The Field Guide to Particle Physics

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-4.0

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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...

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-communication

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.

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 Outsider

Science 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:

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 Complaint

Here 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 Sfermion

The 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 Monopole

Magnetic monopoles are another broad class of particle. An electric monopole is a particle like an electron, proton or even an

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-colliders

This 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.

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-collider

Four 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-physics

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 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 Excess

Space 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...

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/antineutrino

The 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 Decay

Neutrons 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...

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 Antineutron

Like 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 beams

Antiproton 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.

Antiprotons

Antiparticles 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 annihilation

Electrons 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.

The Field Guide to Particle Physics : Season 3
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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!