 0.1 giga electron volts.
Frasier Cain: Right, okay, wow. So that’s, we’ve got four. Are there any more?
Pamela Gay: There are at least two more that we know of. There’s a third generation of particles that we’ve been able to measure to experimentally find using a variety of different reactors. So first of all, there’s that tau particle that is that symmetry with the electron, the muon, so now we have the tau particle which of course doesn’t have a name that fits anything and it’s associated with the tau neutrino. Then in partnership with that is the top and the bottom quark, and it was that top quark that was the last quark to be discovered. It was discovered back in 1995 outside of Chicago at the Tevatron Accelerator at Fermi Lab, and that kind of brought to a close our quest so far for quarks. Now there are people out there saying maybe there’s a fourth generation, but so far we have absolutely no experimental evidence that there’s a fourth generation of particles.
Frasier Cain: But the thought process would be the up and down are just the lowest mass, the place that the more massive quarks want to decay down into as well as spewing a whole bunch of articles that we’re about to talk about, and so theoretically, say someone was to dig up a big chunk of Europe and stick a big ring in there and start accelerating particles around, you could theoretically produce enough energy that maybe you could find a fourth generation, but so far they’ve failed, right?
Pamela Gay: Well, we’re just starting to reach high enough energies, and there isn’t a theoretical, “Thou shalt have a fourth generation,” and there have so far been no hints in the experiments that have been thus far that there’s a fourth generation, but we didn’t know about that second generation of particles until a muon decided to surprise people that were studying cosmic ways, and the tau particle was something that got discovered during accelerator collisions. It’s possible, as we build to higher and higher energies, the top quark is at 173 giga electron volts, and then the Higgs boson is at 126 giga electron volts. These are high-energy particles. It could be that as we start to turn up the accelerators to higher and higher energies, we could find a fourth generation, but it’s not required.
Frasier Cain: And, I mean, does the math predict it?
Pamela Gay: No.
Frasier Cain: Okay, so it’s not like somebody is going, “I can merge gravity in with quantum mechanics if I could just find a fourth class of quarks in the tera electron volt range,” or whatever.
Pamela Gay: No. And the thing is, with the standard model, and this is something that we hit on some in our last episode, the standard model isn’t a predictive theory as much as it is a, “Well, we’ve seen this, this, and this, it seems like there’s a hole here,” kind of like when the periodic table was created. So its predictive factor is more into the, “In order to explain these things we’ve already seen, we need this thing,” not a, “Let’s start at ground zero and build all the way up through the theory.”
Frasier Cain: Right. Okay, so we’ve got the quarks and these are literally the building blocks of all the matter that we know, the up and the down, the ones which are the stable ones that are the building blocks of neutrons and protons, whatever, but there is a whole other class of item which are the electrons, and some of the, and there’s a whole set of leptons that come along with that as well. So we’ve talked about quarks. Let’s talk about leptons.
Pamela Gay: So the leptons are those electrons, those muons, and those tau particles that I brought up. So what we’re looking at is together, all of these things are what are called fermions. These are particles that have spin that is a multiple of a half and we don’t need to go into spin today, it’s just one of those words that people gave for things that particles have. It’s not like an electron is actually spinning, we just say it has something called a spin. So particles that have a spin that is a multiple of ½, so ½, 3/2, 5/2, these are called fermions, and they follow the poly-exclusion principle. So for instance, if you have two electrons in an energy shell in an atom, one will be spin up and one will be spin down, and this is what leads to things like degenerate matter, that we’ve done shows on before, in white dwarfs, where all of the electrons pretty much have to lock into specific energy levels to allow everything to pack in close enough. So fermions are things with spin half. They include leptons, which are neutrinos, and our electron, muon, and tau particles, and they are quarks which are up and down, are charm and strange, and are top and bottom. Now those quarks, they combine – lots of vocabulary today.
The quarks combine to form masons which are two quark combos to form baryons which are three quark combos, so our proton and our neutron, for instance, and it’s those masons and baryons that are all of the heavier things that get built out of these basic particles.
Frasier Cain: So what are some examples of leptons that we’ll be familiar with? Like the electron is a lepton, right?
Pamela Gay: Yes.
Frasier Cain: So really, so the point is that you can’t build, there’s nothing more fundamental than the electron. It’s not like there’s some particles that make up an electron, right?
Pamela Gay: Exactly. So an electron is a fundamental particle that can’t be broken apart. You can turn it into pure energy, but you can’t break it into smaller bits of matter. It is something with a diminutively small mass, and an electric charge of minus one. So the leptons, they all have charge, they actually all have charge of minus one or zero, the neutrinos are charge zero, and spin one half, and they don’t weigh a lot.
Frasier Cain: Right, and so if there was a periodic table, we’d go up the quark periodic table and they decay into – I mean, is that sort of a useful way to kind of think about it?
Pamela Gay: The way that we tend to diagram this out is you have a row that is your first generation where you do all of your leptons down the left. I don’t know why we do this, this is just how we tend to do it. So in that top left hand corner that is your first generation of particles and it’s your leptons, you throw the electron and the electron neutrino in that box, and then next to that you have your column of quarks, and the quarks in that first generation are your up and your down quarks. Then we do the same thing on the next row down for our second generation of particles, so the muon neutrino and the muon particle go into that second row, first column, and the charm and the strange go into that second column, second row. So this is how we fill it out, is it’s like each generation has two quarks, each generation has a neutrino and a lepton that isn’t a neutrino, so that electron, that muon, that tau particle, and so this is just how we piece our way of looking at fundamental physics together.
Frasier Cain: And that first row is the stuff that we’re most familiar with, the, in theory, the up, the down, the electron, obviously the electron we’ve known about for more than 100 years, and then as we kind of move up that periodic table of particle physics, they’re much shorter lived and they only appear in various times. Like you’re not going to get, I mean, are you going to get anything made of muons? Will a muon decay into electrons in the same way that the quarks work?
Pamela Gay: So a muon is generally formed in the upper atmosphere, the ones that we readily detect in undergraduate physics labs, a lot of museums have setups that allow visitors to see muons getting detected. Muons get formed in the upper atmosphere when high energy cosmic ray particles interact with other particles in the upper atmosphere. Muon comes down through the atmosphere, down through the atmosphere, it’s moving at relativistic speeds so even though it’s a very short-lived particle, it makes it to the surface of the planet before it opts to decay, and when it decays, it will decay into an electron, an anti-electron neutrino, and a bit of light. And there’s also a muon neutrino involved as well, so you have to have conservation of all the things.
Frasier Cain: Right, but you just, I guess we can come back around to it, but you just sort of quickly glossed over the really cool idea, right, that as this particle makes it through, thanks to relativity, it survives longer from our perspective than it thinks it did, which is mind bending.
Pamela Gay: Exactly. This is one of my favorite things about muons, and I have to admit that the awesomeness of the fact that they shouldn’t live long enough to make it to the surface of the earth except by the grace of relativity causes me to usually forget pretty much every other fact about muons, because that one fact is so awesome that my brain doesn’t want to retain any more information.
Frasier Cain: But you pretty much are having these high energy particles formed in, what, super novae, in really high energy collisions, and then they’re making their way across the universe, and when they hit the atmosphere, it’s like a natural particle accelerator, right? And that’s what’s getting these high energy collisions, you’re getting these high energy particles to appear when normally you’re not going to bump into muons in your day to day life.
Pamela Gay: No, but it actually doesn’t require things as exotic as supernova explosions to get cosmic rays, and this is both a good thing, if you’re trying to detect muons, and a horrible thing if you’re trying to get to Mars. We actually get cosmic rays from different solar events. We get cosmic rays from occasional things like masers in, well, they can appear in planetary atmospheres, jets, all sorts of different things can create high-energy particles. Now the different kinds of events create different kinds of high-energy particles, but cosmic rays are kind of a way of life, and this is why we’re safe down here at the surface of the planet, those particles hit our upper atmosphere, they hit the Earth’s magnetic field, we’re safe. But if you’re out there trying to get yourself to Mars, you’re not so safe.
Frasier Cain: But I guess I just feel like it’s really kind of fascinating that nature has created this particle accelerator.
Pamela Gay: It’s true.
Frasier Cain: And that the only place that we see these particles further up the periodic table of elementary particles is when they’re generated either through these catastrophic events or in our human created particle accelerators. Like you’re just not going to run into them in any other situation.
Pamela Gay: Yes, and I’d call those human created catastrophic for at least the protons involved events. So it’s kind of cool.
Frasier Cain: Yeah, and that there are events way beyond the energies that we can create in the lab. When you think about what happens, the kinds of accelerations that happen in a supernova, you know, we’re a long way away from being able to crash the energies together like a supernova. So there could very well be a whole pile of particles being formed in these moments that then decay or whatever that we just don’t have access to, which I find very interesting.
Pamela Gay: And we’re slowly starting to find out that there’s entire combinations that we had previously thought maybe kind of could exist, but weren’t sure, so we know that quarks can group up in groups of three. We know they can group up in groups of two, and there’s brand new evidence, fresh out here in 2015, that there are at least two ways that groups of five quarks can come together and form new particles that we’re uncreatively calling penta-quarks. So there’s apparently an entire new class that has what is one of the worst names in particle physics, so now it goes masons, hadrons, and penta-quarks.
Frasier Cain: You don’t like penta-quark?
Pamela Gay: Not when the sibling particles are coolly named masons and hadrons.
Frasier Cain: So I’m just kind of imagining, I guess the point is that you could get matter that is like a proton is made of up and down quarks, you could get some kind of exotic matter that’s made up of a larger combination of ups and downs and charms and stranges and tops and bottoms. Is that the gist of this?
Pamela Gay: Yeah, so far they seem to combine by generation, but yeah.
Frasier Cain: So you could have, like, three ups and two downs and that would create some kind of, now when you say a hadron, you’re talking about matter, right? Like atoms are hadrons, right?
Pamela Gay: Atoms are a combination of hadrons and leptons. Protons and neutrons are hadrons.
Frasier Cain: Okay, so the point being you could end up, but that’s super weird, right? Because it’s not an atom of hydrogen, which is made up of the three quarks like they all are, it’s made up of five quarks. Super weird.
Pamela Gay: But why do you say it’s super weird? We also have masons that are combinations of two quarks and in general we kind of ignore their existence unless we’re in physics.
Frasier Cain: That’s also super weird. They’re all super weird.
Pamela Gay: I love that things that are new ideas to you are super weird.
Frasier Cain: Super weird. Well okay, so we’re kind of reaching the end of the time, so why don’t we just do a quick recap just to make sure that I understand it and everybody else understands it, because we see all these words like quark this and quark that and lepton this and hadron that, right?
Pamela Gay: Yes, so okay.
Frasier Cain: So I’m going to give people the recap and you tell me if I go off the rails right?
Pamela Gay: Okay.
Frasier Cain: All you need to know really for the vast majority of everything out there in the universe, it’s matter, it’s atoms, it’s protons and it’s neutrons and electrons, so all of those protons and electrons, they’re made up of quarks and the two parts you need to care about are the ups and the downs.
Pamela Gay: No, you goofed it. It’s the protons and the neutrons.
Frasier Cain: Sorry, the protons and the neutrons are made of the quarks, and then combined together with them are the electrons and those are leptons.
Pamela Gay: And the electron neutrino. You can’t forget the electron neutrino.
Frasier Cain: And so everything it’s either made of quarks or leptons.
Pamela Gay: Yes.
Frasier Cain: Okay, and you don’t have to worry about all the other ones, the charm, the strange, the muon, the tau, the top, the bottom, the muon neutrino, the tau neutrino, unless you are a particle physicist and you’re actually crashing these things together or studying muon cascades through the atmosphere. Apart from that, don’t worry about those.
Pamela Gay: I think I’m okay with that.
Frasier Cain: You’re all right with that?
Pamela Gay: I’m all right with that.
Frasier Cain: Now this is part two of our journey through the standard model, so what are we going to talk about next week?
Pamela Gay: So next week we’re going to go into more detail on bosons and talk about hadrons and how the forces interact with all of these things, because it’s actually kind of cool because not all the forces choose to interact with all of the particles.
Frasier Cain: Not super strange, just weird.
Pamela Gay: You already knew this fact.
Frasier Cain: No. My understanding of particle physics sort of, I literally have to re-learn it every time when I write some article about particle physics, almost like re-remind myself. I guess because we never spent enough time getting into the fundamental levels to do the math in the level of physics I did at university, so you don’t have to do that much in computer science, so I always sort of run across these and it always sort of pops out of my head again.
Pamela Gay: So fun random fact is I had a professor hand me a glass of champagne the day they announced the top quark, and as she’s shoving it into my face, I’m like, “I’m under 21,” because I didn’t want to get yelled at, and she’s like, “I don’t care, drink it.” So I was forced to drink underage when they found the top quark.
Frasier Cain: It’s totally worth it, and it would have been fine in Canada.
Pamela Gay: That’s true. That’s true.
Frasier Cain: All right, well thanks Pamela.
Pamela Gay: Thank you.
Frasier Cain: Thanks for listening to Astronomy Cast, a non-profit resource provided by Astrosphere New Media Association, Frasier Cain, and Dr. Pamela Gay. You can find show notes and transcripts for every episode at astronomycast.com. You can email us at info@astronomycast.com, tweet us at astronomycast, like us on Facebook or circle us on Google+. We record our show live on Google+ every Monday at 12:00 p.m. Pacific, 3:00 p.m. Eastern, or 2000 Greenwich Mean Time. If you miss the live event, you can always catch up over at cosmoquest.org. If you enjoy Astronomy Cast, why not give us a donation? It helps us pay for bandwidth, transcripts, and show notes. Just click the donate link on the website. All donations are tax deductible for US residents. You can support the show for free too. Write a review, or recommend us to your friends. Every little bit helps. Click “Support the show” on our website to see some suggestions. To subscribe to this show, point your podcatching software at astronomycast.com/podcast.xml or subscribe directly from iTunes. Our music is provided by Travis Serle, and the show is edited by Preston Gibson.
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Duration: 31 minutes
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