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The Reason for Antiparticles.
The Field Guide to Particle Physics : Season 3. Episode 8.
https://pasayten.org/the-field-guide-to-particle-physics
©2022 The Pasayten Institute cc by-sa-4.0The eBook
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References
The definitive resource for all data in particle physics is the Particle Data Group: https://pdg.lbl.gov. This episode also pays tribute to Richard Feynman’s 1986 Memorial Dirac Lecture.
Terrell-Penrose rotation can be viewed from a human perspective in at "A Slower Speed of Light" by MIT's GameLab. That demo also includes the relativistic doppler effect. Some other great videos by Ute Kraus and Corvin Zahn at spacetimetravel.org. See in particular their dice demo.
The Reason for Antiparticles.
Antimatter is uncommon, but it’s not exactly rare. Antiparticles - especially those generated by cosmic radiation - are all around us, all the time. But just what is it doing here?
Antimatter is just like Matter
In a lot of ways, antimatter behaves just like matter does. Quarks make up protons? Antiquarks make up antiprotons… and antineutrons, too!
Antiprotons and antielectrons - that is, positrons - combine to form antihydrogen atoms.
The Antihydrogen Laser PHysics Apparatus - the ALPHA Experiment at CERN - studies the spectroscopic properties of antihydrogen. That is, it uses photons to give a little extra energy boost to those positrons. As those positrons relax to their ground state, they emit distinct wavelengths of light.
Just like regular hydrogen atoms.
Photons, you see, are their own antiparticles. They interact with matter and antimatter in precisely the same way.
If there were any difference between hydrogen and antihydrogen - any difference in mass, spin or the magnitude of their electric charge - those wavelengths of emitted light would also be different. And the ALPHA experiment would be able to detect those differences.
But no such differences have been observed.
So again, what exactly is antimatter doing here in our physical reality?
Antimatter annihilates Matter
The one thing antimatter does *not* do is hang around.
Antimatter annihilates with ordinary matter. Electrons and positrons annihilate to form a pair of gamma rays, a pair of photons.
If the universe were balanced between matter and antimatter, we wouldn’t be here. Or… perhaps worse… we’d rapidly disintegrate into a bursts of gamma radiation as our particles and those antiparticle partners annihilated.
So if antimatter is so uncommon - why is it even here? What is the point, the reason for antimatter? Why does the universe need antimatter?
To understand that, we need to talk about time travel.
The Light Cone
Our reality has four dimensions. Three space and one time. Famously, Einstein’s special theory of relativity tell us that these four dimensions are related.That relationship is nature’s conspiracy to make sure that nothing travels faster than the speed of light.
One way to think about how this works is time travel. Literally traveling through time.
When we are still, we are traveling forward, through time. When we spring up to go for a run, we’re still traveling through time, but we *rotate* our perceived motion through time into space.
This is a four-dimensional sort of rotation. Sometimes this is called a Terrell rotation. There are some stunning visualizations of Terrell rotation linked in the show notes.
The amount of Terrell rotation varies without speed. In a sense, we exchange some of our speed in the time direction to travel through space. The faster we go through space, the slower we go through time.
There is a limit to this kind of rotation. We cannot rotate our motion so deep into space that we travel backwards in time. The most we can do is cause time to stand almost still, which happens when we travel just shy of the speed of light.
Light of course always and only travels at the speed of light, in the absence of matter anyway. And because everything that must travel slower than light - everything that has mass - like protons, electrons, atoms and US - is subject to the ultimate cosmic constraint: the light cone.
To visualize this four-dimensional cone, think of a camera flash. It’s a sphere of light moving outwards from a point. The tip of the cone is us snapping the photo, and the vertical part of the cone corresponds to the dimension of time.
At any moment, our reality can be cut into two regions: inside or outside the light cone. All those points that light can touch - and those that it can’t. Inside the light cone represents everything we can possibly hope to effect later in time. Outside the light cone is outside of our agency to do so.
The light cone - in other words - represents the boundary of causality.
Because we cannot travel faster than the speed of light, any Terrell rotation we experience inside our light cone retains a positive flow of time - however slow.But outside the light cone, that same rotation can cause our perception of time to reverse. Outside our light cone, if we are traveling fast enough, we can perceive time as flowing backwards.
It’s a fun thought exercise to figure out how we might perceive an event outside our own light cone - I’ll leave that one for you to figure out - but here’s a hint: “wait and see”.
If you’re curious, check out our instagram account in the coming days for the answer.
Time flowing backwards might seem terrible for cause an effect. It would literally reverse the two! But time flowing backwards outside our light cone - outside our sphere of influence - has no bearing on our physical reality. As long as our causal influence is restricted to inside the light cone, the observable universe makes sense.
Now let’s tie this back to particle physics. You’d see, the relationship between the world inside and outside the light cone is intimately related to the relationship between matter and antimatter.
The Feynman-Stückelberg Interpretation of Negative Energies
The celebrated Dirac equation - the mathematics which describes particles likethe electron - suggests that positrons are just electrons with negative energy. But what is negative energy? This interpretation was confusing for quite some time.But energy you see is intimately related to time. As time is to space, energy is to motion through space. Energy, in other words, can be thought of as motion through time.
So an antiparticle with negative energy can be thought of as a particle with positive energy moving backwards though time.
In his 1986 lecture commemorating Dirac, Feynman - who is credited with formalizing this interpretation - gave a concise, technical and frankly satisfying explanation for this phen...
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Update! Best place to find associated references are linked in our substack essay:
This is an essay that we originally posted on our substack page:
https://pasayteninstitute.substack.com/p/the-perils-of-science-communicationA Bonus Episode for The Field Guide to Particle Physics : Season 3
https://pasayten.org/the-field-guide-to-particle-physics
©2022 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.
A History Lesson
In the film “Einstein’s Big Idea”, French Scientist Antoine Lavoisier is portrayed just as he discovers how to split water into oxygen and hydrogen gas, thereby realizing the conservation of mass in chemical reactions.
Lavoisier is generally credited with disproving the phlogiston theory of combustion and reframing Chemistry as a quantitive science.
This shift from the qualitative is emphasized in a specific scene where Lavoisier meets with an excited young man who is pitching his apparatus for observing heat. Lavoisier assertively dresses down the man for failing to meet the modern, quantitative standards of scientific experiment.
This man is later revealed to be a revolutionary, and Lavoisier’s final act of the film ends with an escort to the guillotine.
While dramatized, the message was clear:
Science needs popular support, and clear communication is not enough. We need to do more than educate. We need to build community with inspiration, excitement and respect for Science. We also need to share with folks how Science works1.
Respect for Science is a value we share as Scientists. But it’s not universal. Whether or not Science is morally entitled to respect is irrelevant. Without constantly striving to earn and refresh that respect from Society, it can be lost.
The Siren Call of the OutsiderScience Communication is a rapidly professionalizing field that encompasses a spectrum from dynamic professional speakers to university department media managers to science-minded journalists.
From journalists like Natalie Wolchover, to Professors like Tatiana Eurikhamova, there’s a lot of great work being done by people I admire.
The line between #SciComm and marketing is extremely thin, and unfortunately, the internet’s content treadmill incentives their confluence.
Journals and university departments alike publish heroic press-releases about recently accepted scientific publications by department staff as if they were breakthrough results. But more often than not, these results are merely slow, incremental progress.
How is anyone but a specialist supposed to understand the difference?
The SciComm ecosystem, in other words, is full of noise. Especially for the general audience.
Cutting through that noise is tough. But content editors have had a tool for this as long as humans have printed newspapers: headlines.
Here’s a recent one:
“No one in physics dares say so, but the race to invent new particles is pointless.
In private, many physicists admit they do not believe the particles they are paid to search for exist – they do it because their colleagues are doing it”Sabine Hossenfelder - the Guardian Opinion (26 Sept 2022)As a lead generator, this headline and its subtitle are incredible. Given the current intellectual climate around distrusting experts, it hits all the high points: All these experts have no idea what they’re doing, there’s some structural conspiracy and they’re wasting your money.
Taken with the author’s antagonistic, “outsider” persona2, it's direct aim at an established field of study. It's a recipe for clicks, likes and angry shares.
Unfortunately, the piece willfully and violently mischaracterizes the current state of particle physics. It’s so flagrant - and so short - that it’s worth a read.
A Reading Guide to Hossenfelder’s ComplaintHere is a highlighted list of rhetorical and factual errors which both discredit the thesis of Hossenfelder’s piece and demonstrates its disservice to the endeavor of Science Communication.
Broadly, high energy or “particle” physics is the study of what constitutes matter and energy, as well as the forces that govern their dynamics. Like any good science, it involves the study of both what particles we see as well as how those forces work.
Hossenfelder’s piece begins with a collection of names of physical models at various stages of generality. As written, it conflates them with concrete models for actual, physical particles. Doing so betrays such a misunderstanding of how Particle Physics works in practice that it was almost certainly an editorial decision.
Let’s consider some examples.
The SfermionThe sfermion is a very broad class of particle, a collective noun akin to saying “cats” or even “mammals”. They are particles associated to fundamental fermions - particles of matter like the electron, muon or up quark - by a general class of models related to the idea of Supersymmetry3. Whence the name s(uper)fermion.
The Magnetic MonopoleMagnetic monopoles are another broad class of particle. An electric monopole is a particle like an electron, proton or even an
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The rest of season three is still under development! We wanted to improve the clarity before publishing. Parity violation just isn't that easy to talk about! In the mean time, here is the second episode in a short bonus series about the state and future contemporary particle physics. I hope you enjoy it!
This is an essay that we originally posted on our substack page:
https://pasayteninstitute.substack.com/p/the-physics-of-muon-collidersThis is a follow up to our 4 Reasons to Build a New Particle Collider
You can also get the bumper sticker version here!A Bonus Episode for The Field Guide to Particle Physics : Season 3
https://pasayten.org/the-field-guide-to-particle-physics
©2022 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.
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The rest of season three is still under development! We wanted to improve the clarity before publishing. Parity violation just isn't that easy to talk about! In the mean time, here is the second episode in a short bonus series about the state and future contemporary particle physics. I hope you enjoy it!
This is an essay that we originally posted on our substack page:
https://pasayteninstitute.substack.com/p/we-should-build-a-muon-colliderFour Reasons we should build a new particle collider:
1. We still have more science to do!
2. Technology transfer to Medicine and Industry
3. Institutional memory is valuable
4. Even more science comes with it!Share these reasons with someone, especially if they doubt the need for more Scientific funding!
You can also get the bumper sticker version here!
A Bonus Episode for The Field Guide to Particle Physics : Season 3
https://pasayten.org/the-field-guide-to-particle-physics
©2022 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 rest of season three is still under development! We wanted to improve the clarity before publishing. Sphalerons just aren't easy to talk about! In the mean time, here is the first in a short bonus series about the state and future contemporary particle physics. I hope you enjoy it!
This is an essay that we originally posted on our substack page:
https://pasayteninstitute.substack.com/p/do-we-really-need-new-particle-physicsA Bonus Episode for The Field Guide to Particle Physics : Season 3
https://pasayten.org/the-field-guide-to-particle-physics
©2022 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 Field Guide to Particle Physics : Season 3
https://pasayten.org/the-field-guide-to-particle-physics
©2022 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 Positron ExcessSpace is not a safe place. Matter and energy take on a totally different form than is familiar from our planetary lifestyle. Radiation is everywhere, and with it we find high energy particles flying all over the place. One of the biggest challenges in a voyage to Mars is shielding the travelers from all that radiation. Our magnetosphere and atmosphere do an outstanding job of filtering out the most of the high energy particles flying at us from all directions.
Many energetic particles come from the sun. Fast moving protons and electrons that boil off our friendly plasma ball get trapped in the van Allen belts of our earth’s magnetic field. Way above the atmosphere, we can see them sometimes as the Aurora.
Other energetic particles come to us from inside the Milky Way galaxy. Exploding stars, neutron stars and other monsterous astrophysical objects can shed or accelerate their own high energy particles. Often these particles have more energy than those put off by the sun, but it’s the same story: A lot of protons, a few electrons, and also some heavier nuclei: like alpha particles. Much less often, we see cosmic rays made up of even bigger things, like the nuclei of Carbon, Silicon or even Iron!
Some particles come from outside our galaxy. These can sometimes have outrageously high velocities, and are observed as miles-wide particle showers by large, ground based detector arrays. They aren't common. One of the biggest of these was observed by the Fly’s Eye camera back in 1991. It had over 50 J of energy packed into a single particle - probably a proton. That’s about the same kinetic energy as baseball being thrown around… in a single particle.
Fast moving high energy particles - the ones flying in from outside our solar system - are typically called Cosmic Rays. A tiny fraction of these Cosmic Rays are actually antimatter. Antiprotons and positrons, specifically. Understanding where all these cosmic rays come from is an important scientific question in its own right, but understanding where the antimatter comes from - and how much of it there is - has been a truly fascinating question. Especially of late.
Where does the cosmic antimatter come from?
The ratio of matter to antimatter in Cosmic Rays is small, and varies with particle speed. Typical numbers are 1 or 2 antiprotons for every ten thousand protons. The ratio of positrons to electrons is higher, closer to a few parts in a hundred. One thing we haven't seen? Bigger antiparticles. No antideutrons or antialpha particles have been observed - at all - let alone bigger antinuclei. But of course, we see big nuclei in Cosmic Rays all the time.Because Cosmic Rays come from other parts of the galaxy - or even outside of it - these ratios are basically consistent with our typical assumption that all observed antimatter is secondary. It is created - in other words - through collisions or decay of so-called “normal” matter.
Really fast Cosmic Rays occasionally interact with other particles in our galaxy: the tiny, sparse bits of gas and dust in the large voids between stars, sometimes called the interstellar medium. Those collisions often generate more particles, and just like in our own atmosphere, antiparticles are part of that collision debris.
Just like the proton and the electron, to the best of our knowledge, the antiproton and the positron are stable particles. So unless they annihilate, these particles of antimatter just hang around. The collective effect of all these Cosmic Rays bounding around our galaxy is a very small - but measurable - population of antiprotons and positrons flying at us as secondary cosmic rays.
If we were to assume that all antimatter is secondary - that is, if antiprotons and positrons are created only from collisions in the interstellar medium - we can use that assumption to calculate how much of it we expect to see. In these calculations, the number of antiprotons pretty much lines up expectations. While on the high side, the population of antiprotons in our galaxy essentially agrees with what you'd expect from collisions of other cosmic rays in the interstellar medium.
While it is possible that antideutrons and antialpha particles can be also created in these collisions, they are rare. The expected number of them is currently far below current experimental sensitivity.
Positrons are a different story. What’s fascinating astroparticle physicists these days is that the number of positrons observed in Cosmic Rays is noticeably higher than we expect from these calculations. In particular, the number of positrons at higher energies is much bigger than we’d expect if they were only created in collisions, upwards of 10 percent or more!
In short, we see too many positrons flying at us as Cosmic Rays and we don't know why!
What we do know about Cosmic Rays
Earth's atmosphere is much denser than interstellar space, so Cosmic Rays that make it to Earth typically collide dramatically with molecules in our upper atmosphere. With land-based detectors, we can see the resulting showers of particles down on Earth. We can calculate how much energy they had, but we can't exactly say what kind of particle they were.
To assess the species of particle that's slamming into the Earth, we need to capture, identify and count them before they strike the atmosphere. We need, in other words, particle detectors on satellites.
Older experiments like the Fermi Gamma Ray Telescope and the PAMELA detector were put in orbit around the earth on satellites. The current state of the art, the AMS-02 Cosmic Ray experiment is literally in a box attached to the side of the International Space Station.
All these experiments agree: Cosmic Rays follow a somewhat predictable pattern. Most particles come equally from every direction in space, so as a population of particles, they're very likely diffused around the entire galaxy. The number of particles we see depends on their energy. Roughly speaking, the more energy a particle has, less common it is to see. But this trend is also true by particle species. In aggregate, simpler particles are also more common than complex ones. And of course, antimatter is far, far less common than matter.
There are...
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The Field Guide to Particle Physics : Season 3
https://pasayten.org/the-field-guide-to-particle-physics
©2022 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.Also check out the links embedded this description. Or also check out those same links at:
https://pasayten.org/the-field-guide-to-particle-physics/antineutrinoThe Pasayten Institute is on a mission to build and share physics knowledge, without barriers! Get in touch.
The Antineutrino
The neutrino is a curious particle. As fundamental as the electron or the muon, but rarely interact with other particles. This makes the study of these neutrini quite challenging. But also quite interesting.
Are there antineutrini? Yes, surely. But, a better question is what are antineutrini?
Antiparticles with an electric charge are easier to identify. Positrons and electrons have opposite charges and behave oppositely in most respects.Photons and neutral pions do not have any electric charge. They are their own antiparticle partners! But this isn’t always the case with neutral particles. As we have antineutrons and two distinct kinds of neutral kaons: the K0 and K0bar which are antiparticles of each other.
Neutrini - those smallest of massive matter particles in the Standard Model - are electrically neutral. So it is natural to ask: are they their own antiparticle? Or are there distinct antineutrini? And importantly, how can we tell the difference?
The short answer is, we don’t know yet. End of story. But the short answer is boring.
Neutrini are famously shy and interact only via the weak nuclear force - and gravity - so detecting them so detecting them is no small task.
So without further ado, let’s go ahead with the long answer.
Beta DecayNeutrons decay to protons by emitting an electron. This is usually called beta decay, and is mediated by the W- boson. Other nuclei experience it as well.
Detailed studies of beta decay suggest that the neutron should decay into two particles rather than one. That second particle was need to make sure that energy, momentum and spin angular momentum was conserved. As it should be.The neutrino - the small neutral one - was discovered nearly 26 years after their proposal.
Now, electric charge is conserved in beta decay. The uncharged neutron decays to a positively charged proton and a negatively charged electron and a neutrino. The neutrino also has no electric charge, but carries away some of the energy and some of the momentum.
So far as we can tell, energy, momentum and spin like electric charge, is always conserved. Such conservation laws are useful organizing principles for understanding the laws of particle physics. Some might argue they are foundational.Another thing that seems to be conserved in nature - usually anyway - is the number of leptons in the universe. There are actually quantum effects that can change the number of leptons, but in ordinary decays - like beta decay - they seem to conserve the number of leptons.
Neutrini - like electrons, muons and taus - are leptons. Naively you might think that beta decay creates two leptons: a neutrino and an electron. The thing is, the neutron actually emits an electron and an antielectron neutrino. Like electric charge, antineutrinos count as minus one lepton.The math also works in reverse. If a nucleus absorbs an electron - which sometimes happens in certain isotopes of Vanadium, Nickel and Aluminum - it will convert a proton to a neutron, and spit out a regular neutrino. Conserving the number of leptons.
Now, before your eyes glaze over, I know. Talking about weird conservation rules like lepton number is tricky, because it seems like a bunch of silly rules the details quickly spiral out of control. Neutrino physics is nothing if not complicated.
So let’s talk more about some of the reactions.Flavors of Antineutrini
Each electrically charged lepton: the electron, the muon and the tau, has it’s own flavor of neutrino. There’s an electron neutrino. A muon neutrino and a tau neutrino. Each electrically charged antilepton also has its antineutrino partner: antielectron neutrino. anti muon neutrino. Anti tau neutrino.
When a muon decays into an electron, it actually emits three particles: the electron, the antielectron neutrino and a regular muon neutrino.
Given that there are so many cosmogenic muons around us, muon neutrinos - and anti electron neutrinos - are also fairly ubiquitous here on Earth.And of course you might remember the famous experimental result that neutrinos can change their flavor as they move. So neutrinos flavors can get all mixed up, just like antineutrino flavors can get all mixed up. But do neutrini get mixed up with antineutrini?
They would if they were the same particle, wouldn’t they? Let’s think about it another way. In terms of annihilation.
Do Neutrini and Antineutrini annihilate each other?When an electron and positron collide, a pair of photons usually comes out. The antiparticle partners annihilate into pure electromagnetic energy. What do you suppose happens when a neutrino collides with an antineutrino?
A neutrino and an antineutrino - assuming it exists - would not annihilate to form photons. They have no electromagnetic charge and therefore no chance. They could p...
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The Field Guide to Particle Physics : Season 3
https://pasayten.org/the-field-guide-to-particle-physics
©2022 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 AntineutronLike the antiproton, the antineutron is a composite particle made up of antiquarks. It looks a lot like the neutron, and that’s pretty interesting because both of those particles have no electric charge!
The antineutron is made from two antidown quarks and an antiup quark. The antineutron’s mass is a bit over 939 MeV, and the mass difference ratio between the neutron and the antineutron is essentially consistent with zero.
Because it’s electrically neutral, it is really hard to measure properties of the antineutron. You can’t really use electric or magnetic fields to confine, shape or cool collections of antineutrons in any meaningful way.
We don’t have a working measurement of the antineutron’s magnetic dipole moment. We haven’t really studied their decay.
Left to its own devices, the neutron decays in about 15 minutes to a proton, and electron and a neutrino. We’d expect the antineutron to decay similarly, but with a positron. But again. It’s a serious experimental challenge.
We barely have a handle on the antineutron’s mass! But there have been experimental antineutron beams and there is still plenty of interesting physics that can be done with them.
Antineutron beamsAntiproton and antineutron technologies are linked. The antiproton was discovered in 1955 , and the antineutron was found in 1956. In the 1980s, The Low Energy Antiproton Ring at CERN fired a slow beam of antiprotons at liquid hydrogen to create a secondary beam of anti neutrons.
Low energy proton-antiproton collisions proceed by the exchange of a single pion. Because the hydrogen was kept super cold, and the antiprotons had such low energy, the two particles exchanged a single, virtual neutral pion, which afforded a conversion of the proton antiproton pair to a neutron antineutron pair.
This secondary beam of neutron/antineutron pairs was aimed at an iron slab for a target. The neutron and antineutron interact with iron differently, but expecting to find both particles simultaneously made the measurement pretty tractable.
Again. Antineutrons are hard to work with, so any trick you can find to help is welcome!
Antinuclei
Of course, there’s more.Antineutrons have been created in atomic nuclei. Or antinuclei, if you like. Deuterium - a hydrogen atom with a bonus neutron in the nucleus has a theoretical antimatter cousin, antideuterium. The nucleus of anti deuterium was created in experiments way back in the 60s, although cooling those nuclei down enough to accept an orbiting positron has not yet occurred. But hey, ,antihydrogen was only really successfully studied in 2016!
The relativistic heavy ion collider has observed the anti helium-4 nucleus. In other words, there’s also an anti alpha particle!
All these discoveries point to to the fact that there is very little difference between matter and antimatter, which makes the overall dearth of antimatter in our observed universe even more confusing. -
The Field Guide to Particle Physics : Season 3
https://pasayten.org/the-field-guide-to-particle-physics
©2022 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.
AntiprotonsAntiparticles are everywhere. They’re just part of life. The electron has its positron partner. Muons and antimuons are both routinely created in the upper atmosphere. They’re so familiar that we often just call them mu plus or mu minus. The antiparticle nature of mu plus just isn’t that big a deal.
If you’ve been paying attention to our series, you know we’ve talked about antiparticles quite a bit, at least in passing. Up and down quarks sometimes associate with anti-up and anti-down quarks to form pions. Other mesons like kaons form similar quark-antiquark pairs.
It’s fun to see composite particles made up from particles and antiparticles. The neutral pion - for example - is a bound state of particle/anti particle partners - uubar & ddbar - not unlike positronium: where an electron and a positron orbit each other like an atom.
Of course, all these composite particles are unstable.
Arguably what separates antimatter from antiparticles is finding a composite particle that is stable. Or at least really long lived. Something that looks and behaves like ordinary matter. Something like atoms.
Enter the antiproton.
Just like the proton, the antiproton is a tiny bag of subnuclear goo. Virtual pions and gluons and other quantum effects are all dressed up in the antiproton package around three valance antiquarks. That’s two anti-up quarks and one anti-down quark. The antiproton looks virtually identical to the proton - except that it has a negative electric charge.
Like the proton, the antiproton has a mass of about 931 MeV. In fact, it’s difference from the proton’s mass has been measured, and at present it looks like they’re the same up to less than one part in a million!In fact, everything they measure from the antiproton seems to to line up exactly with the proton. The magnetic moment - a measure of a little dipole magnetic field generated by the anti proton - still appears to be equal and opposite to that of the proton.
Antihydrogen
And yes, the negatively charge antiproton can pick up a positively charge positron and form an atom. Like hydrogen. You know, Antihydrogen! Antihydrogen has been studied and confirmed to look and behave exactly like hydrogen. The positron energy levels of thes anti atom and the associated electromagnetic spectra are all the same. Even the fancy, hyperfine splitting of those energy levels have been experimentally shown to be identical with ordinary hydrogen, at least up to experimental precision.
Antiproton decay
By all observations so far, the proton appears to be a stable particle. If the proton did decay, it would be big news and a boon for folks looking to study physics beyond the standard model.
The antiproton - so far as we can tell - is also stable. Which is good - our theory is self consistent - but it does present the question: if they don’t decay, then where are all the antiprotons in nature?!
Sources of antiprotons
Nobody knows why there’s so little antimatter in the universe, but there definitely is some.
Antiprotons impinge upon the Earth’s upper atmosphere all the time. They’re secondary cosmic rays that currently appear to be associated with super high energy protons smashing into gas and other material sitting in between the stars in our own galaxy.It’s a by-product - in other words - of cosmic ray collisions.
We can make them here on Earth too. The ALPHA experiment at CERN has an antiproton source made by smashing protons into iridium. The Tevatron at FermiLab had an antiproton source that used Nickel instead.
The Tevatron was an interesting particle accelerator in that - unlike the LHC, which colliders protons together - the Tevatron collided protons against antiprotons, to give it a little extra boost in energy from quark-antiquark annihilation when those two, composite particles collided.
The fact that there is so much more matter in the universe than antimatter means that antimatter is simply going to annihilate against any matter that it runs into. But how protons and antiprotons annihilate is a complicated issue.
Antiproton annihilationElectrons and positrons annhilate cleanly into a pair of gamma rays. The antiproton and the proton do not cleanly annihilate. There is no easy, super clean signal when they annihilate. They’re composite particles. Worse, they’re both really messy composite particles.
Typically what happens when a proton meets an antiproton is that one of the quarks meets up with one of the antiquarks and interacts from there. All kinds of particles can come out, things like pions, more protons, and other emissions from the subnuclear goo. The details all depend on how quickly those particles are moving when they meet each other.If they’re moving slowly, their quantum clouds of subnuclear goo might overlap, and a pion might be exchanged.
If they’re moving quickly, like they were at the Tevatron, those antiquarks - who carry the highest fraction of the antiproton’s momentum - will collide with the quarks in the proton, and all kinds of things can - and have! - come out.
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The Field Guide to Particle Physics : Season 3
https://pasayten.org/the-field-guide-to-particle-physics
©2022 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 Positron
The positron is the antiparticle partner to the electron.
Ostensibly, positrons have the same mass as the electron, around 511 keV. They also have the same electric charge - at least up to a minus sign. The positron is of course positively charged.
Positrons also carry equal and opposite magnetic dipole moments to the electron: that little magnetic field carried often carried by elementary particles.
Like the electron, positrons are stable. They do not decay. But of course, we don’t see may of them around. When electrons and positrons collide, they annihilate each other! That is, they convert into a pair of photons, each with 511 keV of energy.
Because it is *extremely* rare for photons to interact with each other, this reaction almost never goes in reverse, which explains why positrons don’t accumulate here on Earth.
As you might be aware, the matter to antimatter ratio of our universe is way out of whack - which is great for us! - but makes it a little hard to study antimatter particles like the positron.
Sources of Positrons
Some positrons are produced by the decay of cosmogenic muons - or antimuons, more precisely - that are formed when the pi-plus - the positively charged pion decays. Those pions are in turn produced in collisions with cosmic rays in the upper atmosphere.
Sometimes positrons are produced in nuclear decays, like an antimatter version of beta decay. Fluorine-18 - which has 9 protons and 9 neutrons - is one such unstable nucleus. Oxygen-15 - which has 8 protons and 7 neutrons is another. A more exotic case is Rubidium-82, which forms when a strontium-82 nucleus absorbs an electron, converting one of its 38-protons into a neutron. Rubidium-82 then decays by positron emission, converting another proton to a neutron, resulting in the noble gas Krypton-82.
Because the mass of the neutron is higher than that of the proton, positron emission is a form of radioactive beta decay that requires *extra* input energy, which is typically supplied by the remainder of the nucleus. It’s a curious concept that we’ll come back to in a future episode.
In medicine
Because the photons emitted by the annihilation of a positron-electron pair have a very specific energy, scientific instruments can be calibrated to detect them. Positron Emission Topography is an imaging technique that specifically looks for these pairs of 511 keV photons - these gamma rays if you like. By injecting a radioactive substance that decays by positron emission, PET devices back calculate the gamma ray trajectories to build a three-dimensional model of whatever that tracer was injected into. Typically the human body!
Fluorine-18, oxygen-15 and rubidium-82 are manufactured by particle accelerator for direct use in medical PET imaging. Sometimes those accelerators are RIGHT INSIDE THE MEDICAL FACILITY.
That’s right. Particle physics isn’t just for lab rats or abstruse aloof theorists. It’s crucial for medicine too! You can be a medical doctor AND study particle physics.
Positronium
Finally, electrons and positrons can form a bound state - an atom if you like - called positronium. Positronium doesn’t last very long - typically it decays by annihilation into an assorted number of gamma rays in a time that’s measured in nanoseconds .
The precise dynamics of positronium decay is a well studied science used in precision tests of quantum electrodynamics. We’ll learn more about positronium later this season!
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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.
Introducing Season 3 : Antimatter!
I hope you enjoyed Season 2, and the bonus episodes on cosmic rays that followed shortly after.
This is just a short note to let you know that we’re still hard at work on developing Season 3, and the Season 3 will be all about ANTIMATTER.
We’ve mentioned antimatter in brief before, for example, how the positron and the electron can collide, annihilating each other to turn into a pair of photons.
In seasons one and two, antimatter was used for taxonomy; it was used to organize the particles we know.
This season, we’re going to dig much deeper. We’ll explore what it means for a particle to have an antiparticle partner.
Questions we’ll discuss include:
Where does antimatter come from, and what’s with that name? Why is there so little of it in the universe? Does antimatter always annihilate matter? What does it mean to be your OWN antiparticle partner? Finally, what can we USE it for?
We’ll finish season 3 with what we DON’T YET know about antimatter. Like just who, exactly, is the antiparticle partner of the neutrino.At the Pasayten Institute, antimatter fills our minds with wonder - and a little bit of terror. By the end of next season, we hope to fills yours with curiosity to explore more.
We’re excited to share these ideas - and a few stories - with you!
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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 Primary reference for this piece on the Lunar Surface:
https://pubs.er.usgs.gov/publication/70034108Dr. Jean-Philippe Combe's professional website at the PSI.
The asteroid 4-Vesta:
https://en.wikipedia.org/wiki/4_VestaA video on the basics of sputtering as applied to nanotechnology:
https://www.youtube.com/watch?v=GgD6G3B-2WUA note on the creation of Lunar Soils
Micrometeorites - incident space rocks that are less than 1 millimeter in size, and discussed by Dr. Combe below - play a large role in the formation of lunar soils. They play the role grinding and melting rocks and minerals into a dusty material that set the stage for particles from the sun and deep space to impact.Below is Dr Combe's full essay.
Interaction of the solar wind with the surface of the Moon
Planetary science contribution to the Paysayten Institute
by Jean-Philippe Combe, PhD, Winthrop, Washington State
Senior scientist at the Planetary Science Institute, Tucson, Arizona
April 1, 2022The coming narration comes largely from one scientific article entitled “ Sources and physical
processes responsible for OH/H2O in the lunar soil as revealed by the Moon Mineralogy Mapper (M3).“
by Thomas. B. McCord, Lawrence. A. Taylor, Jean‐Philippe Combe, Georgiana Kramer, Carle M.
Pieters, Jessica M. Sunshine, and Roger N. Clark published in 2011 in the JOURNAL OF GEOPHYS ICAL RESEARCH PLANETS, VOL. 116.
aurorae in the upper Earth’s atmosphere terrestrial magnetic stormstheir correlations with solar flaresand comet’s tails always pointing away from the Sun .
The Solar Wind
The Solar Wind was postulated in the mid-19th century, as a flow of particles and energy (photons) traveling away from the Sun into the Solar System. The observations that led scientists to this theory wereThe solar wind was first observed directly by the Soviet satellite Luna 1 in 1959 and verified by measurements from Luna 2, Luna 3 and Venera 1, and then it was observed by a U.S. spacecraft, Mariner 2, in 1962.
The solar wind is composed mostly of protons and electrons, with about 4% helium and smaller amounts of heavier element ions. The flux at the Earth is about four hundred thousand particles per square centimeter and per second, with average energy of around half of a kilo-electronvolt per atomic mass unit, and a flux energy half width between 300 and 1500 keV as measured in 2009 by the Indian lunar orbiter Chandrayaan‐1. For this energy range, protons (H+) have a penetration depth in the surface grains of 5 to 10 nm. The solar wind plasma is almost completely absorbed by the Moon’s illuminated surface. However, up to 20% of the impinging solar wind protons are reflected from the lunar surface back to space as neutral hydrogen atoms.
The solar wind impacts the lunar surface, and the resulting i nteraction depends largely on the nature of the lunar soil exposed to space at the molecular level. Most known minerals of the solar system are made of molecules that contain large amounts of oxygen, mostly in oxides. The lunar surface has two major types of terrains that can be distinguished with the naked eye: the bright ones are called the highlands, and the dark ones are called by the latin word mare, which means seas, although they are not made of water, but are instead made of volcanic rocks, with various types of minerals rich in iron oxides and magnesium oxides. The lunar soil surface consists in a layer of crushed rock, minerals, and glass called “regolith”.
On the Moon, regolith formation results from combination of all the physical and chemical factors that occur on airless bodies, and that is called space weathering . This is unlike the processes that occur on Earth, and that are largely driven by tectonic activity and erosion due to the water cycle and atmospheric circulation. On the Moon, the agents of space weathering include a wide range of types and sizes of impactors, such as meteorites, micrometeorites (<1 mm), solar wind particles, solar‐ ultraviolet photons and galactic cosmic rays.
Solarwindparticlesbombardtheexposedsurfacesoflunarsoilgrains,producinganamorphouslayer and effecting atoms, ions or molecules to be ejected from a lattice site in the target material; this process is called sputtering . Such ejected particles from the source material can be redeposited as a thin film on a surface. The sputtered particle can be charged, but is most often neutral. The sputtering energy is inherent to the solar wind velocity and particle mass. Protons (H+), which make ∼95% of solar wind particles, have an incident energy range of ∼300–1500 keV and average energy of ∼500 keV under normal solar wind velocities of 300–800 km/s. Heavier ions such as He+ and other heavy species, with greater incident energies, also play a significant role in the sputtering process . Sputtering of cations with the lowest crystalline binding energy occur preferentially, such as for magnesium.
Effects of the Solar Wind
Interaction of the lunar surface with solar wind particles occurs in a context where micrometeorite impacts locally melt the soil to form layers of amorphous glass at the surface of mineral grains, and agglutinates composed of small fragments of minerals in a matrix of glass. As a consequence, the solar wind is able to implant protons (H+) in a lunar soil that is rich in iron oxide FeO, by reducing the FeO component in the soil melt to metallic iron. As a result, in agglutinitic glass of soil grains, nanophase metallic iron particles are ubiquitous.
The texture and major element compositions of these thin, amorphous rinds on the surface of lunar soil grains is a testament to the process of vaporization and subsequent deposition of these silica‐rich patinas, with their myriads of nanophase iron particles.
UV Photons from the Sun
Now let’s talk about photons, and specifically about solar ultraviolet photons. This is a radiation that triggers the emission of electrons from the lunar dayside surface, which makes the surface charged several volts positive and leaves many dangling positive ions. The released photo -electrons move to the unlighted side of the Moon and into the solar wind plasma wake forme d by the Moon absorbing most of the solar wind. Some of these effects were observed by the Electron Reflectometer onboard the Lunar Prospector spacecraft, and they have stimulated the study of the electrical charging of objects on the Moon, such as astronauts. The photoejection of electrons is yet another effect that ...
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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 Particle Data Group's write up on cosmic rays. See Figure 29.8 for a representation of the "ankle" feature in the spectrum.
https://pdg.lbl.gov/2019/reviews/rpp2019-rev-cosmic-rays.pdfAnother representation of the power laws can be found in Professor Peter Gorham's Coursework on Ultra High Energy Cosmic Rays: http://www2.hawaii.edu/~gorham/UHECR.html
Natalie Wolchover has written two great articles in Quanta on Cosmic Rays, both which talk about what might accelerate these particles.
The Particle That Broke a Cosmic Speed Limit and Cosmic Map of Ultrahigh-Energy Particles Points to Long-Hidden Treasures
Colussi & Hoffmann
In situ photolysis of deep ice core contaminants by Çerenkov radiation of cosmic origin
Gephysical Research Letters: https://doi.org/10.1029/2002GL016112Guzmán, Colussi & Hoffmann
Photolysis of pyruvic acid in ice: Possible relevance to CO and CO2 ice core record anomalies
Atmospheres: https://doi.org/10.1029/2006JD007886A quick primer on Cherenkov Radiation: https://www.radioactivity.eu.com/site/pages/Cherenkov_Effect.htm
Theme music "Sneaking Up on You" by the New Fools, licensed by Epidemic Sound.
Cosmic Rays
Part 4 - Paleoclimatology and Muons
Our atmosphere is one giant filter for cosmic rays. The sparse molecules near the top of our atmosphere begin the process of catching the energy of those energetic particles from space and transferring it into heat or muons. These cosmogenic muons that typically make it all the way down to the surface.Near the surface, the atmosphere is a lot thicker, but it’s still just a collection of ballistic molecules bashing into each other at 1000 miles per hour. Some of those molecules hit us, and some hit the ground. We perceive these molecular impacts as air pressure.
By contrast, cosmogenic muons are moving through this mess at over 600 million miles per hour.
To those muons, the surface of the Earth is barely noticeable. They fly through a lot of things: hundreds of meters of rock, oceans, plants and animals before colliding or decaying. By contrast, those particles of atmospheric gas typically reflect off the surface of the Earth. Rocks just aren’t that permeable to most gas. As we explained in the ALPHA particle miniseries, helium gas generated from radioactive decay deep within the earth collects underground, trapped by rocks.
One thing gas can permeate is surface water.
Quite a bit of our atmospheric gases get dissolved into the ocean. Oxygen in the air allows the fish to breathe too, once dissolved into the water so it can be picked up by their gills. Increased carbon dioxide levels also imply more CO2 gets put under water.
When the water on Earth’s surface freezes, as it might do near the polar ice caps, it traps some of that dissolved gas with it.
This has been happening for millions of years, and until somewhat recently at least, that ice has been compounding. New ice forms above, pushing old ice down.
This has resulted in a LOT of ice.In Antarctica there are areas where the ice is over four kilometers deep! That’s miles of ice! Greenland also carries massive glaciers, two to three kilometers deep, built up in same fashion.
The gases trapped in that glacial ice is a frozen relic of an older atmosphere. The deeper the ice, the older the dissolved gases. As the mixture of molecules in our atmosphere changes over time, it sets down a record in the glacial ice. The deepest ice, millions of years old, can tell us what the atmosphere was like millions of years ago.Extracting that ice is quite the scientific adventure!
This all easy to say in theory - but the practice of Science requires a lot of gory, technical detail. Different measurements from different samples of ice at different depths from different parts of the world need to be calibrated. Ice can form at different rates in different places under different conditions.
But, at least averaged over a given year or decade or so, the atmosphere should be well mixed. Huge weather patterns around the world mix the air, ensuring should be about the same.
And so the Scientific logic goes like this:
Assuming older ice is usually below the younger ice and the atmosphere is well mixed, then given any two ice sheets on earth, there should be a way compare them. The concentrations of different gases dissolved at different times should sequentially be the same. Like multi-colored stripes on a pole. The stripes may be different sizes, but they should be in the same order.
If we can find the same sequences in gas concentrations across different ice sheets then we can start to put together a history of the Earth’s atmosphere.
Near the turn of the 21st century, geophysicists were working on exactly this problem. They were trying to calibrate the gas concentrations trapped in ancient ice samples by comparing ice from Antarctica with Greenland. And things just weren’t adding up. The sequences didn’t align. The gas concentrations were just too different. There was some kind of missing variable in the data.
As it turned out, that variable involved cosmogenic muons.
The Speed of Sound and Light
To understand how muons resolved this Paleoclimatology puzzle, we need to go back to the source. The source of cosmic rays.
In episode two of this series we talked about Fermi Acceleration - the process by which electrically charged particles like protons get accelerated to outrageous velocities by SHOCKWAVES in astrophysical plasmas.
And shockwaves occur in glacial ice too.
To understand shockwaves, let’s think about sound waves.
Sound usually travels in the atmosphere like a wave. A wave of air pressure. Those atmospheric particles slam against each other in an organized and oscillating way, spreading out away from source.
The speed of those waves depends on the amount and types of molecules present, as well as the overall temperature of the atmospheric gas. The sound waves we experience travel at around 343 meters per second, which is about 767 miles per hour.
Here’s the thing, humans routinely fly supersonic jets that travel faster than that.
Supersonic jets - like fighter jets - travel faster than the speed of sound. They travel faster than noise they make. You can’t hear them coming until they’re already past you. And when you do finally hear them, it’s a tremendous noise.
It’s a s...
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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 Particle Data Group's write up on cosmic rays. See Figure 29.8 for a representation of the "ankle" feature in the spectrum.
https://pdg.lbl.gov/2019/reviews/rpp2019-rev-cosmic-rays.pdfAnother representation of the power laws can be found in Professor Peter Gorham's Coursework on Ultra High Energy Cosmic Rays: http://www2.hawaii.edu/~gorham/UHECR.html
Natalie Wolchover has written two great articles in Quanta on Cosmic Rays, both which talk about what might accelerate these particles.
The Particle That Broke a Cosmic Speed Limit and Cosmic Map of Ultrahigh-Energy Particles Points to Long-Hidden Treasures
MIT's GameLab has a fun example of how Special Relativity works. See also Gamow's popular science book on Special Relativity.CERN's DIY Cloud Chamber Design
Cloud Chamber without Dry Ice (see also references within)Other References:
Measurement of muon flux as a function of elevation
ICRP Paper on Aviation and Radiation
Radiation Exposure During Commercial Airline Flights
Radiation from Air Travel as per the CDC
Calculate Your Radiation Dose (EPA)Cosmic Rays
Part 3 - Cosmogenic Muons and Special Relativity
Muons - those heavy, unstable cousins of the electron - are all around us. All the time.On average, every square centimeter of Earth sees a muon about once a minute. While that might not seem like a lot, if you consider your personal space. Say, about square meter around you - you know, 10 square feet . Over 160 muons pass through your personal space per second! Per second!
Those muons coming form the upper atmosphere. They are the debris left over from the constant bombardment Earth experiences from high energy cosmic rays.
If only there was a way to see them.
Do you remember when I said that a particle physicist will look for particles WHEREVER they can find them? Well, before weather balloons, before particle colliders, there were cloud chambers.
Cloud chambers are boxes full of super saturated vapor or some kind. Any little disturbance will cause that vapor to condense, as clouds do up in the sky.
High energy particles blasting through a cloud chamber leave tracks. Little clouds form around the path of the particle, just like the contrails of a jet flying through the sky.
The muon and the positron were both discovered this way!
Cloud chambers are fun because you can build them yourself at home! The main thing you need is a sustained temperature gradient and tiny bit of very pure isopropol alcohol.
We’ll link to two great examples of DIY cloud chamber designs in the show notes.
Building a cloud chamber at home is a great way to come face to face with the fact muons - the debris from cosmic rays - are passing through us all the time.
The Atmosphere as a Muon Filter
The magnetic field generated by the Earth’s core protects us from many incident particles from space. Especially all that plasma in the solar wind.
But those high energy cosmic rays blast straight through the magnetic field. It’s just not strong enough to contain them.
Our upper atmosphere is our next layer of defense. Cosmic rays collide with its molecules tens of miles above the Earth, creating a shower of debris that itself can be miles across.
In some sense, the atmosphere serves as a filter, converting all those particles like protons and pions into muons. Muons comprise the bulk of what we see down here at the surface.
Muons are unstable particles. They decay to electrons after about 2.2 microseconds. This means that while many muons make to the ground, not all of them do. The higher you are above sea level, the more muons you’re likely to see.
At 10,000 ft above sea level, this number can triple! Given that commercial airline flights typically occur above 40,000 ft, it’s important to realize that flying exposes you to more Cosmogenic Muons.
Fortunately for you frequent flyers, the extra does radiation exposure is still a very small amount of radiation exposure! The International Commission on Radiological Protection has well established professional limits to protect even commercial flight crews from exposure to all those cosmogenic muons.
Long Lived Muons
Despite the atmospheric filter, those Cosmogenic Muons are still traveling really, really fast. Like 99.9 percent of the speed of light fast. Muons moving that fast don’t behave like you’d expect. For one thing, they take far longer than they should to decay.
How do we know that?
As you might recall from their eponymous episode, muons only live for about 2.2 microseconds. That’s 2.2 millonths of a second. Even traveling near the speed of light, that’s simply not enough time to get from the upper atmosphere to anywhere near the surface of the Earth.
That’s a bit over 9 miles - or 15 kilometers. It takes light about 50 microseconds to travel that far.
Muons that make it to Earth, then, live over 22 times as long as they should.
Why that happens - what causes the muons to live so long - requires a small digression on the theory of relativity.
On Special Relativity
As they say, Nothing travels faster than the speed of light. Which is true, at least, in outer space and to some extent in the air around us. You see, it’s not so much that LIGHT is the fastest thing around. It’s that the universe itself has a maximum possible speed - a speed limit, if you like - which is just shy of 300 million meters per second.
When left to its own devices, light - or any particle with zero mass - travels at that speed.
That universal speed limit is just a fact of life, but we don’t notice is much because a typical human moves at about 1 meter per second. Not 300 million meters per second.
But having a speed limit like the speed of light leads to some pretty strange paradoxes.
For example: you cannot race a photon. Photons, you might recall, are particles of light.
If you ran towards a photon, the photon sill still move away from you at the speed of light.
If you drove towards the photon at 100 miles and hour, the photon will still move away from you at the speed of ...
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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 Particle Data Group's write up on cosmic rays. See Figure 29.8 for a representation of the "ankle" feature in the spectrum.
https://pdg.lbl.gov/2019/reviews/rpp2019-rev-cosmic-rays.pdfAnother representation of the power laws can be found in Professor Peter Gorham's Coursework on Ultra High Energy Cosmic Rays: http://www2.hawaii.edu/~gorham/UHECR.html
Natalie Wolchover has written two great articles in Quanta on Cosmic Rays, both which talk about what might accelerate these particles.
The Particle That Broke a Cosmic Speed Limit and Cosmic Map of Ultrahigh-Energy Particles Points to Long-Hidden TreasuresCosmic Rays
Part 2 - Plasma PhysicsWhat might you say is the most insightful law in theoretical physics? E = mc2? The general theory of relativity? The quantum nature of the atom? The debates could rage for days. Looking back on my own education, I’d isolate two really important ones.
The first is Newton’s FIRST LAW of motion:
A body at rest will stay at rest or a body moving in a straight line with a constant speed will not change in its motion unless acted upon by a force.
The second is probably Dalton’s Law of Multiple Proportions, otherwise interpreted as the modern theory of atoms. You know, that everything in nature is made of up individual molecules and those molecules are made up of atoms.
These ideas run counter to much of our direct, daily experience. At least that kind of experience we’ve had in common with our ancestors for thousands of years.
So please don’t ask me to pick between the two. Both Newton and Dalton’s laws are crucial.
Putting those ideas together - which involves a lot of mathematical work - physicists arrived at the modern, kinetic theory of gases.
There are LOTS of details and lots of implications, but one way to understand it goes like this:
Gases - like the air we breathe - is made up of molecules and those molecules move at different speeds. Their average speed tells us the temperature. The higher the temperature, the higher the average speed. But also - and importantly - the higher the temperature the wider the spread on molecular velocities.
In other words, all around you there are gazillions of tiny molecules. At room temperature, they’re moving at 1000 miles per hour, on average. Of course, some are moving really very slowly, and some are moving quite fast. A tiny fraction of those molecules are moving, really, really quickly, more than twice as fast as average.
But we can’t see any of it because they’re just too small.
Plasmas
When gases get really hot, the individual atoms inside the gas begin to break down. Their collisions have too much energy. The impacts are too powerful. The electrons and nuclei split apart and form separate components of the gas. Perhaps not surprisingly, this often - but not always - coincides with a very low density of atoms.When a gas has its charged particles ripped apart, we call that gas a plasma. Plasma’s are kind of a BIG DEAL in astrophysics.
If you’ve stood around a bonfire, you’ve seen a plasma. Those tongues of fire are little pockets of air whose atoms have been ripped apart by the intense heat. The intense speeds of electrically charged particles zipping past each other is what causes those tongues of fire to give off electromagnetic radiation - otherwise known as light.
We discussed another sort of plasma in our last mini-series on the ALPHA particle, where we discussed the solar wind and the Earth’s magnetosphere. Of course, the outer bits of the sun itself are in a plasma, hence all the glowing we see every day. And that giant plasma ball we call our sun spits a constant stream of charged particles our way - the solar wind. The magnetic field generated by our Earth’s spinning core captures much of those charged particles well before they hit the Earth’s atmosphere. Thereby protecting both it and us.
Those particles are confined so the so-called Van Allen belts which hold the plasma - a very low density plasma compared to what you’d see in a bonfire - thousands of miles above the Earth’s surface.
Magnetic fields contain that solar wind by bending the trajectories of the individual particles - it curves their motion. That’s just what magnetic fields do. The strength of the magnetic field means that those particles can - at best - move in circles. The faster the particle, the bigger the circle. Approximately anyway.
Like any gas of particles, the van Allen belt plasma has particles moving at very low speeds and very high speeds. Very small circles and very large circles. The average speed - in part - determines the approximate size of those van Allen radiation belts.
Particles moving stupidly fast through a magnetic field - like cosmic rays from space - will also bend, but not enough to get trapped. Instead they fly through the magnetosphere and into the upper atmosphere. Breaking apart by spreading their energy around, leaving us to content with that debris of particles.
Plasmas in Space
You might wonder where those high energy particles from space - those cosmic rays - come from.Well, there’s a lot of stars in space and subsequently a lot of plasmas. Stellar winds blow off particles all the time. But that’s not really enough energy to generate cosmic rays. But sometimes, when stars explode as supernovae, even more charged particles get ejected into space.
Those astrophysical gases - plasmas - often give us beautiful photographs to look at here on Earth. But don’t be fooled. The density of those gorgeous gas clouds - even in star forming reasons like the Horsehead Nebula - aren’t really that visible to the naked eye. Even if you were right up on it, you’d probably have to leave the camera shutter open for a bit to capture all that light.
That is to say, that astrophysical plasmas are pretty sparse. By comparison our atmosphere feels like a thick, pea soup. The particles inside those astrophysical plasmas don’t really smash into each other like they do down here on Earth. Rather, the particles interact via the longer range, electromagnetic force.
Astrophysicts will sometimes call them Colisionless plasmas to emphasize that fact. The gas behaves less like a game of billiards and more like… traffic… or a flock of birds.
Fermi Acceleration
In a diffuse, astrophysical plasma there are really three components to worry about. The electrons with negative charge, the ions with positive charge and the magnetic field itself.The importance of the magnetic field can be felt even here in our solar system. Like the Earth, the sun has a magnetic field. A bit one. Unlike the Earth, the sun is constantly producing a large stream of energetic partic...
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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 Particle Data Group's write up on cosmic rays. See Figure 29.8 for a representation of the "ankle" feature in the spectrum.
https://pdg.lbl.gov/2019/reviews/rpp2019-rev-cosmic-rays.pdfAnother representation of the power laws can be found in Professor Peter Gorham's Coursework on Ultra High Energy Cosmic Rays: http://www2.hawaii.edu/~gorham/UHECR.html
Cosmic Rays
Part 1 - Particles from SpaceWell before the gigantic particle accelerators like the Large Hadron Collider at CERN or the Tevatron at Fermilab, particle physics was studied with balloons.
Well. It still is. I don’t want to give an overly simplified take on the history of particle physics - but it’s fair to say that physicists will study high energy particles WHEREVER they can find them. And it just so happens that a large number of really high energy particles are constantly bombarding us from space.
In our prior series on the ALPHA PARTICLE, we learned about the solar wind and how the Earth’s magnetic field catches much of that ionizing radiation, which we occasionally see displayed as the Aurora.
Well. That magnetic field is no match for these cosmic rays, which come flying at us from deeper in space with much higher velocities than anything in the solar wind. These particles smash right though the magnetosphere and into the molecules of the upper atmosphere.
From our perspective on Earth, these cosmic rays appear as showers of debris left over from those high altitude collisions. But they’re happening all the time.
There’s so much debris out there that hundreds of particles - particles of that debris - have passed through you since you first hit play on this episode.
### Cosmic Rays
On the 15th of October, 1991, a particle with enormous energy entered our upper atmosphere.
A particle with this much energy had never been seen before on Earth. At least by humans. It had tens of millions of times more energy than anything produced at the Large Hadron Collider or FermiLab.
All told. It had the kinetic energy of a baseball moving at around 60 miles per hour. All Packed. Into. A single. Particle. And it was heading right for us.
The first thing it found upon arrival at Earth was the magnetic field. Traveling at such a high speed, it barely noticed the deflecting force and smashed through through into the atmosphere.Even in the rarified air of our upper atmosphere, tens of kilometers above the Earth’s surface, there are plenty of molecules to go around. Way more than you’d find in outer space. And what do particles from space with a lot of energy do when suddenly surrounded by a bunch of molecules? They spend it.
That poor first molecule it encountered didn’t stand a chance. Whether it simply lost an electron or got completely blown apart is hard to say, but that incident cosmic ray quickly broke apart into a shower of particles high above the Earth.
Pions were certainly created, but with that much energy - 50ish Joules of energy - all kinds of particles - from Lambda Zero to Kaons to the Cascades and Sigma baryons - could have been present.
And all of them decayed as they usually do.
The resulting shower of decays grew wider and wider, until the final, resultant charged pions decayed into muons. And the neutral pions decayed into photons. And any high energy photons decayed into electron-positron pairs who would in turn radiate the rest of the energy away.
That final blast of radiation filled a circle kilometers wide that slammed into the US Army’s Dugway Proving ground in Utah desert and - as luck would have it - right into the detectors of a physics experiment.
The detectors measured resultant spray and were able to back-calculate the energy of the original, impinging particle from space.
This aptly-named Oh-my-god particle was far and away the highest energy particle ever detected. To date, it’s not entirely clear what could accelerate a particle to such outrageous speeds.
And the current candidates are, frankly, terrifying.
## The Power Law
In some respects, cosmic rays are kind of like earthquakes. There’s a lot more little ones than there are big ones. For earthquakes, big ones with lots of shaking - magnitude 7 or 8 - are, mercifully, fairly uncommon around the world. Small ones, like magnitude 1’s or 2’s happen almost every day.Cosmic rays follow a similar law.
Cosmic rays with an energies around 1000 MeV arrive almost continuously, from every direction, where as cosmic rays with energies a million times that might strike near you a few times per year. Those ultra high energy cosmic rays, a bit like the oh my god particle, whose energies can be measured macroscopic units like Joules? They might strike your whole TOWN maybe once a year, if that.
As scientists would say, the frequency depends inversely on energy.
More precisely, the frequency is derived from a POWER LAW. A power law for cosmic rays implies that the relative likelihood of two events, with energy E1 and E2 is proportional to their ratio, raised to some exponent.
Power laws are simple enough to understand, but difficult to explain. Bigger cosmic rays are less common, sure. But what’s difficult to explain about a power laws is that little number. The scaling exponent. Where does it come from?
It’s a collective effect. All the things inside and outside our galaxy that throw cosmic rays at us contribute to that effect. Other stars and planets sometimes get in the way and capture some of them. Cosmic rays in interstellar space sometimes decay or interact en route, turning into other particles with somewhat lower energy, muddying things a bit. Magnetic fields in space - like the one surrounding our Earth - deflect cosmic rays ever so slightly.
It’s a bit like studying Earthquakes. There are just so many moving parts and we don’t know all the details.
## The Ankle
Despite all this complexity, the shape of the cosmic ray power law can tell us a LOT about the nature of cosmic rays.
For example.
At the highest end, for those particles coming in with the HIGHEST energies, the power law for cosmic rays actually changes a bit. It flattens out. It becomes slightly less sensitive to Energy.
Ultra high energy Cosmic Rays aficionados call this change the “ankle”.
The ankle represents a cut in the cosmic ray spectrum at about half a joule of kinetic energy. There’s a pretty good plot physicists who study these things have made. I’ll include links to a picture in the show notes.
It’s a small effect on a small fraction of the total number cosmic rays we see on Earth. But to astrophysicists who study these things, it says a lot.
Namely, this “kink” or “ankle” in the power law suggests a possible change in where those cosmic rays are coming form. The leading explanation is that those ultra high energy cosmic rays - those cosmic rays with kinetic energy greater than half a Joule or so - are coming from OUTSIDE our galaxy.
Which is just as well. Becau...
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The Field Guide to Particle Physics : Season 2
https://pasayten.org/the-field-guide-to-particle-physics
©2022 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 Omega Baryon
Introduction
The Omega Baryon is the strangest particle we have encountered so far. It may also be the strangest particle known to Science, literally.
With a mass of 1672.4 MeV, the Omega Baryon is heavy. As well it should be. It is comprised of three, strange quarks. The three strange quarks gives the Omega an electric charge of three times minus one third, or minus one.
Those strange quarks also gives it the unusually long lifetime of about 8% of a nanosecond.
While short by our standards - even a bit shorter than some other strange particles - a solid fraction of a nanosecond is an enormous lifetime for a particle with such an enormous mass.
The Decay of Strange Quarks
As if on brand, this strangest of the strange particles lives for so long precisely because its made from only strange quarks. The strange quarks, you might recall, struggle to decay. They wouldn’t decay- actually - if not for a mild identity crisis.
The strange quarks talk to other particles both by photon, gluon and by W boson. That is, in addition to the electromagnetic force, strange quarks communicate via both the strong and weak nuclear forces. From the strong force’s perspective, strange quarks are distinct. Just like the up and down quarks. Nobody is confused, all that that subnuclear gu respects their identity as strange quarks.
The weak force hedges a bit. The W-boson in particular is a little confused on who is who, and from its perspective down and strange quarks are a little mixed. Just like North and West mean slightly different things to a compass or a cartographer, down and strange quarks appear slightly different to the strong and weak forces. They’re almost aligned, but not quite.
As a result, the strange quark decays by W boson as if it were a down quark. That decay is amplified by the strange quark’s heavy mass, but its still a small effect. The weak nuclear force is… well… weak.
Being made of three strange quarks, the Omega baryon decays once one of its constituents does.
Omega Baryon Decay Channels
The Omega minus decays when one of its strange quarks throws out a W boson, changing its identity to an up quark. Typically the W-boson then decays to a pair of quarks itself, an antiup and a down quark. This all happens quickly inside the baryon itself, which subsequently explodes into a pair or triplet of particles. There there are a number of possible results.
Two-thirds of the time, that anti-up quark SCORES and runs away with one of the Omega’s other strange quarks, creating one of those tricky K minus mesons that we’ve discussed previously. What’s left over? An up, a down and a strange quark, which manifests as a Lambda zero baryon.
Twenty-three percent of the time, that anti-up quark isn’t so lucky. It remains stuck so the down quark that came from W boson, which together run away as a negatively charged pion. The quarks that remain - two strange and an up - comprise the neutral cascade or Xi baryon, which of course leads to its own cascade of particle decays.
Almost all the rest of the time - that’s about 8% for you bean counters out there - the Omega baryon spits out a neutral pion, decaying to a Xi minus instead. For this to happen, that down quark has to hold on tight to that pair of strange quarks that didn’t decay.
On extremely rare occasions, instead of a neutral pion, the Omega decays to Xi minus by spitting out a pi+ pi- pair. This could happen, for instance, the resulting up and antiup quarks happened to find a down-anti down pair inside the subnuclear goo.
Spin and the Decuplet
In addition to having three strange quarks, the Omega baryon also has three times the angular momentum of most baryons we’ve encountered so far. Inside the baryon, those three strange quarks are all zooming around each other, extra fast.
We’ve seen this behavior before, when we looked into the Delta baryons. The Delta Plus Plus baryon, you might remember had three up quarks. And the Delta Minus baryons, which had three down quarks.
In a sense, the Omega baryon is the strange version of those beasts. Because of that simple, three strange quark arrangement, the Omega baryon was predicted to exist well before it was found.
Well, that’s… not exactly right. In fact, it’s exactly backwards. Back in the 50s and early 60s nobody knew what a quark was, or how baryons were even organized. They just had all those wild names: Delta. Sigma. Xi. This zoo of strange particles was something of a mystery.
The physicist Gell-Mann (and, independently Ne'eman) chased the patterns of all the particles and their decays and divsed the quark model to fit those data. There was only one problem: one particle was missing.
The Omega baryon was discovered as a short stub of a line which appeared on a photographic plate at Brookhaven national Lab. It had essentially the same mass, spin and charge that Gell-Mann predicted, ushering in the first of many experimental verifications of the quark model of subnuclear physics.
And that concludes our second - STRANGE - season of the Field Guide to Particle Physics! We’ve got a few bonus episodes, stories and other extras in store, including a bonus series on Gell-Mann’s Eightfold Way and more details on how particles like the Down and Strange quarks mix. So please stay tuned and subscribe for more!
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The Field Guide to Particle Physics : Season 2
https://pasayten.org/the-field-guide-to-particle-physics
©2022 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 Cascade Particles
Prepare for trouble! And make it double! Today we confront the two Cascade or Xi /ksee/ baryons which each have a PAIR of strange quarks.Xi minus checks in with a mass of about 1322 MeV, making it the heaviest baryon we’ve encountered so far. This is just as well, as it comprised of two of those heavier, strange quarks. Together with a third, down quark, it also has a total electric charge of minus 3 thirds or… minus one.
Xi 0 is just a little bit lighter with a mass of 13 hundred and 14 MeV. Its two strange quarks are paired with an up quark, which gives it an electric charge of twice minus one third PLUS one third, or… zero.
Decays of the Xi minus
Like many strange particles, the cascades take quite a while to decay. The Xi minus takes a solid fraction of a nano-second, the usual time it takes to convince one of those strange quarks to decay into an up quark. The result? The strange-strange-down bag of quarks converts to up-strange-down bag, otherwise known as the Lambda 0 baryon. As usual, that decay is accompanied by some other junk, and in this case the net result is a pi minus.
As we’ve already seen, the Lambda 0 and the pi minus are both unstable themselves. The former converts to either a proton or a neutron and the latter typically decays to a muon and then an electron.
If you tried to sketch that all out, you’d find a LONG decay chain of a LOT of different particles. This gives the cascades their name. Producing ONE Xi baryon results in a cascading SHOWER of particles all the way down to that familiar, stable stuff like protons, neutrons and electrons.
Now the Xi minus ALMOST ALWAYS decays to the Lambda 0 with a pi minus. Like 99.8 percent of the time! The rest of the time we find some cuter decays, each incredibly rare, happening less than a thousandth of a percent of the time.
These rarer decays shed some rather alarming light on the very identity of the strange quark. But BEFORE we get to that, we should talk about the Xi 0.
Decays of the Xi 0
The Xi 0 takes about TWICE as long as the Xi minus to decay, which is still, only a third of a nano-second. What’s short for humans is a seriously overripe old age for an elementary particle.
Like it’s partner, the Xi 0 decays into a Lambda0 with a pion. This time a neutral pion. This happens 99.5% of the time. These decays are a little twisted.
Its is the same thing we saw with those charged sigma baryons. We need to convince an a bag of of three quarks: up-strange-and-strange, to decay in something that looks like the up-down-strange bag of the Lambda zero. This is troublesome precisely because the strange quark ONLY EVER decays to an up quark.
As with decays of the Sigma baryons, things just need to rearrange a little bit. The Sigma Plus you might recall decays into a proton and a pi zero thanks to a W boson. Similarly, one strange quark of the Xi0 decays to an up quark by emitting a W- boson. The W- decays to a down-anti-upquark pair so fast the rest of the quarks barely notice. The down quark runs away with the other “up and strange” quarks from the original Xi 0, and the anti-up quark leaves with the freshly minted up quark as a neutral pion.
If that sounds convoluted, it is. It helps to have a diagram to look at, which you can do on our website, pasayten.org.
Incidentally, the other, rarer decays of the Xi zero match up quite nicely what you’d expect from those rare decays of the Xi minus. More on those in a later episode.
Spin Angular Momentum
The spin angular momentum of both Xi 0 and Xi minus is simply hbar over 2, just like the proton and the neutron. Well those and the three Sigma baryons AND the Lambda baryon.
That’s a total of eight distinct particles, which is a lot, but you can rest easy knowing there are NO other particles with that spin angular momentum made from combinations of those three quarks. And this is no accident.
Any OTHER, three particle combination of up, down and strange quarks - like a baryon with three of the same quarks, like the Delta Baryons we’ve already seen - will have a higher spin angular momentum.
You could think of that as if the quarks were orbiting each other inside that subnuclear goo at a faster and faster pace. And we’ll meet some examples of these soon enough.
The story behind WHY this should be is fascinating, nerdy and otherwise adds some useful scaffolding to an ever expanding ZOO of wild, subatomic particles.
More on that, next time.
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The Field Guide to Particle Physics : Season 2
https://pasayten.org/the-field-guide-to-particle-physics
©2022 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 Sigma Baryons
Introduction
Weighing in at 1192 MeV, the middle-weight sigma baryon is also the the electrically neutral one.The Sigma Baryons are a trio of strange, slightly heavy cousins to everyday particles like the proton and the neutron. We’ve already talked at length about the charged Sigma baryons. Today, we’re focusing on their electrically neutral sibling, Sigma Zero.
While the decay resistant charged sigma baryons - with their unusually long lifetimes - certainly qualify as “strange” particles, the sigma zero feels far less strange. At least at first.
The Sigma Zero decays rapidly. Tens of trillions of times faster than its charged siblings Sigma Plus or Minus. If you’re into really small numbers, or just to measure time in seconds, that’s a decimal point followed by 19 zeroes before you get 7 and then a four.
0.000000000000000000074 seconds
That’s too short a time for us to fathom, but its about right for an unstable particle that heavy.Remember, it is STRANGE that the typical lifetime for strange baryons like Lambda Zero or the Charged Sigmas can be measured in nanoseconds.
So why does the sigma zero baryon decay so quickly? OR why do we even consider it to be in the “Strange” family?
Decays
One reason to consider sigma zero “strange” is because it decays to a strange particle. Specifically, it decays, 100 percent of the time, to a lambda zero.In the process, the sigma zero throws out a photon - that is, a gamma ray - which itself might be hard to explain. You see, photons carry the electromagnetic force. Photons are passed around like baseballs between particles that have an electric charge. Photons can be thought of as building blocks for electric and magnetic fields. SO what business does the uncharged Sigma Zero - or Lambda Zero for that matter - have interacting with a photon?
Electrically neutral pions, you might recall, decayed into a PAIR of photons. So perhaps it’s not weird. But pi zero decays were something of an anomaly. Literally. You might recall that pi zeros decayed to two photons because of the chiral anomaly. It involved these wild, quantum mechanical beasts known as instantons. Very nonlinear, very intricate, unusual stuff. In some sense, the neutral pion just vaporized into the electromagnetic field.
This is decidedly NOT what happens with Sigma zero. It doesn’t vaporize. It just decays like any other particle. So what gives?
To understand HOW an electrically neutral particle could spit out a photon, we have to look inside the baryon to that subnuclear goo of quarks and gluons.
Innards
The Sigma baryons are all bona fide strange particles, they all have a strange quark. Sigma Plus had two up quarks and a strange quark. Sigma minus shad two down quarks and a strange quark.
Can you guess what a Sigma Zero has?
One of each. Up, down and strange.
But wait. Wasn’t the Lambda Zero ALSO made up from an up quark, a down quark and a strange quark? Well yes. And that fact explains in fact, why the sigma zero decays so quickly. It decays to the lambda zero because they both share the same number and kind of internal or valance quarks.
As it turns out, the Sigma Zero is something of an “Excited” version of the Lambda zero. Internally, you might say that the up and down quarks are buzzing around in a slightly different configuration. A configuration with slightly more energy. They’re a little more spun up, as it were. That bit of spin energy gets released by the emission of a photon, leaving that bag of quarks and gluons with lower internal energy, otherwise known as the particle Lambda Zero.
E = mc^2 after all just means that the MASS is proportional to ENERGY.
Including Photons
The internal structure of the Sigma Zero also explains why an electrically neutral particle can throw out a photon. It’s just electrically neutral on AVERAGE. The average value of the electric charges of all the quarks is zero. But individually, they each have a charge.
This brings us back to the story of the neutron. While the AVERAGE electric charge of a neutral baryon is zero, the electromagnetic field need not be identically zero.
Like the neutron or the earth, the Sigma Zero baryon has a nonzero magnetic dipole moment. It probably should also has an electric dipole moment. All this means is that the electromagnetic fields kind of averages out to zero, but are still smeared out, in a way.
And it’s these smeared out configurations that allow the Sigma Zero to throw out a photon and decay to a lambda zero. Or at least, that’s another fun way to think about it.
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The Field Guide to Particle Physics : Season 2
https://pasayten.org/the-field-guide-to-particle-physics
©2022 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 Sigma Baryons
Introduction
The Sigma Baryons - that’s a capital Sigma - are a trio of slightly heavy cousins to everyday particles like the proton and the neutron.With masses of almost 1200 MeV each, it may surprise you that the physics of Sigma baryons feels much closer to a comparatively puny trio of familiar particles: the pions.
The pions form a triplet of mesons: pi plus, pi zero and pi minus. So too, do the Sigmas: Sigma Plus, Sigma Zero and Sigma Minus. The similarities are helpful for building an intuition, but the differences are stark. While the charged pions are antiparticle partners, the charged Sigmas are anything but.
Today we’ll focus on that fact as we explore the pair of particles Sigma Plus and Sigma Minus.
Charged Sigmas
The charged Sigma baryons are your typical strange particle. They live much longer than they should, given their mass. Their lifetime is a sizable fraction of a nanosecond. Like the Lambda Zero, the charged Sigma baryons live so long because they have to wait for their constituent strange quark to decay.
The strange quark can only decay to an up quark, and while possible, it takes a while. It’s a quantum bottleneck that in particle decays that has come to be known as the technical term “Strangeness”.
While the down quark and the strange quark have separate identities as far as the strong nuclear force is concerned, they mix slightly under the weak nuclear force. That slight mixing is what gives the strange quark a chance to decay.
And it always decays to an up quark.
Keep an eye on this fact. It’s what makes the Sigma baryon decays so tricky.
What’s fun about the charged Sigma Baryons - that is markedly DIFFERENT than the charged pions - is that they are NOT antiparticles for one another. ¿The ANTI sigma plus is NOT the sigma minus. Not even close.
The Sigma Plus has two up quarks and a strange quark. That gives it’s electric charge of two thirds plus two thirds minus one third or one. The Sigma Minus has two down quarks and a strange quark, which contribute a charge minus one third each.
So, despite having opposite electric charges, they have very different quarks inside:up up and strange versus down down and strange. And with that constitutional difference comes more mundane ones: the Sigma Plus and Sigma Minus have slightly different masses AND slightly different lifetimes. They are, in other words, very different particles.
Still. The Sigmas try their best to behave like pions. Isn’t it nice how neatly organized Nature at least tries to be?
At 1197 MeV, the Sigma minus is just a little bigger than the Sigma plus, whose mass is about 1189 MeV. Bigger masses usually imply short lifetimes, but the Sigma Baryons are strange in this sense too. The heavier, sigma minus baryon has a lifetime around 15% of a nanosecond. The lighter sigma plus baryon decays about twice as fast, living on average for about 8% of a nanosecond.
Charged Sigma Baryon Decays
Why does this slightly lighter, sigma plus baryon decay twice as fast?
Sigma Plus has two major ways to decay whereas Sigma Minus has only one.
Sigma Minus only really decays to a neutron and a pi minus. There are other options - including muons, electrons, neutrini and, rarely, a lambda zero-electron pair - which all together occur less than 1% of the time.Similarly, 99% of the time Sigma Plus will decay into a familiar nucleon and a pion. But here’s a slight imbalance between these two options. The proton and pi zero appears just over 51% of the time. The neutron and pi plus happens a bit of 48% of the time.
Amusingly, the other 1% of stuff looks exactly like the anti particle versions of the rare Sigma minus decays. You know, antimuons, positrons and neutrinos. Notably, there’s also a rare Lambda zero with positron decay. Charge has to be conserved, after all.
Because the sigma plus has two ways to decay - two decay channels, in the parlance of particle physics - it’s not surprising that it decays twice as fast as its negatively charged sibling.
Why the sigma minus only has one decay channel relates back to the fact that is NOT the anti particle partner of sigma plus. Despite its negative charge, it’s made of QUARKS and not ANTIquarks. Because there is no negatively charged analog for the proton, there’s nothing else for the sigma minus to decay into.
Some Gory, Decay Details
The details of these decays are fun to examine.The sigma minus - down, down, strange - decays when the strange quark does. The strange quark emits a W boson and leaves behind an up quark. That essentially converts the Sigma minus into a neutron - down, down, up. The W boson promptly decays into a down quark-antiUp quark pair - that is, a negatively charged pion.
Did you get that? Sigma minus decays to a neutron with a pi minus.
The sigma plus - up, up, strange - is a bit more complicated. The strange quark again decays, but the final combination of quarks: up, up, up, down, anti-up, can be rearranged to form a proton: up, up, down and a neutral pion: up / anti-up. Because the W-boson lives for such a short time, that rearrangement all essentially happens at once.
The other possibility for the sigma plus is even more wild. The strange quark decay as usual, laving behind an up quark, but the emitted W-boson is immediately absorbed by one of the other up quarks, which then converts it into a down quark. If a gluon just happen to be emitted at around the same time, it can convert to a down quark-anti-down quark pair, giving a final combination of quarks: up, down, down, anti-down, up. This can be rearranged to form a neutron (up-down-down) and a pi plus (up-anti-down).
Was that complicated enough for you? Converting a sigma plus to a neutron is a little more complex so it doesn’t happen quite as often. To get a better sense of visualization, check out our drawing on the website. But suffice it to say, gluons aren’t hard to find given all that nuclear goo those quarks live with. It’s not all that surprising things work out this way.
We should say that these descriptions are something of a sketch or skeleton of what is actually going on. Physicists doing the full calculation using Quantum Field Theory would call it a tree-level approximation. Quantum effects can sometimes be dramatic, as we saw with the pi zero. Mercifully, not in this case.
Particle physics is nothing if not messy.
Why no Lambda Zero?
If you’re numerically minded - like you accountants out there - you might wonder why these charged Sigma baryons do not decay into a Lambda zero baryon. After all, the mass of the charged sigmas is around 1190 MeV, but the mass of the Lambda zero is only just shy of 1116 MeV.
Energetically, it’s more than possible! But the details matter.
Both Sigma plus and sigma minus can and do decay to Lambda 0 with either a positron or an electron, respectively. But it’s a needle in a haystack. For every MILLION charged sigma baryons ...
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