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