One of the most difficult aspects of particle physics is how to impose a sense of order on all of the elementary particles that have been discovered. With only a few pages left in this section I hope that is something I can do for you without leaving you wondering what the heck it's all about. So for sake of clarity I'm going to give you a list of some of the ways all of these pieces in the microcosmic puzzle of elementary particles can be classified. By the time we get to the end we'll see if we can come away with some simple descriptions.
A lepton is a particle that is involved with the weak interaction and sometimes the electromagnetic. All charged particles are affected by electromagnetism. Lepton is the Greek word for “small,” and the electron is the most well known lepton.
A hadron is any particle that is involved with the strong interaction. It's another Greek word and it means “strong.” Hundreds of hadrons have been discovered.
A baryon (heavy one) has a proton as an end product in its decay. All baryons are hadrons and the most common ones are the proton and neutron, which make up most of the mass of ordinary atoms. For this reason everyday matter is sometimes called “baryonic matter.”
A meson is any particle that has leptons and photons, but no protons, as the final decay particles. Remember that the muon was originally called the mu-meson, so this is a new definition of the word. They are all hadrons as well and are therefore involved in the strong interaction, too.
There are other methods of classification we could consider, but I think you get the idea of how it can be done. And to put it simply, we can say that all matter is made up of two kinds of particles, hadrons or leptons. But there's a little more to our puzzle that we need to complete the picture, the funny things called quarks and the particles that carry the four fundamental forces.
Excerpted from The Complete Idiot's Guide to Theories of the Universe © 2001 by Gary F. Moring. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.