The last word Field Guide to Subatomic Particles [Io9 Backgrounder]

Muons, neutrinos, supersymmetric partners, the infamous Higgs boson – with so various subatomic particles flying about, it’s no wonder theoretical physics will likely be so confusing. That’s why we made this (reasonably) simple guide to the whole different elementary particles.

This is, as you may imagine, a fairly large topic, so we’re splitting it into (no less than) two posts. Today we’re going to accommodate just the particles that physicists are certain (or, at the least, reasonably certain) exist, and then tomorrow we’ll get into the even stranger world of particles which have been hypothesized but may or would possibly not actually exist. I’ve also made a handy cheat sheet listing each of the elementary particles and their vital statistics, that you may find here. But to grasp what all of that suggests, you’ll really need to read on.

The Basics: Time To bear in mind Your Middle School Physics Class

Part of what makes this task difficult is that there’s so a variety of how one can organize the subatomic particles, counting on which particular property you’re fascinated about. So let’s go back to middle school physics and start with the three subatomic particles many people have heard of: electrons, protons, and neutrons, the three components of any atom. Which means those three are the elemental building blocks of practically all matter – a minimum of, matter of the non-dark variety.

Physics has been shopping for the smallest possible style of matter since the time of the ancient Greeks – the word ” atom” has its origins within the Greek word for ” indivisible.” In fact, the work of early twentieth century physicists like J.J. Thomson and Ernest Rutherford proved the atom actually was divisible, but it surely wasn’t until the late 1960s that scientists discovered protons and neutrons themselves were composite particles, the end result of combining still smaller particles referred to as quarks.

This brings us to 1 of probably the most basic divisions of subatomic particles: there are elementary particles, like electrons and quarks, and composite particles, like protons and neutrons. Elementary particles don’t have any substructure and no smaller constituent parts…no less than, none that theoretical physicists can currently agree upon. (Here is where string theory enters the picture, but since I’m only writing a post and not a complete book, we’ll leave that to at least one side for now.)

Now, one of the crucial basic properties of electrons, protons, and neutrons is their charge. Electrons are negative, protons are positive, and neutrons are, unsurprisingly, neutral. It’s the horny electromagnetic force between protons and electrons that holds atoms together, and it’s different charges of assorted ions that causes atoms to combine into molecules. But let’s keep this subatomic.

Quarks of Every Flavor

The thing to remember is that each one particles should have an electromagnetic charge of -1, 0, or 1 – the costs of electrons, neutrons, and protons respectively. (Actually, certain particles could have a charge of 2 or -2, but the really crucial bit is that each one charges need to be integers, and -1, 0, and 1 are by far the most typical.) Anyway, that’s all simple enough, right? Good, because I’m about to contradict that totally. This would be a recurrent theme.

Like I said, protons and neutrons are made from different combinations of quarks. Quarks are one of the most three major groups of known elementary particles. There are six types, or flavors, of quarks, half of which can be up-type quarks with positive charge and the opposite half are down-type quarks with negative charge. Here’s the list of the flavors and their charges: up, charm, and top quarks are +2/3, while down, strange, and bottom quarks are -1/3.

I imagine you’ve spotted the contradiction. I just said all particles have an integer charge of -1, 0, or 1, but these types of quarks have fractional charges. What gives? It is because quarks don’t exist independently in nature – they’re always found combined with one or two other quarks. These combinations of quarks are held together by the strong nuclear force and are collectively often known as hadrons, a word you may recognize from CERN’s starcrossed particle accelerator, the massive Hadron Collider. (By the style, it’s worth noting that quarks once did exist independently, and there were free particles with fractional charge. But this only occurred in the course of the universe’s first 10^-12 seconds, when the laws of physics didn’t quite work within the same way they do now.)

Hadrons make up the majority of composite particles, and both protons and neutrons are hadrons. Specifically, a proton is made from two up quarks and one down quark, which supplies it its +1 charge (since 2/3 + 2/3 – 1/3 = 1). Neutrons have the other structure, that’s one up quark and two down quarks, which makes for a neutral charge (2/3 – 1/3 – 1/3 = 0). You will discover the proton above and the neutron below. Any hadron it truly is made from three quarks is really a baryon. We’ll come back to the alternative kind of hadron in slightly, but first let’s return to electrons.

Electrons, Neutrinos, and Other Leptons

Electrons are the right known members of another major group of elementary particles, the leptons. A dead ringer for quarks, there are six flavors of leptons. These flavors can also be divided into three generations, with each successive generation so much more massive than the only before it. An analogous is right of quarks – up and down quarks are the lightest, then charm and unusual, then top and bottom. Only the lightest leptons and quarks are stable, that’s why up and down quarks form protons and neutrons, and it’s why electrons are found in atoms and not their heavier counterparts.

What are the six flavors of lepton? You already know the electron, and you’ve probably heard of the opposite member of its generation, the neutrino. The neutrino – or, more accurately, the electron neutrino – is a virtually massless particle proposed by Wolfgang Pauli in 1930 to account for the slight loss of total energy and momentum in a process often called beta decay, where a neutron decays into an electron, a proton, and a neutrino. (Technically speaking, it’s actually an electron antineutrino it is emitted, but let’s keep this as straightforward as we will.)

The electron and the electron neutrino are the first generation of leptons, and the opposite four leptons are the muon, muon neutrino, tau, and tau neutrino. The electron, muon, and tau all have the charge -1, while the three neutrinos have a neutral charge. So that’s the quarks and the leptons, which together form a gaggle is called the fermions. Exactly what makes a fermion a fermion is a bit of complicated, but suffice it to claim that fermions are all of the particles that take care of matter. So what in regards to the last group of elementary particles, the ones that don’t take care of matter?

Creating the Forces: The Bosons

These are the bosons, and they handle the basic forces of the universe. We’ve already mentioned a number of them – there’s the electromagnetic force, the strong nuclear force, and the weak nuclear force, that is mostly liable for the beta decay process we were just discussing.

(I should clarify here that there are two varieties of bosons – bosons which can be elementary particles and those who are composite particles. The elementary bosons are more important for the purposes of this discussion, and I’ll be using the term ” boson” as a shorthand for just the elementary bosons. The composite bosons act in more complicated ways, but we don’t wish to worry about that here.)

All of these forces create fields that the matter particles pass through, and the bosons are accountable for carrying these fundamental forces between the matter particles. Without the bosons to act as mediators, the opposite particles wouldn’t have the ability to experience the consequences of a few of the forces, which would mean quarks couldn’t combine into protons and neutrons without the strong force, and neither of these could combine with electrons to make atoms without the electromagnetic force.

There are four known elementary bosons: photons, which mediate the electromagnetic force, gluons, which take care of the strong force, and W and Z bosons, which can be liable for the weak force. All of these have neutral charge, aside from the W bosons, which have -1 charge. Both gluons and photons have zero mass, that means, consistent with general relativity, both of them must travel at the velocity of light. That seems a chunk obvious, I suppose, considering photons are light particles.

There are a couple other bosons that many physicists think exist but have thus far eluded detection: the infamous Higgs boson, which theoretically gives all particles their mass, and the graviton, which would mediate gravity. The usual Model of physics only requires the Higgs Boson, as gravity isn’t considered a fundamental force in that model. However, other theories of particle physics argue gravity is quantized, which requires the existence of a graviton boson to carry that force between particles. (We covered the Higgs Boson in some depth in a previous post .)

And Now For a short Recap

Well then, I feel we’ve got the complete known elementary particles sorted out. Let’s check the table (be happy to click on it for a closer look):

The three values represented in each particle’s box are, from top to bottom, its mass in electron volts , its charge, and its spin. What’s spin? We’ll get to that tomorrow, but suffice it to assert…it’s complicated. Anyway, let’s seriously look into how the quarks, leptons, and bosons fit together to form the matter and forces we experience every day (again, which you can click for a closer look):

Flipping the Script: Antiparticles

We’ve nearly reached the top of of the known particles, but there are two more groups we want to house: mesons and antiparticles. Mesons are the opposite kind of hadron, which feature only two quarks as opposed to the three seen in baryons like protons and neutrons. Now, in the event you’ll have a look at the quark charges, you’ll notice your entire up-type quarks have +2/3 charge, and your complete down-type quarks have -1/3 charge. That suggests there’s no method to combine only two quarks to get a composite particle with an integer charge.

The way out of this mess is that it isn’t actually two quarks that combine to form a meson – it’s a quark and an antiquark. Antiquarks are one group of antiparticle, in conjunction with the antileptons and antibosons, and all of these antiparticles have the other charge of their particle counterpart. As an example, probably the most common mesons, the pion, is made from an up quark and a down antiquark, which provides it a favorable charge (2/3 + 1/3 = 1).

Other than that reversed charge, antiparticles are more often than not the image of their particle counterparts. I say ” basically” because there are still a couple of reversed features. These rather subtle properties include the baryon number , the lepton number , and color.

Those first two properties are fairly easily calculated by subtracting the number of antiquarks from the number of quarks and the number of antileptons from the number of leptons, but what about color? Basically, color is the strong force equivalent of electromagnetic charge, and it’s often called color because it has three basic aspects (compared to the two aspects of charge – positive and negative), that have been called ” blue” , ” red” , and ” green” after the main colors.

But besides these more subtle features and the charge, antiparticles really are practically indistinguishable from particles. They’ve an identical mass, an identical spin, an identical lifetime…indeed, a whole antimatter Earth could theoretically exist, and it’d be exactly kind of like our Earth except all of the charges can be flipped (and a couple of other minor things, but charge stands out as the only obvious difference). In fact, there isn’t an antimatter Earth because, for reasons we’re only now commencing to understand , matter managed to dominate antimatter inside the universe’s earliest moments, and so any antimatter that now comes into existence is readily annihilated when it runs into its matter counterpart.

Here’s the tricky part (well, the even trickier part, at any rate). Antiquarks aren’t just antiparticles – they’re elementary antiparticles, and an analogous goes for other antiparticles like the antineutrino or the antielectron, that is better called the positron. But there are also composite antiparticles, like the antiproton or the antineutron, and the relationship between elementary and composite antiparticles isn’t always straightforward.

To see what I mean, let’s take another examine the pion. It’s a meson, which implies it’s a composite particle. However it’s fabricated from a quark and an antiquark, meaning it’s equal parts elementary particle and elementary antiparticle. And that’s only 1 form of pion – the negatively charged antipion is made from a down quark and an up antiquark (-1/3 – 2/3 = -1). Again, one elementary particle and one elementary antiparticle, but this time the outcome is a composite antiparticle.

This also explains how something like an antineutron can exist. I’ve said antiparticles have the other charge of their particle counterparts, but neutron has no charge. So what’s the variation between a neutron and an antineutron? It’s all to do with the elementary parts. Like I discussed earlier, a neutron has one up quark and two down quarks (2/3 – 1/3 – 1/3 = 0), but an antineutron has one up antiquark and two down antiquarks (-2/3 + 1/3 + 1/3 = 0). Though the web charge is similar, the components are reversed, and that’s why neutrons and antineutrons can annihilate each other just in addition as electrons and positrons or protons and antiprotons can when matter and antimatter meet.

But that only explains the existence of composite neutral antiparticles – what about elementary neutral particles, like the neutrino? We’ve already mentioned the antineutrino, so it should have something that distinguishes it from the neutrino. Well, the quick answer is that…it doesn’t, at the least not as far as physicists can tell. The antineutrino and neutrino are seemingly identical, that means they may be Majorana particles, that is any particle it truly is its own antiparticle.

All the neutrally charged bosons are definitely Majorana particles, but the neutrino often is the only fermion to earn that title. However, the current consensus is that, though it’s not impossible, neutrinos probably aren’t Majorana particles, partly owing to those quantum properties I mentioned earlier. It looks as though the lepton numbers of neutrinos and antineutrinos are different: +1 for neutrinos and -1 for antineutrinos.

There are also composite Majorana particles, comparable to the third form of pion, that is made from either an up quark and antiquark or down quark and antiquark. Since this particular meson is combining a particle with its own antiparticle, it’s not exactly surprising that it’s its own antiparticle.

The End of What We Know

And that almost does it for the complete particles and antiparticles we learn about. Like I said at the start, I’ve made a cheat sheet for the whole known elementary particles – be happy to seek advice from it if you mix up your muons and your gluons. Unfortunately, there are simply too numerous combinations of composite particles to make a cheat sheet for them, but I did include the two most vital hadrons within the elementary particles cheat sheet.

Also, if any of it truly is still confusing – that is perfectly understandable – be happy to depart questions inside the comments or email them to me at alasdair@io9.com . If there are enough questions to warrant it, I’ll do a post next week making sense of everything we’ve discussed this week.

But enough housekeeping. Onward and upward to tomorrow’s topic – the undiscovered particles! I’ll provide you with a warning right this moment, things are going to get weird…

Huge because of our resident physicist, Dr. Dave Goldberg, for clearing up several technical points!

Source

This post is tagged: composite particles, electrons protons and neutrons, elementary particles, ernest rutherford, protons and neutrons, theoretical physics, three subatomic particles