Subatomic particles, the elementary building blocks of matter, come in dozens of varieties. First to be discovered was the negatively charged electron, followed by the positively charged proton and the neutral neutron. Protons and neutrons bind together to form a nucleus, which when surrounded by electrons constitutes an atom. The number of protons, neutrons, and electrons in an atom determines the element (like oxygen, carbon, or uranium) it comprises. The photon, the subatomic particle responsible for electromagnetic energy, is the basic unit of light. By the 1970s, however, more than 100 other subatomic particles had been discovered. Most exist only at the very high energy states produced by particle accelerators. Many last only a fraction of a second. And each is thought to have an anti-particle associated with it: The positron, for example, is the anti-particle of the electron, having a positive rather than a negative charge. The whole collection is sometimes referred to by physicists as the ``particle zoo.'' These particles are classified as bosons if they obey a form of statistical mathematics known as the Bose-Einstein statistics, and fermions if they obey the Fermi-Dirac statistics. The are also classified as leptons (fermions which includes electrons, neutrinos, and muons) if they do not interact with the so-called strong force that binds the nucleus together, and hadrons (which includes the proton and neutron) if they do.
In an effort to explain this bewildering variety of particles, physicists have postulated the existence of sub-subatomic particles called quarks. They are thought to be the basic constituents of all conventional hadrons. Quarks, which appear not to exist in isolation but always bound together in sets of three, come in several varieties (known among physicists as flavors) called up, down, top, bottom, charmed, and strange.