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 The eBook The Field Guide to Particle Physics eBook is now available! If you're looking to support the show, we've got some fun options for you here, or you could buy us a coffee! 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: “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 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 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 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!

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 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!

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!

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.

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.

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 ...

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 Kaons Before we can describe neutral kaons in the wild, we need to take a quick detour to the city. South of Downtown Seattle resides the historic Pioneer Square. Bricks line the sidewalks and the ends of metal, reinforcement bars stick out of the old, stone buildings. Approaching the iconic Smith Tower, you cross Yesler Way, and something happens. The direction of the streets change. South of Yesler, the streets all run north-south. The grid, in other words, matches the cardinal points of the compass: north south, east west. When you cross Yesler, confusingly, everything rotates by 45 degrees. The streets of downtown Seattle - between Yesler Way and Denny Way - all point Northwest. The rotation of streets is made to match the contour of Elliot Bay. The waterfront goes NW to SE. Streets run parallel and perpendicular to it. It’s often easier for us to think in terms of north south east west, but sometimes the geographic features of the terrain forces a change. There are still two directions, it’s just a shift in our frame of reference. This idea of a 45-degree shift in our frame of reference, mixing North with West and South with East, is very similar to how the neutral kaons behave. Introducing the Neutral Kaons There are two, electrically neutral kaons. K0 and K0bar. Like the eta and eta prime mesons - which are also electrically neutral - these neutral kaons are mesons that include a strange quark. The strange quark is like the down quark, only quite a bit heavier. Because it’s heavier, the particles comprised of them like the kaons, also tend to be heavier. Mesons - like the kaons - are quark molecules made from one quark and one antiquark. The strange quark is a heavy version of the down quark, and so both have an electric charge of -1/3. Neutral kaons are combinations of to quark-antiquark pairs: down-antiStrange and strange-antiDown. These two combinations are called K0 and K0bar, respectively. With masses just shy of 500 MeV, these neutral kaons are heavy. Unlike the heavy and short-lived eta and eta prime mesons, the lifetime of these neutral kaons is considerably longer. Like the Lambda zero baryon, the strange quark makes it difficult for the neutral kaons to decay. How they decay brings us back to the streets of Seattle. The Long and Short of it The charged kaons presented a mystery because they could decay to both three pions and two pions. This confused particle physicists for quite some time. Neutral kaons also share this curious property, with nuance, of course. Like the street map of Seattle, K0 and K0bar mix. While moving through space, a K0 can spontaneously change into a K0bar and vice versa. This mixing is something of an artifact of an even stranger phenomena. Strictly speaking, neither the K0 nor K0bar mesons interact with Weak nuclear force. Only mixtures of them interact the W bosons. Just like North and East can combine to Northwest and Northeast, we think of these kaon combinations as K0 plus K0bar or K0 minus K0bar. It’s like a rotation by plus or minus 45 degrees. How is the possible? In a word, Quantum Mechanics. You might say that the Strong nuclear force - the thing that binds quarks together into bigger particles - respects the individual identities of those quarks: up, down and strange. The Weak nuclear force, however, does not. These particle-antiparticle oscillations are similar to the flavor oscillations experienced by the three flavors of neutrini. The “minus” combination is sometimes called K-short, because it decays relatively quickly: just shy of 8 percent of a nanosecond. The “plus” combination is sometimes called K-long, which takes 50 nanoseconds,. Any nanoseconds is an eternity by particle physics standards, but it’s remarkable that K-short decays 1700 times faster than K-long given that they’re made of the same things. In some sense the K0-K0bar mixing occurs because the “K-short” combination wants to decay so much faster than “K-long”. Given a beam full of ONLY K0 particles, it will eventually turn into ONLY a beam of K-long particles. The K-shorts will all decay. K-short decays into pairs of pions, either two pi0s or a pi plus-pi minus combination. K-long decays into three pions: either one of each or three pi zeros. K-longs actually decays into electrons or muons too - or their associated antiparticles - along with a single charged pion of opposite charge. In Summary The number of fine details in the mixing of neutral kaons is staggering. I’ll give you three examples. First: if a K0 decays into an electron-like particle, it’s virtually always going to be the anti-electron with a pi minus. Second, and oppositely, if K0bar decays like that, it will always output an electron with a pi plus. Finally, there is an extremely small mass difference between K-long and K-short. 3 parts per trillion. The quantum mixing of the two neutral kaons is just the first hint that particle physics only gets more complex and strange as we delve deeper into it. So take a breath. Relax your shoulders. And then get excited. There’s still plenty more to see.

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 Eta and Eta Prime Mesons The Eta and Eta Prime particles are a pair of electrically neutral particles that were - for a moment anyway - the center of a fierce debate among physicists. That debate ended as quantum chromodynamics - the mathematical theory which describes how quarks and gluons interact - was enshrined into the standard model of particle physics. Those mathematical details involved in describing eta and eta prime are almost as fierce as the debate over how they worked. And those details are what we’ll describe today. With the strange quark, three are three kinds of quarks that can combine into all sorts of particles: up, down and strange. Of course, they each have antiparticle partners: antiup, antidown and antistrange with opposite electric charge. Mesons are particles that mix quarks and antiquarks. The neutral pion is a pretty clean mixture of direct quarks/antiquarks pairs: up-antiup with down-anti-down. Nature can also make nice, clean, symmetric combinations of all three quarks: up-antiup, down-antidown, strange-antistrange. Actually, it can make two of them, because, you know, strangeness. Those two combinations are known as the eta and eta-prime mesons, respectively. Like the pi0, both eta and eta prime have zero electric charge, but these strange mesons are heavy. Eta itself weighs in at 547 MeV, a good four times the mass of the neutral pion. Eta prime’s mass is an outrageous 957 MeV, heavier than both the proton and the neutron. The reason the eta mesons are so heavy is related to the fact that the neutral pions decay so quickly. You might remember that the neutral pions decay much faster than the charged pions. In some sense, the charged pions are protected from decaying by the conservation of charge. To be precise, pi0 mesons decay via the chiral anomaly. Quantum mechanics gives rise to sudden host of all the quarks appearing all at once which vaporizes into a pair of photons. Like the pions, the eta and eta prime mesons are electrically neutral. They have no electric charge to conserve and also decay through that quantum mess at at a much more reasonable 10^-19 and 10^-21 seconds , respectively. The lighter, eta meson typically decays just like the neutral pion, that is into a pair of photons. Sometimes it will decay into triplet of pions! Either all three pi0 or one of each, pi + pi - and pi0. The heavier, eta prime meson typically decay into the eta meson and a pair of pions: oppositely charged or neutral. Not infrequently, eta prime will decay instead to a photon plus an unstable, neutral rho meson, which is like the neutral pion, except its constituent quark antiquark pairs are orbiting each other. The same mess of quantum effects that causes all these neutral mesons to decay quickly has one more interesting effect on the eta and eta prime mesons. It explains their heavy masses. The cloud of quantum particles - all those quarks appearing all at once - collectively act to impede their physical motion through space. Physicists have a word for this phenomena. It’s called a mass. Different particles experience this mess in different ways, which explains their different masses. The pions barely notice. The eta meson feels it a bit. The eta prime meson feels it the most, which is also why it is the heaviest of the bunch. The debate amongst particle physicists amounted to precisely how these masses came about, and how the Chiral Anomaly was involved. We now understand that the quantum mess of quarks which causes the pi zero and eta to decay to photons and which gives eta prime its enormous mass, are related to instantons - which are like kinks, wrinkles and textures in the quantum subnuclear goo we’ve been talking about. You know, that amorphous stuff that surrounds all these quarks inside particles.

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. Charge Kaons Strangeness - as a property of particles - was an attempt to explain why some particles took a really long time to decay. By that measure, the charged Kaons are definitely strange. Weighing in at 493 MeV, the charged kaons are heavy. Three times as heavy as the pions. And yet, given their mass, it’s surprising that they’re lifetime is also measured in nanoseconds. About 12 nano seconds, actually. Quite long for such heavy particles. The charged kaons are composed of a strange quark and an antiup quark. Or an up quark and an anti-strange quark. Kind of like pions. Only STRANGE. Kaons are strange in more than the technical sense. Their decays confused everyone for quite some time. In the late 1940s, particle physicists discovered a few strange particles that all seemed to have about half the mass of the proton, but decayed very differently. Something physicists originally called the tau+ (a historical name, which should not to be confused with the tau lepton) decayed into THREE pions. A pi+ and two neutral pions. Something originally called the theta+ also decayed into pions, but only one pi+ and ONE pi 0. Confusingly, these taus and theta appeared to be IDENTICAL otherwise. They should have been the same, actually, except for those different decays. And up until that time, no particle had ever been seen decaying to BOTH two AND three pions. How could the tau+ and the theta+ be the same particle? It would be as if you were BOTH totally left handed AND totally right handed. Like you were literally your reflection in your mirror, but only sometimes. These ideas are captured by the idea of PARITY, nothing more than a twist on the idea of left and right handedness. Decaying to THREE pions suggested that the kaon parity was ODD. But decaying to TWO pions suggested it had even parity. Numbers can’t be BOTH EVEN AND ODD, how could particles? This might seem like an abstruse problem to have, but to physicists at the time, conservation of parity seemed as vital as the conservation of angular momentum. We know better now, as kaon decay involves strange quarks which each via the weak nuclear force. The weak nuclear force - carried by W and Z-bosons - violates parity explicitly. Maximally, as it turns out. The charged kaons decay to muons about 63% of the time. Those two pion decays? That’s just over 20%. The three pion decays? Just about 5.6%. There are crazy things too, like the so-called semileptonic decays which include BOTH pions AND electrons or muons. Things get complicated when the masses get large. But this is only the tip of the strangeness iceberg. There’s plenty more to come.

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. Strange Quarks Quarks make up baryons like the proton and the neutron. Or more exotic things like the Lambda0 or the Delta++. Previously, we’ve learned about the up and down quarks - those fundamental constituents of matter like protons and neutrons. Today we’re learning about the third of the three light quarks - the strange quark! The stability of quarks against decay is a quirky thing to talk about because the context matters a lot. One of the down quarks in the neutron will decay - in about fifteen minutes or so - to an up quark, shooting out an electron and leaving behind a proton. But that’s only if the neutron is alone. If its bound together with a proton in an atomic nucleus, it is stable. The strange quark will also decay - also typically to an up quark - and it’s lifetime also depends on the context. The Lambda baryon - like the proton - is a big bag of nuclear goo with three quarks banging around inside. It has an up, a down and a strange quark. The thing about strange quarks is: particles that contain one tend to live longer than you’d expect. The reason for this? There are a few. First, it necessarily decays via the weak force, which is always slower to move. But it’s worse than that. There’s just not much for a strange quark to decay into. Heavier particles can only decay into lighter particles, and the strange quark is just not heavy enough to provide many good options. There is literally one option: the up quark. To decay into an up quark, strange quark must emit a W-boson. It must behave like a down quark. The window for that decay is small! It might be 13 percent the size of a down quark. But between us, it’s kind of surprising that this happens at all! That 13 percent suppression is related to something astonishing called the Cabibbo angle, which we’ll come to at the end of the season. In any case, getting the right window to decay to an up quark takes awhile, which is why strange particles tend to live so long. Just like it’s awkward to talk about the lifetime of a strange quark, it’s also awkward to talk about it’s electric charge. Quarks always show up in groups - and its their collective, electric charge that matters - but for the bean counters out there, the strange quark has an electric charge of minus 1/3. Just like the down quark. In a lot of ways, the strange quark is just a heavy version of the down quark. How heavy? Fairly. Compared to the humble 4.7 MeV of the down quark, the strange quark’s mass is around 95 MeV. It’s possible - in some corners of our universe - that strange quarks could be stable. But this is probably only true at stupidly high pressure. For example, It’s possible that they exist inside neutron stars. Neutron Stars are what’s left over after a star goes supernova. A fragment. A tight ball of matter that fell in on itself. Without the constant outward pressure given by a star’s normal nuclear furnace, what’s left of star falls inward, pulled together by gravity. Neutron star matter so tightly bound it’s basically like one titanic nucleus. These are stars that are just on the precipice of becoming black holes. In this context. It is possible that the strange quarks can exist stably. For the experts, the Pauli Exclusion principle creates an effective, statistical pressure - modeled say by the Lennard-Jones potential - on the up and down quarks in the neutron star that can be relieved - in part - by the inclusion of strange quarks to literally mix things up. At least, that’s what the models say. Whether we’d ever be able to test those models directly, is another matter.

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. For more technical details on the Lambda 0 baryon, including how why it decays to an up but not a down quark, check out this short, informal video. The Lambda 0 Baryon With a mass of well over 1115 mega electron volts, about 20 percent more than the proton, the Lambda 0 is a rare, but historic particle to find. Inside the Lambda Baryons you’ll find an up quark, a down quark and… something else. The first Lambda Baryon was first observed in 1950. In what amounts to a weather ballon. Way up high in the atmosphere. On a photographic emulsion plate. Particle physics was a different game back then! Physicists KNEW it was a baryon because it decayed into a proton. And it was heavy. Heavy particles decay quickly, and the Lambda was heavy. But it did not decay quickly enough. It stayed around for quite a while, a bit less than a nanosecond, which is rather long by particle physics standards. Especially for a baryon. Very strange. The long lifetime of the Lambda 0 was so strange that physicists knew there was something special about that particle. It had a special property. And in the 50’s this new property of particles was showing up in more and more experiments. Because they had no idea what it could be, that called this property strangeness. For better or for worse, that technical name stuck. The new particle that makes up the Lambda 0 baryon is called a strange quark, and we’ll visit them soon enough. Suffice it to say, a Lambda 0 baryon is composed of an up quark and a down quark and a strange quark. The Lambda 0 has no electric charge. It typically decays to either a proton or a neutron. If it’s a proton - which happens about 63 percent of the time, it spits out a negative pion in the process. If it’s a neutron - which accounts for most of the rest of the decays - it spits out a neutral pion instead. Using this quark model, we can explore the decay of the Lambda 0 in sharper detail. Inside the Lambda 0, the strange quark decays into an up quark, which converts that big bag of nuclear go into that proton or neutron. To do that, it must emit a W boson. Those familiar force carriers then decay into to a pion. As per usual. The Lambda 0 was just the first of the strange family of particles to be discovered. This entire season will be devoted to these strange particles.