The Field Guide to Particle Physics

The Field Guide to Particle Physics

By Sean Downes

This is your informal guide to the subatomic ecosystem we’re all immersed in. In this series, we explore the taxa of particle species and how they interact with one another. Our aim is give us all a b... more
  • 5
  • 5
  • 5
  • 5
  • 5

5

16 ratings

Download on the App Store

Best of The Field Guide to Particle Physics

The most played episodes among Podcast App listeners.

  1. Number 1: The Delta Baryons

    The Field Guide to Particle Physics https://pasayten.org/the-field-guide-to-particle-physics ©2021 The Pasayten Institute cc by-sa-4.0 The definitive resource for all data in particle physics is the Particle Data Group: https://pdg.lbl.gov. The Pasayten Institute is on a mission to build and share physics knowledge, without barriers! Get in touch. The Delta Baryons The Delta baryons are combinations of up and down quarks. They’re like the proton and the neutron, only much more heavy and much less stable. They all seem to have about the same mass - around 1232 MeV, but we don’t really have a good handle on it. That is mostly because the Delta baryons lives are so short. They decay extremely quickly, in 5.6×10^−24 seconds. There are four delta baryons, corresponding to all triplet combinations of up and down quarks:Δ++ = uuu, made from three up quarks, which has an electric charge of +2eΔ+ = uud, made from two up quarks and one down, which has an electric charge of +eΔ0 = ddu, made from two up quarks and one down, which has an no electric chargeΔ- = ddd, made from three down quarks, which has an electric charge of -eWhat makes these combinations different from protons and neutrons is their spin. The uud of the Δ+ might look like a proton, but it has a higher intrinsic angular momentum. The quarks are zooming around each other - and all that subnuclear goo - with more energy than they do in the proton, which explains the higher mass. In other words, Δ-baryons are excited states of the proton or neutron. And excited states decay. They decay so quickly because the strong nuclear force - the same force that holds them together - is so strong. The electron cloud of an atom can be similarly excited: their electrons can bounce up to a higher energy orbital state, often with more angular momentum. When they decay, they tend to emit photons. If you’ve seen glow in the dark toys - or pictures that glow under UV light - you’ve seen electron cloud decays with your own eyes. The Δ-baryons decay similarly, although being bags of quarks they emit pions. Δ baryons play a crucial role in limiting the speed of cosmic rays. Cosmic rays are mostly comprised of protons flying through deep space - some at outrageously high velocities. Once those protons are accelerated up to enough energy - 5×10^19 eV - which is about the kinetic energy of a child walking (contained in SINGLE PARTICLE!) - they tend to collide with the photons left around from the Big Bang. Those collisions, from the perspective of those ridiculously fast protons, are extremely energetic, and Δ-baryons are produced. These of course decay almost immediately into proton, neutrons and pions, effectively converting that stupidly high kinetic energy into pion radiation. In other words, intergalactic friction in the universe is provided by Δ-baryon production and decay. Of course, we still see a few cosmic rays with energies far beyond that limit, so its not a perfect speed limit, but a pretty effective one.

    5min
    Listen Later
  2. Number 2: The Neutral Pion

    The Field Guide to Particle Physics https://pasayten.org/the-field-guide-to-particle-physics ©2021 The Pasayten Institute cc by-sa-4.0 The definitive resource for all data in particle physics is the Particle Data Group: https://pdg.lbl.gov. The Pasayten Institute is on a mission to build and share physics knowledge, without barriers! Get in touch. The Neutral Pion In many ways, the neutral pion - sometimes call the π0 meson - is closely related to the charged pions. In others, the neutral pion is a truly bizarre little beast. There are so many kinds of pions because there are so many ways to combine up and down quarks into particle/antiparticle pairs. The neutral pions are kind of a mixed bag of up quarks paired with antiup quarks, and down quarks paired with antidown quarks. The electric charges are always canceled between these antiparticle partners. The neutral pion is a great example of how antimatter isn’t always a destructive thing. Owing to this special structure, the neutral pion is its own antiparticle. π0 mesons decay much, much faster than their charged siblings, a literal billion times faster. They only stick around for about 8.5×10^−17 =0.000000000000000085 seconds. Yikes! That’s so small it probably doesn’t mean anything to most of us! By particle standards, it’s a properly fast decay. The charged pions decay via the weak nuclear force. The neutral pions decay via the electromagnetic force, which is much, much stronger. Which is why they decay much faster. However uninspiring their short lifetimes might be, how these particle decay is truly fascinating. Neutral pions decay into a pair of photons almost all the time. When they don’t, its because one or more of those photons converted implicitly to an electron/positron pair. This decay happens because of the so-called chiral anomaly, which is an extremely subtle feature of the laws of our universe. The chiral anomaly emerges as an event - a blip or moment in time - where the quarks that make up the neutral pion quantum mechanically convert into all kinds of quarks - up down strange top bottom whatever - all at once. It’s kind of reminiscent of the nuclear goo that’s inside a pion or a proton. That goo collectively and cleanly self annihilates into a pair of photons - and it does so with remarkably efficiency. This is a purely quantum phenomenon - microscopic correction at that - which has a lot of real world consequences. Tthe chiral anomaly depends on all the quarks all at once, even ones that might be so heavy we haven’t seen yet. As it turns out, to correctly compute the measured pion lifetime, the total number of quark species must be six. Precisely six. As it also turns out, we've already observed six different kinds of quarks which, on the one hand, is a bummer. On the other, in an amazing consistency check on the known laws of physics. It's how we know our understanding of nature is on the right track. Because these neutral pions decay to photons of a very specific energy, they are very easy to look for. Astronomers will routinely see them while looking into space! π0 decays are often associated with supernovae - that is, exploding stars - which suggests two things. First, exploding stars explode with such power that they act as gargantuan particle accelerators. Second, having giant, cosmic particle accelerators explains the large number of ultra high energy cosmic rays that are responsible for the cosmogenic muons raining down upon all the time.

    4min
    Listen Later
  3. Number 3: The Charged Pion

    The Field Guide to Particle Physics https://pasayten.org/the-field-guide-to-particle-physics ©2021 The Pasayten Institute cc by-sa-4.0 The definitive resource for all data in particle physics is the Particle Data Group: https://pdg.lbl.gov. The Pasayten Institute is on a mission to build and share physics knowledge, without barriers! Get in touch. The Charged Pions Quarks make up more than big, triplet particles like protons and neutrons. Sometimes they come in pairs. A pair of up and down down quarks is called a pion. There are three kinds of pions: pi plus, pi minus and pi zero. Compared to the proton, they’re quite small and a little quirky. They’re certainly unstable. Today we’ll talk about those charged pions, π±. Like the proton, pions are mostly comprised of nuclear goo. Unlike the proton, that goo surrounds only two quarks. Or, really, a quark/antiquark pair. A π+ has an up quark together with an antidown quark. That gives is an electric charge of 2/3 + 1/3 = 1, That is to say, π+ has exactly the same charge as the proton. Being the antiparticle, π− is made up of a down quark, with an anti up quark. And of course all that nuclear goo. Its electric charge is precisely the opposite. The charged pions have a mass of 139.6 MeV, making them just a tiny bit heavier than the muon. They are unstable particles, and given their mass typically decay into such muon, emitting a neutrino in the process. The muon, of course, is also unstable. It too decays to an electron and a pair of neutrinos. That’s pretty typical. Pion to muon to electron. A π+ will decay to an antimuon, who has a positive charge, and an antimuon decays to a positron. When cosmic rays smash into the molecules of the upper atmosphere, creating the showers of muons we deal with on a day to day basis, pions are the first on the scene. They're some of the first particles produced, and they don't have around for too long. Charged pions decay within about 30 nanoseconds. Pions belong to a class of nuclear particles called mesons, which were originally thought up by Hideki Yukawa way back in 1934. To some extent, the play the role of communicating between the protons and neutrons. In that way, they are partially responsible for the residual nuclear force that binds atomic nuclei together. You can kind of think of them like photons, only for the nuclear force. Occasionally a bunch of pions are created together, and you can see little pion atoms - pionium - where pi plus and pi minus orbit each other for a short time. Curiously, the pionium atom typically decays not to the pair π+ π-, but to a pair of π0’s. This is a kind of of matter/antimatter annihilation. Less frequently, pionium annihilates into a pair of photons.

    5min
    Listen Later
  4. Number 4: The Neutron

    The Field Guide to Particle Physics https://pasayten.org/the-field-guide-to-particle-physics ©2021 The Pasayten Institute cc by-sa-4.0 The definitive resource for all data in particle physics is the Particle Data Group: https://pdg.lbl.gov. The Pasayten Institute is on a mission to build and share physics knowledge, without barriers! Get in touch. The Neutron With a mass of 939.565 MeV, the neutron is the second lightest baryon - that is, a particle made up of three quarks. The neutron's quarks include one up and two downs, so it is total electric charge is zero: 2/3e − 2×1/3e = 0 The neutron may be electrically neutral, but don’t let that fool you, it still interacts with the electromagnetic field. Like the earth - and the proton - the neutron has a tiny magnetic field. It's a dipole field: with a north and south pole. Because moving electric charges generate magnetic fields, you might expect a spinning electric charge, like the proton, to have a magnetic field, but not the neutron. And yet it does. What’s curious about the neutron’s magnetic field is that its so strong. It’s about 60% that of proton, although oppositely orientated. This fact was probably the first clue that the neutron wasn’t a fundamental particle, but was made up of some other junk. The neutron is just a tiny bit heavier than the proton, which is fitting given that the down quark is just a tiny bit heavier than the up quark. Because the up and down quarks are related by the weak nuclear force, the neutron is unstable! Left to its own devices, one down quark will eventually decay to an up, emitting an electron and a neutrino in the process. This is all very hard to see, given that the neutron’s quarks swim in a bag of nuclear goo, but the net effect is that an electron sails away with a negative charge, leaving behind a positively charged proton. Alone, the average life expectancy of a neutron is something like 15 minutes - which is an eternity by particle physics standards. Given that a neutron decays in the time it takes to drink a cup of coffee, you might wonder why we have any neutrons left in the universe. Suffice it to say, neutrons are like cattle. When they’re herded together in a nucleus, they’re far more stable. In fact, the number of neutrons goes up as the size of the nucleus does too. Helium-4 has two protons and two neutrons, and it’s pretty much stable. Iron-60 has 26 protons and 34 neutrons. It lives for millions of years. Uranium-238 has 92 protons and 146 neutrons. It lives for billions of years. Unstable alone, yet stable in packs, the humble neutron is all around us, and makes up a good chunk of our mass, too.

    4min
    Listen Later
  5. Number 5: The Proton

    The Field Guide to Particle Physics https://pasayten.org/the-field-guide-to-particle-physics ©2021 The Pasayten Institute cc by-sa-4.0 The definitive resource for all data in particle physics is the Particle Data Group: https://pdg.lbl.gov. The Pasayten Institute is on a mission to build and share physics knowledge, without barriers! Get in touch. The Proton With a mass of 938.27 MeV, the proton is the lowest energy configuration of a trio of quarks. It has two up quarks and a down, but remember most of that mass is made up of subnuclear goo. You can find protons literally everywhere: from the nuclei of atoms on Earth to collisions at ultra high speeds from deep in outer space. From our accounting of electric charge, the proton has two copies of the up charge, and one of the down, which means that electric the charge of the proton is 2/3e + 2/3e − 1/3e = e. So far as we can tell, the proton is stable. It’s the only stable quark triplet or baryon because it has the lowest mass. Anything heavier would decay… to it. There are a host of experiments looking for evidence that protons decay, but so far all we can say is that it’s average lifetime longer than 10^34 =10000000000000000000000000000000000 years. That’s like expecting to find only a single proton decay out of all the atoms in one million people per year. There are a lot of reasons to expect the proton to decay - at lot of really compelling, interesting ones. But they’re all hypothetical extensions to the standard model of particle physics. So again, as far as we know, the proton is stable. And all quark triplets - the baryons - eventually decay to the proton. Given that the down quark mass is slightly heavier than the up quark mass, one might think that the lightest baryon would be a triplet of up quarks: uuu. If it were, the electric charge would be 3×2/3e = 2e. But it is not. Again, it is important to remember that a baryon's mass is made up almost entirely by its internal energy. The uuu-baryon does exist - its called a Δ++ , but its internal energy - and therefore its mass - is quite a bit larger than that of the proton.

    4min
    Listen Later

The Field Guide to Particle Physics episodes:

FAQs about The Field Guide to Particle Physics:

How many episodes does The Field Guide to Particle Physics have?

The podcast currently has 49 episodes available.

More shows like The Field Guide to Particle Physics

Sean Carroll's Mindscape: Science, Society, Philosophy, Culture, Arts, and Ideas by Sean Carroll

Sean Carroll's Mindscape: Science, Society, Philosophy, Culture, Arts, and Ideas

4,168 Listeners