[Physics FAQ][Copyright]

Updated July 2007 by John M. Dlugosz.
Updated July 1995 by MCW.
Original by Matt Austern.

The Particle Zoo

Overview

If you look in the Particle Data Book, you will find more than 150 particles listed there. It isn’t as overwhelming as you might suppose though, because there are some powerful organizing principles.

The (observed) particles are divided into two major classes: the material particles, and the gauge bosons. The material particles in turn fall into three categories: leptons, mesons, and baryons.

Fundamental Particles
Fundamental Particles of Matter
ChargeGen.1Gen.2Gen.3
−1eμ (mu)τ (tau) + antiparticles
having opposite
charge
0νeνμντ
−1/3downstrangebottom
+2/3upcharmtop
Gauge Bosons
ChargeForce
0photonelectromagnetism
0gluons (8 colors)strong force
±1W+ and W−weak force
0Z

In the above table, three groups of particles are boxed. The top box is the 6 leptons. There are another 6 matching anti-particles. The middle box is the 6 quarks. There are another 6 matching anti-particles. The quarks are never seen alone, but are used in pairs to form the mesons and in triplets to form the baryons. The bottom box are the bosons. The photon and Z are each their own “anti” particles. The W+ and W− are anti to each other. The gluons are discussed on their own page.

Names

First, some background on the category names, and a summary of how the categories overlap.

Fermions and bosons are both named after people, Enrico Fermi and Satyendra Nath Bose. All particles are one or the other: Fermions have half-integer spin, and bosons have full integer spin.

The gauge bosons are named because of gauge theory, which is itself named gauge like in a railway gauge referring to a size or scale factor.

The names lepton, meson, and baryon are from Greek, meaning (approximately) light, medium, and heavy. Hadron is also Greek meaning bulky.

So, baryons are fermions, and mesons are bosons. Mesons and baryons combined are the hadrons.

Leptons

The lepton you are most familiar with is the electron. It has a spin of ½ and a negative charge. There is also the anti-electron which is the same except for having a positive charge.

Associated with the electron is the neutrino, and of course the matching anti-neutrino. The symbol symbol for the neutrino is the Greek letter “ν” which is also spelled “nu”, presumably for “nu”-trino. However, the Greek letter is actually pronounced like the English word “knee”. Neutrinos and electrons change into each other as part of the weak force.

The same pattern is then repeated twice! The muon is exactly like the electron except it is heavier. It has a neutrino too, that is also heavier than the electron’s neutrino. And of course there are the anti-particles of those. Then there is the tau particle and all the accessories, that are heavier yet.

Mesons and Baryons

Mesons and baryons (also collectively known as hadrons) are both made up from quarks. In the table above, you’ll notice that the quarks are just like the leptons in some interesting ways. They too have a spin of ½. There are also 12 of them (6 shown and 6 anti-particles). The table has two rows, and the items in the rows are related in the same way: they differ by an electric charge of 1, and the weak force can flip-flop between them. The columns are related in the same way too: each generation is just like the previous only heavier.

What is really unique about the quarks is that they never exist in isolation. The strong force keeps them bound together in groups of two or three, called mesons and baryons respectively.

Mesons

A meson is made up of two quarks, which must be chosen so that one is a quark from the table above and the other is an anti-quark. The top quark is too heavy and is never used. So you might suppose that this makes it easy: 5 × 5 is 25 possible combinations.

If you start to fill in a chart, it goes well at first but then you run into complications. Here is an example showing only the first generation:

u huh??π+, ρ+
dπ, ρ huh??

The charged pion is ud̅ (up and anti-down) for the positive, and du̅ (down and anti-up) for the negative. So far, so good. Continuing through the list of mesons, the rho (ρ+ and ρ) have the same quark combinations: ud̅ and du̅!

From there it gets worse: There is also a neutral pion (π0) that is listed as being (uu̅−dd̅)/√2.

The first complication is easy to understand. There are two reasons for duplicates, where more than one meson has the same pair of quarks. First, recall that quarks have a spin of ½. A grouping of two quarks can be formed so that both spins are the same (so the total is 1) or they point in opposite directions (so the total is 0). The other issue is that the two quarks can be in an excited state. Just like electrons in an atom arrange themselves in orbitals, the two quarks in a meson can have some orbital angular momentum. Higher orbitals form (usually heavier) mesons.

The other complication is harder to explain. In the example above, neither the uu̅ nor the dd̅ cell in the table have mesons attributed to them. Instead, both of those states are mixed together to form the neutral pion. It is a superposition of both states. Superposition is just one of those things that quantum objects do. Without getting into the mathematics, the best you can make of this is two practical observations: the superposition is distinct from any of its parts— it is not neither and it is not both; and if you look at the superposition it will have a probability of becoming one or the other. In this case, if you observe a neutral pion (by having it participate in a weak force interaction, for example) there will be a 50% chance it will come up uu̅ and a 50% chance it will come up dd̅.

People who do quantum mechanics are used to this sort of mixing going on. In the organization table, some of the cells don’t contain actual mesons but are mixing ingredients for mesons. The cells can be added or subtracted, and then the whole thing is divided by a number to make the total probability come out to be 1.

Among the mesons, they were originally all given names. As patterns emerged, so did naming conventions. The result (list of mesons at Wikipedia) is that we have π, η, J/ψ, φ, ρ, υ, etc., with superscripts to indicate the charge, a few other marks for interesting variations, and finally subscripts like b to indicate that it uses bottom quarks instead of what the symbol was originally used for.

That semi-mess is for mesons that are “flavorless”, meaning they only contain quarks from the first generation or they contain the quark and corresponding antiquark from any generation, e.g. cc̅ so they “cancel out” the flavor. For flavored mesons, the naming convention is much simpler (see Meson at Wikipedia).

Each of the 4 flavored quarks (that is, those not in the first generation) have a letter associated with it. The meson starts with the symbol for the heaver of the two quarks in it. Then it uses a subscript for the other quark (no symbol if it is in the 1st generation), and other superscripts and subscripts for the angular momentum details.

Suffice to know that lots of particles are mesons. If you read about one, you can always look up the quark composition based on the main part of the symbol, and you know the rest of the name has to do with orbital details.

Baryons

A baryon is made up of three quarks. All three are from the table above, or all three are anti-quarks. You can’t have a mixture of regular and anti-quarks in a baryon.

The common proton and neutron are baryons, with the composition uud and ddu respectively. The Δ+ is also uud. The difference, as with the mesons, is the spin. In the proton one of the quarks is pointing in the opposite direction from the other two. In the Δ+, they all point the same way. Sometimes it matters which quark is pointing the opposite way: the Lambda (Λ) and the Sigma (Σ) particles have the same quarks but in one case it is the flavored (non-first generation) quark that is pointing the other way and in the other case the two first-generation quarks are pointing in opposite direction and the flavored quark is pointing in the same direction as one of those two. That is the only real complication, as they don’t get all mixed up like mesons and they don’t have different internal orbital states.

The baryon names include n and p (for neutron and proton), Σ, Λ, Ξ, Δ, and Ω. Except for the long-known proton and neutron, all the baryons are named as capital Greek letters and have superscripts indicate the charge. While the n and p have different symbols, all the others use a naming convention by family so you have, for example, Δ, Δ0, Δ+, and Δ++. Heavier ones are given subscripts, for example Σ+b means it is just like the Σ+ except that it has a bottom quark.

In addition, the baryons can be arranged in interesting geometric patterns (see illustrations on Wikipedia) that illustrate regularities in their properties.

Gauge Bosons

Bosons are any particle with a spin that is an integral number like 0 and 1, as opposed to fermions with a spin that is half of an odd number, like ½ or 3/2. The reason the value of spin is so important is because fermions collect together to form extended lumps, and form what we call matter, while bosons smear together into coherent fields and act as the forces between pieces of matter.

You should recognize now that mesons are bosons. In fact, the pions are used as the interneucleon force that hold the protons and neutrons together in the core of an atom. But, they are not on the Table of Fundamental Particles because they are not fundamental: they are made out of quarks.

The bottom box of the table list the gauge bosons, which are not made out of anything else. They all have a spin of 1.

The most familiar boson is the photon, which is responsible for electromagnetic interactions. They form light and radio waves, and also are responsible for making electric and magnetic fields.

Closely related to the photon are the particles that carry the weak force (more properly called the weak interaction because it is not responsible for holding anything together). These are the W+, W− and Z. At high energy, it becomes clear that the photon and the other 3 are all in one family and there is one unified electro-weak force. At normal energy (even the core of the sun is normal at this scale!) you’d never know that they were related.

What the weak interaction does, fundamentally, is transform the particles in the upper two boxes of the table. It will turn one kind of particle into the one in the other row directly above or below it in the same group. An e can turn into a νe by emitting a W− or absorbing a W+. There is much more to it than that, but you can appreciate from this description how the particles in the table are organized the way they are.

The last entry in the table are 8 gluons. The gluons hold the quarks together in hadrons. More about the color of gluons is found on its own page in this FAQ.

There is one more particle in the Standard Model that is not on the table and (as of July 2007) has not yet been detected in particle accelerator experiments. It is the only particle in the Standard Model that has not yet been observed, yet it plays a key role in the theory. The Higgs mechanism is a family of 4 fields that make the W+, W− and Z bosons so different from the photon and give them and the leptons mass. The theory predicts one leftover field that will allow the higgs boson to exist as a particle.

Non-Standard Particles

More complicated theories (going beyond the Standard Model) predict additional particles, including, for example, gauginos and sleptons and squarks (from supersymmetry), W’ and Z’ (additional weak bosons), X and Y bosons (from GUT theories), Majorons, familons, axions, paraleptons, ortholeptons, technipions (from technicolor models), B’ (hadrons with fourth generation quarks), magnetic monopoles, e* (excited leptons), etc. None of these exotica have yet been seen. The search is on!

References

The best reference for information on which particles exist, their masses, etc., is the Particle Data Book. It is published every two years. The Web version can be accessed through http://pdg.lbl.gov/

There are several good books that discuss particle physics on a level accessible to anyone who knows a bit of quantum mechanics. One is Introduction to High Energy Physics, by Perkins. Another, which takes a more historical approach and includes many original papers, is Experimental Foundations of Particle Physics, by Cahn and Goldhaber.

For a book that is accessible to non-physicists, you could try The Particle Explosion by Close, Sutton, and Marten. This book has fantastic photography.