updated 4-JUL-1995 by MCW
original by Matt Austern

The Particle Zoo

If you look in the Particle Data Book, you will find more than 150 particles listed there. It isn't quite as bad as that, though...

The (observed) particles are divided into two major classes: the material particles, and the gauge bosons. We'll discuss the gauge bosons further down. The material particles in turn fall into three categories: leptons, mesons, and baryons. Leptons are particles that are like the electron: they have spin 1/2, and they do not undergo the strong interaction. There are three charged leptons, the electron, muon, and tau, and three corresponding neutral leptons, or neutrinos. (The muon and the tau are both short-lived.)

Mesons and baryons both undergo strong interactions. The difference is that mesons have integral spin (0, 1,...), while baryons have half-integral spin (1/2, 3/2,...). The most familiar baryons are the proton and the neutron; all others are short-lived. The most familiar meson is the pion; its lifetime is 26 nanoseconds, and all other mesons decay even faster.

Most of those 150+ particles are mesons and baryons, or, collectively, hadrons. The situation was enormously simplified in the 1960s by the "quark model," which says that hadrons are made out of spin-1/2 particles called quarks. A meson, in this model, is made out of a quark and an anti-quark, and a baryon is made out of three quarks. We don't see free quarks (they are bound together too tightly), but only hadrons; nevertheless, the evidence for quarks is compelling. Quark masses are not very well defined, since they are not free particles, but we can give estimates. The masses below are in GeV; the first is current mass and the second constituent mass (which includes some of the effects of the binding energy):

```      Generation:       1             2            3
U-like:     u=.006/.311   c=1.50/1.65   t=91-200/91-200
D-like:     d=.010/.315   s=.200/.500   b=5.10/5.10
```
In the quark model, there are only 12 elementary particles, which appear in three "generations." The first generation consists of the up quark, the down quark, the electron, and the electron neutrino. (Each of these also has an associated antiparticle.) These particles make up all of the ordinary matter we see around us. There are two other generations, which are essentially the same, but with heavier particles. The second consists of the charm quark, the strange quark, the muon, and the muon neutrino; and the third consists of the top quark, the bottom quark, the tau, and the tau neutrino. These three generations are sometimes called the "electron family", the "muon family", and the "tau family."

Finally, according to quantum field theory, particles interact by exchanging "gauge bosons," which are also particles. The most familiar on is the photon, which is responsible for electromagnetic interactions. There are also eight gluons, which are responsible for strong interactions, and the W+, W-, and Z, which are responsible for weak interactions.

The picture, then, is this:

```                FUNDAMENTAL PARTICLES OF MATTER
Charge        -------------------------
-1          |  e    |  mu   |  tau  |
0          | nu(e) |nu(mu) |nu(tau)|
-------------------------       + antiparticles
-1/3         | down  |strange|bottom |
2/3         |  up   | charm |  top  |
-------------------------

GAUGE BOSONS
Charge                                                Force
0                  photon                          electromagnetism
0                  gluons (8 of them)              strong force
+-1                 W+ and W-                       weak force
0                  Z                               weak force
```
The Standard Model of particle physics also predicts the existence of a "Higgs boson," which has to do with breaking a symmetry involving these forces, and which is responsible for the masses of all the other particles. It has not yet been found. More complicated theories 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 most recent edition is Physical Review D vol.50 No.3 part 1 August 1994. 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.

For a Web introduction by the folks at Fermilab, take a look at http://fnnews.fnal.gov/hep_overview.html