## Glossary

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accelerator - a machine designed to increase the momentum of particles. At Fermilab, we use five accelerators, the last and most powerful of which is the Tevatron. The Tevatron accelerates protons and antiprotons to within one one-hundredth of a percent of the speed of light.

antiparticle - A particle and its antiparticle have equal mass and opposite charge. Every type of particle has an antiparticle. For example, an electron's antiparticle is a positron.

baryon - a hadron that is made of three quarks or three antiquarks. Examples include protons (uud) and neutrons (udd). (The word baryon was taken from the Greek baros, which means "heavyweight.") Baryons include protons and neutrons, as well as particles named after the Greek letters lamba, sigma, xi (also called "cascade" because it decays into a cascade of lighter particles), delta, and omega.

boson - category of particles that includes those with integer spin (mesons and mediators). Bosons were originally called "Bose particles." Physicist S.N. Bose set up a statistical system which these particles obey.

charmed meson - also known as a D meson. Charged charmed mesons are made of a combination of a charm quark and an antidown (positive charmed meson) or an anticharm and a down (negative charmed meson). The neutral D meson and the antineutral D meson are charm-antiup and up-anticharm, respectively.

color - a physical property of quarks which differentiates them. Quarks are not actually colored, but physicists use the terminology to make it easier to communicate. The possible colors of a quark are red, cyan (anti-red), green, magenta (anti-green), blue, and yellow (anti-blue). The colors of the quarks that make up a hadron have to sum to white.

decay - a process by which a particle splits into two or more other particles.

fermions - category of particles that includes those with half-integer spin (baryons, leptons, and quarks). The statistical system they obey was discovered by Enrico Fermi. Thus, they were originally called "Fermi particles."

flavor - method of describing the quarks. Up, down, strange, charm, bottom, and top are the flavors of quarks.

hadron - category of particles that includes one of two combinations of quarks: three quarks or three antiquarks (a baryon) or a quark and an antiquark (a meson).

half-life - the amount of time it takes half of the particles in a given set to decay. Every type of particle that decays has a characteristic half-life, independent of the number of particles present in any given sample.

kaon - see strange meson.

lepton or antilepton - a fundamental fermion. A lepton or antilepton carries a charge of zero, +1, or -1 (opposite an electron's charge or equal to an electron's charge). Leptons include electrons (e-), negatively charged muons and taus, and each of their neutrinos (electron neutrino, muon neutrino, and tau neutrino), which are neutral. Antileptons include positrons (e+), positively charged muons and taus. Each antilepton has a corresponding antineutrino. (Lepton means "lightweight")

mediator - particle that is exchanged in the mediation of an "action at a distance." For example, the graviton mediates gravity, while the photon is the mediator in electrodynamics. Other mediators include W (for "weak") and Z (for "zero") bosons as well as intermediate vector bosons.

meson - a hadron that is made of a quark and an antiquark. A meson can have a charge of +1, -1, or 0. (Meson means "middleweight.")

photon - the mediator for the electromagnetic force. A photon is a particle of electromagnetic (EM) radiation. We refer to EM radiation in many different ways, depending on its energy. From low energy to high energy, those types of EM radiation are radio waves, microwaves, infrared light, visible light, ultraviolet light, X-rays, gamma rays, and cosmic rays.

pion - a meson made of an up or down quark and an anti-up or anti-down quark. They are the lightest mesons. Positively charged pions are made of an up quark and an antidown quark. Negatively charged mesons are made of a down quark and an antiup quark. (Negative and positive pions are each other's antiparticles.) About half the time neutral pions are detected, they are observed to be an up and an antiup quark, and the other half the time, they are observed to be a down and an antidown quark.

quark - a fundamental fermion. It is structureless and pointlike. The quarks are known as up, down, strange, charm, bottom, and top (u, d, s, c, b, and t). They have fractional charge, either 2/3 of a proton's charge or -1/3 of a proton's charge. d, s, and b quarks have charge -1/3. u, c,and tquarks have charge +2/3. Each quark is also associated with an antiquark. Antidown quarks have charge +1/3, antiup quarks have charge -2/3, and so forth. The word "quark" means curds or slop in German and was taken from James Joyce's Finnegan's Wake.

strange meson - also known as a K meson or kaon. Charged kaons are made of a combination of a strange quark and an antiup (positive kaon) or an antistrange and an up (negative kaon). The neutral kaon and the antineutral kaon are down-antistrange and strange-antidown, respectively.

## Sources

Griffiths, D. Introduction to Elementary Particles. Wiley & Sons, New York, 1987.

Patt, B. "Re: Neutral Pions." E-mail to the author. 31 Aug 1998.