The Ultimate Field Guide to Subatomic Particles
Muons, neutrinos, supersymmetric partners, the infamous Higgs boson - with so many different
subatomic particles flying about, it's no wonder theoretical physics can be so confusing. That's why
we made this (reasonably) simple guide to all the different elementary particles.
This is, as you might imagine, a pretty big topic, so we're splitting it into (at least) two posts. Today
we're going to deal with just the particles that physicists are certain (or, at least, reasonably certain)
exist, and then tomorrow we'll get into the even stranger world of particles that have been
hypothesized but may or may not actually exist. I've also made a handy cheat sheet listing all the
elementary particles and their vital statistics, which you can find here. But to understand what all of
that means, you'll really want to read on.
The Basics: Time To Remember Your Middle School Physics Class
Part of what makes this task difficult is that there's so many different ways to organize the subatomic
particles, depending on which particular property you're interested in. So let's go back to middle
school physics and start with the three subatomic particles most people have heard of: electrons,
protons, and neutrons, the three components of any atom. That means those three are the basic
building blocks of pretty much all matter - at least, matter of the non-dark variety.
Physics has been searching for the smallest possible form of matter since the time of the ancient
Greeks - the word "atom" has its origins in the Greek word for "indivisible." Of course, the work of
early twentieth century physicists like J.J. Thomson and Ernest Rutherford proved the atom actually
was divisible, but it wasn't until the late 1960s that scientists discovered protons and neutrons
themselves were composite particles, the result of combining still smaller particles known as quarks.
This brings us to one of the most basic divisions of subatomic particles: there are elementary
particles, like electrons and quarks, and composite particles, like protons and neutrons.
Elementary particles have no substructure and no smaller constituent parts...at least, none that
theoretical physicists can currently agree upon. (This is where string theory enters the picture, but
since I'm only writing a post and not an entire book, we'll leave that to one side for now.)
Now, one of the most basic properties of electrons, protons, and neutrons is their charge. Electrons
are negative, protons are positive, and neutrons are, unsurprisingly, neutral. It's the attractive
electromagnetic force between protons and electrons that holds atoms together, and it's the
different charges of various ions that causes atoms to combine into molecules. But let's keep this
subatomic.
Quarks of Every Flavor
The thing to keep in mind is that all particles must have an electromagnetic charge of -1, 0, or 1 - the
charges of electrons, neutrons, and protons respectively. (Actually, certain particles can have a
charge of 2 or -2, but the really crucial bit is that all charges must be integers, and -1, 0, and 1 are by
far the most common.) Anyway, that's all simple enough, right? Good, because I'm about to contradict
that completely. This will be a recurrent theme.
Like I said, protons and neutrons are made up of different combinations of quarks. Quarks are one of
the three major groups of known elementary particles. There are six types, or flavors, of quarks, half
of which are up-type quarks with positive charge and the other 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 all these quarks have fractional charges. What gives? This 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 known as
hadrons, a word you might recognize from CERN's starcrossed particle accelerator, the Large
Hadron Collider. (By the way, it's worth noting that quarks once did exist independently, and there
were free particles with fractional charge. But this only occurred during the universe's first 10^-12
seconds, when the laws of physics didn't quite work in the same way they do now.)
Hadrons make up the vast majority of composite particles, and both protons and neutrons are
hadrons. Specifically, a proton is made up of two up quarks and one down quark, which gives it its +1
charge (since 2/3 + 2/3 - 1/3 = 1). Neutrons have the opposite structure, which is one up quark and
two down quarks, which makes for a neutral charge (2/3 - 1/3 - 1/3 = 0). You can see the proton
above and the neutron below. Any hadron that is made up of three quarks is known as a baryon.
We'll get back to the other type of hadron in a little bit, but first let's return to electrons.
Electrons, Neutrinos, and Other Leptons
Electrons are the best known members of another major group of elementary particles, the leptons.
Just like quarks, there are six flavors of leptons. These flavors can be divided into three generations,
with each successive generation much more massive than the one before it. The same is true of
quarks - up and down quarks are the lightest, then charm and strange, then top and bottom. Only the
lightest leptons and quarks are stable, which is 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
other member of its generation, the neutrino. The neutrino - or, more accurately, the electron neutrino
- is a nearly massless particle proposed by Wolfgang Pauli in 1930 to account for the slight loss of
total energy and momentum in a process known as beta decay, in which a neutron decays into an
electron, a proton, and a neutrino. (Technically speaking, it's actually an electron antineutrino that is
emitted, but let's keep this as straightforward as we can.)
The electron and the electron neutrino are the first generation of leptons, and the other 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 group known as the fermions. Exactly what makes a fermion a fermion is a bit
complicated, but suffice it to say that fermions are all the particles that deal with matter. So what
about the last group of elementary particles, the ones that don't deal with matter?
Creating the Forces: The Bosons
These are the bosons, and they deal with the fundamental forces of the universe. We've already
mentioned a couple of them - there's the electromagnetic force, the strong nuclear force, and the
weak nuclear force, which is mostly responsible for the beta decay process we were just discussing.
(I should clarify here that there are two types of bosons - bosons that are elementary particles and
those that 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 need to worry about that here.)
All of these forces create fields that the matter particles pass through, and the bosons are responsible
for carrying these fundamental forces between the matter particles. Without the bosons to act as
mediators, the other particles wouldn't be able to experience the effects of the various 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 deal with the strong force, and W and Z bosons, which are responsible for the weak force. All
of these have neutral charge, except for the W bosons, which have -1 charge. Both gluons and
photons have zero mass, which means, according to general relativity, both of them must travel at the
speed of light. That seems a bit obvious, I guess, considering photons are light particles.
There are a couple other bosons that many physicists think exist but have so far eluded detection: the
infamous Higgs boson, which theoretically gives all particles their mass, and the graviton, which
would mediate gravity. The Standard 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 Quick Recap
Well then, I think we've got all the known elementary particles sorted out. Let's check the table (feel
free 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 say...it's complicated.
Anyway, let's take a look at how the quarks, leptons, and bosons fit together to form the matter and
forces we experience on a daily basis (again, you can click for a closer look):
Flipping the Script: Antiparticles
We've nearly reached the end of of the known particles, but there are two more groups we need to
deal with: mesons and antiparticles. Mesons are the other type of hadron, which feature only two
quarks instead of the three seen in baryons like protons and neutrons. Now, if you'll take a look at the
quark charges, you'll notice all the up-type quarks have +2/3 charge, and all the down-type quarks
have -1/3 charge. That means there's no way 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, along with the antileptons and antibosons,
and all of these antiparticles have the opposite charge of their particle counterpart. For instance, one
of the most common mesons, the pion, is made of an up quark and a down antiquark, which gives it a
positive charge (2/3 + 1/3 = 1).
Other than that reversed charge, antiparticles are pretty much identical to their particle counterparts. I
say "pretty much" because there are still a few 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 known as color
because it has three basic aspects (compared to the two aspects of charge - positive and negative),
which have been called "blue", "red", and "green" after the primary colors.
But besides these more subtle features and the charge, antiparticles really are practically
indistinguishable from particles. They have the same mass, the same spin, the same lifetime...indeed,
an entire antimatter Earth could theoretically exist, and it would be exactly the same as our Earth
except all the charges would be flipped (and a few other minor things, but charge would be the only
obvious difference). Of course, there isn't an antimatter Earth because, for reasons we're only now
starting to understand, matter managed to dominate antimatter in the universe's earliest moments,
and so any antimatter that now comes into existence is quickly 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 the same goes for other antiparticles like the antineutrino or the
antielectron, which is better known as 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 look at the pion. It's a meson, which means it's a composite
particle. But it's made of a quark and an antiquark, meaning it's equal parts elementary particle and
elementary antiparticle. And that's just one kind of pion - the negatively charged antipion is made of a
down quark and an up antiquark (-1/3 - 2/3 = -1). Again, one elementary particle and one elementary
antiparticle, but this time the result is a composite antiparticle.
This also explains how something like an antineutron can exist. I've said antiparticles have the
opposite charge of their particle counterparts, but neutron has no charge. So what's the difference
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 net charge is the same,
the components are reversed, and that's why neutrons and antineutrons can annihilate each other
just as well 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 must have something that
distinguishes it from the neutrino. Well, the short answer is that...it doesn't, at least not as far as
physicists can tell. The antineutrino and neutrino are seemingly identical, which means they might be
Majorana particles, which is any particle that is its own antiparticle.
All the neutrally charged bosons are definitely Majorana particles, but the neutrino would be the only
fermion to earn that title. However, the current consensus is that, though it's not impossible, neutrinos
probably aren't Majorana particles, in part because of 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, such as the third type of pion, which is made of 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 just about does it for all the particles and antiparticles we know about. Like I said at the
beginning, I've made a cheat sheet for all the known elementary particles - feel free to refer to it
whenever you mix up your muons and your gluons. Unfortunately, there are simply too many different
combinations of composite particles to make a cheat sheet for them, but I did include the two most
important hadrons in the elementary particles cheat sheet.
Also, if any of this is still confusing - which is perfectly understandable - feel free to leave questions in
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.