Updated by MCW, 1995.
Original by Matt Austern.
If you look in the Particle Data Book, you will find more than 150 particles listedthere. 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 farther down. Thematerial 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 notundergo 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 areboth short-lived.)
Mesons and baryons both undergo strong interactions. The difference is thatmesons 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 areshort-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 thathadrons are made out of spin-1/2 particles called quarks. A meson, in this model, ismade out of a quark and an anti-quark, and a baryon is made out of three quarks. Wedon'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 welldefined, since they are not free particles, but we can give estimates. The massesbelow are in GeV; the first is current mass and the second constituent mass (whichincludes some of the effects of the binding energy):
Generation: 1 2 3 U-like: u=0.006/0.311 c=1.50/1.65 t=91-200/91-200 D-like: d=0.010/0.315 s=0.200/0.500 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, theelectron, and the electron neutrino. (Each of these also has an associatedantiparticle.) 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 heavierparticles. The second consists of the charm quark, the strange quark, the muon, andthe muon neutrino; and the third consists of the top quark, the bottom quark, the tau, andthe 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 "gaugebosons," which are also particles. The most familiar on is the photon, which isresponsible for electromagnetic interactions. There are also eight gluons, which areresponsible for strong interactions, and the W+, W-, and Z, which are responsible for weakinteractions.
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 responsiblefor the masses of all the other particles. It has not yet been found. Morecomplicated theories predict additional particles, including, for example, gauginos andsleptons and squarks (from supersymmetry), W' and Z' (additional weak bosons), X and Ybosons (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 beenseen. The search is on!
There are several good books that discuss particle physics on a level accessible toanyone who knows a bit of quantum mechanics. One isIntroduction to High EnergyPhysics, by Perkins. Another, which takes a more historical approach andincludes many original papers, isExperimental Foundations of Particle Physics,by Cahn and Goldhaber.
For a book that is accessible to non-physicists, you could tryThe ParticleExplosion by Close, Sutton, and Marten. This book has fantasticphotography.
For a Web introduction by the folks at Fermilab, take a lookathttp://fnnews.fnal.gov/hep_overview.html