Theories of the Universe

The Particle Zoo

Theories of the Universe

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.

Universal Constants

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.

Universal Constants

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.

  • Classification by interaction: leptons and hadrons There are three types of interactions that can affect an elementary particle: electromagnetism, the strong force, or the weak force. The electron, the muon, and the neutrino aren't part of the strong interaction at all. They are involved with the weak interaction and have been put into a small group of particles called leptons. All of the other particles discovered, and there are literally hundreds, are involved one way or the other with the strong interaction. This huge set of particles is made up of hadrons. The photon is usually put in a class by itself since it only mediates the electromagnetic interaction.
  • Classification by decay product: mesons and baryons All hadrons will eventually decay into a collection of stable particles—the proton, electron, photon, and neutrino. Particles in which a proton appears in the end product (the decay process often occurs as a cascade, similar to the ones created by cosmic rays entering our atmosphere) are called baryons. And particles whose final collection of particles is made up entirely of leptons and photons (no protons) are called mesons.
  • Classification by internal dynamics: spin Another classification for particles is the direction of rotation it has around its axis. It is really defined in terms of its angular momentum. Spin is defined directionally by up, down, or sideways.
  • Classification by electrical charge: isospin These particles are defined by two quantities, their spin direction and their electrical charge. The combination of these two gives a new quantity called isospin.
  • Classification by speed of decay: strange vs. nonstrange Some particles take longer to decay than others, so this allows another means to classify them. Strange particles decay in times on the order of 10-10 second and nonstrange particles decay in 10-23 seconds.

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.

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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.

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