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Glossary . Accelerator. A device that increases the energy of charged particles such as electrons and protons. AGS. The Alternating Gradient Synchrotron, a 33-GeV proton accel- erator at the Brookhaven National Laboratory. Annihilatior'. See Antiparticle. Antimatter. Matter composed of antiparticles, i.e., antiprotons, antineutrons, antielectrons, instead of, i.e., the ordinary protons, neutrons, electrons. Antiparticle. Each particle has a partner, called an antiparticle, which is identical except that all chargelike properties (electric charge, strangeness, charm. for example) are opposite to those of the particle. When a particle and its antiparticle meet, these properties cancel out in an explosive process called annihilation. The particle and antiparticle can then disappear and other particles be produced. Antiprotorl. The antiparticle partner of the proton. Astrophysics. Physics applied to astronomy and astronomical phe- nomena such as the evolution of stars and the formation of galaxies. Asymptotic freedom. The concept that the strong force between quarks gets weaker as the quarks get close together. Atom. The smallest unit of a chemical element, approximately 1/100.000,000 centimeter in size, consisting of a nucleus surrounded by electrons. Bar~'o''. A type of hadron. The baryon family includes the proton, 213

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214 GLOSSARY neutron, and those other particles whose eventual decay products include the proton. Baryons are composed of three-quark combina- tions. Beam. A stream of particles produced by an accelerator. Beauty . See Bottom. BEPC. A circular electron-positron collider with a total energy up to 6 GeV and high luminosity, under construction near Beijing, China. Beta decay. The decay of a particle or nucleus by the emission of an electron or positron through the weak interaction. Bevatron. A circular accelerator at the Lawrence Berkeley Labora- tory, Berkeley, California; previously used to accelerate protons up to 6 GeV and now part of a complex for accelerating nuclei. BNL. Brookhaven National Laboratory. Bottom. The distinguishing characteristic of the fifth type of quark, also called the b quark or beauty quark. Each quark is characterized by a number of properties including familiar ones like mass and electric charge, and less familiar ones that were arbitrarily given names like bottom and charm. Broken symmetry. The failure of a symmetry principle owing to the presence of an additional force or phenomenon. Bubble chamber. A particle detector in which the paths of charged particles are revealed by a trail of bubbles produced by the particles as they traverse a superheated liquid. Hydrogen, deuterium, helium, neon, propane, and Freon liquids have been used for this purpose. Calorimeter. A particle detector in which the total energy carried by a particle or group of particles is measured. Cerenko`, counter. A detector of Cerenkov radiation, which is elec- tromagnetic radiation emitted by a charged particle when it passes through matter at a velocity exceeding that of light in that material. CERN. The European Center for Nuclear Research, located near Geneva, Switzerland, and supported by most of the nations of Western Europe. CESR. The Cornell Electron Storage Ring, an electron-positron collider with a maximum total energy of 16 GeV located at Cornell University. Charm. The distinguishing characteristic of the fourth type of quark, also called the c quark. Each quark is characterized by a number of properties, including familiar ones like mass and electric charge, and less familiar ones that were arbitrarily given names like charm and bottom. Charmonium. The family of hadronic particles composed of a charm quark and an anticharm quark.

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GLOSSAR Y 2 15 Circular accelerator. An accelerator in which the particles move around a circle many times. being accelerated further in each revolution around the circle. Collider. When a high-energy particle collides with a stationary target' a large portion of the energy resides in the continuing forward motion. Only a small portion of the energy is available for creating new particles. In a collider. collisions take place between high- energy particles that are moving toward each other. In such an arrangement, most of the energy. is available for creating new particles. Colliding-bec~m ac c elevator. See Collider. Color. A property of quarks and gluons. analogous to electric charge, which describes how the strong force acts on a quart; or gluon. Conserl atiO'I lint'. A physical law that states that some quantity or property cannot be changed in a reaction. The law of conservation of energy states that the total energy cannot change in a reaction. Cosmic rays. Energetic particles such as protons that come from outside the Earth's atmosphere. Cosmolo OCR for page 213
216 GLOSSARY . particles. This force is intermediate in strength between the weak and strong force. The carrier of the electromagnetic force is the photon. Electron. An elementary particle with a unit negative electrical charge and a mass 1/1840 that of the proton. Electrons surround an atom's positively charged nucleus and determine the atom's chemi- cal properties. Electrons are members of the lepton family. Electron volt. The amount of energy of motion acquired by an electron accelerated by an electric potential of one volt: MeV, million electron volts; GeV, billion electron volts; TeV, trillion electron volts. Electroweak force or interaction. The force and interaction that represents the unification of the electromagnetic force and the weak force. Elementary particle. A particle (piece of matter) that has no other kinds of particles inside of it and no subparts that can be identified. Hence the simplest kind of matter. Elemerztary-particle physics. The area of basic science whose goal is to determine and understand the structure and forces of the most basic constituents of matter and energy. Fermilab. Fermi National Accelerator Laboratory. Flavor. A general name for the various kinds of quarks, such as up. down, and strange. Also sometimes applied to the various kinds of leptons. Gamma rays. A term used for the energetic photons that are emitted in the decay of unstable particles and nuclei. Gauge theory. A type of general theory of forces, modeled on the immensely successful modern theory of electromagnetism. Generation. The classification of the leptons and quarks into families according to a mass progression. The first generation consists of the electron and its neutrino and of the up and down quarks. The second generation consists of the muon and its neutrino and of the charm and strange quarks. The third generation consists of the tau and its neutrino and of the bottom and expected top quarks. GeV. (Giga electron volt) A unit of energy equal to one billion (109) electron volts. Gluon. A massless particle that carries the strong force. Grand unified theory (GUT,. A hoped-for unification of the electro- weak force with the strong force into a single gauge theory. Gravitational force or interaction. The weakest of the four basic forces and the one responsible for the weight of matter and the motion of the stars and planets.

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GLOSSARY 217 Gra~'iton. A proposed massless particle that is assumed to carry the gravitational force. Hadron. A subnuclear. but not elementary' particle composed of quarks. The hadron family of particles consists of baryons and mesons. These particles all have the capability of interacting with each other via the strong force. HERA. An electron-proton circular collider being constructed at the DESY laboratory in the Federal Republic of Germany. Higgs mechanism and particle. A mechanism that may explain the origin and value of the mass of all or some of the elementary particles. The mechanism includes a proposed set of particles called -Higgs particles. High-energ~ physics. Another name for elementary-particle physics. This name arises from the high energies required for experiments in this field. IHEP. A 76-GeV circular proton accelerator in Serpukhov, USSR. intermediate vector boson. The general name for the W and Z part- icles that carry the weak force. invariance. A property of physical laws and equations such that they do not change when changes are made in reference or coordinate systems. J. A particle made of a c quark (see Charm) and an anti-c-quark. It is also called the psi particle and is three times as massive as the proton. Jet. A narrow stream of hadrons produced in a very-high-energy collision. K meson or koon. The next to the lightest meson. It is the lightest hadron that contains a strange quark. KKK. A 12-GeV circular proton accelerator at Tsukuba, Japan. LAMPF. An 800-MeV linear proton accelerator at Los Alamos National Laboratory, used for nuclear and elementary-particle phys- iCS. LEP. A circular electron-positron collider with a maximum design energy of about 200 GeV being constructed at CERN, Switzerland. Lepton. A member of the family of weakly interacting particles, which includes the electron, muon, tau, and their associated neutri- nos and antiparticles. Leptons are not acted on by the strong force but are acted on by the electroweak and gravitational forces. Lifetime. A measure of how long an unstable particle or nucleus exists on the average before it decays. Linac. An abbreviation for linear accelerator. Linear accelerator. In this type of accelerator, particles travel in a

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218 GLOSSARY straight line and gain energy by passing once through a series of electric fields. L``mi'`osit~. A measure of the rate at which particles in a collider interact. The larger the luminosity the greater the rate of interaction. Magnet. A device that produces a magnetic field and thus causes charged particles to move in curved paths. Magnets are essential elements of all circular accelerators and colliders, as well as of many particle detectors. Magnetic mon`'pc~le. A hypothetical particle that would carry a single north or south magnetic pole. All known particles with magnetic properties carry both a north and a south magnetic pole. Mass. The measure of the amount of matter in a particle and an intrinsic property of the particle. Meson. Any strongly interacting particle that is not a baryon. Mesons are composed of quark-antiquark combinations. Me V. (Mega electron volt) A unit of energy equal to one million electron volts. Molec'`le. A type of matter made up of two or more atoms. Muon. A particle in the lepton family with a mass 207 times that of the electron and having other properties similar to those of the electron. Muons may have positive or negative electric charge. Neutrino. An electrically neutral and massless particle in the lepton family. The only force experienced by neutrinos is the weak force. There are at least three distinct types of neutrinos, one associated with the electron, one with the muon, and one with the taut Neutron. An uncharged baryon with mass slightly greater than that of the proton. The neutron is a strongly interacting particle and a constituent of all atomic nuclei except hydrogen. An isolated neutron decays through the weak interaction to a proton, electron, and antineutrino with a lifetime of about 1000 seconds. Nucleon'. A neutron or a proton. Nucleus. The central core of an atom, made up of neutrons and protons held together by the strong force. Particle. A small piece of matter. An elementary particle is a particle so small that it cannot be further divided it is a fundamental constituent of matter. Particle detector. A device used to detect particles that pass through it. PEP. An electron-positron circular collider with a maximum energy of 36 GeV, at SLAC. PETRA. An electron-positron circular collider with a maximum en- ergy of 46 GeV, at DESY, Hamburg, Federal Republic of Germany.

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GLOSSARY 219 Photon. A quantum of electromagnetic energy. A unique massless particle that carries the electromagnetic force. Pion. The lightest meson. Positron. The antiparticle of the electron. Proton. A baryon with a single positive unit of electric charge and a mass approximately 1840 times that of the electron. It is the nucleus of the hydrogen atom and a constituent of all atomic nuclei. PS. A circular proton accelerator with a maximum energy of 28 GeV at CERN, Switzerland. . Psi. A panicle made of a Cal quark (see Charm) and an anti-c -quark and three times as heavy as the proton. It is also called the J particle. Quantum chromodvnamics (QCD). A theory that describes the strong force among quarks in a manner similar to the description of the electromagnetic force by quantum electrodynamics. Quantum electrodynamics (QEDJ. The theory that describes the electromagnetic interaction in the framework of quantum mechanics. The particle carrying the electromagnetic force is the photon. Quantum mechanics. The mathematical framework for describing the behavior of photons, molecules, atoms, and subatomic particles. According to quantum mechanics, the forces between these particles act through the exchange of discrete units or bundles of energy called quanta. Quarks. The family of elementary particles that make up the hadrons. The quarks are acted on by the strong, electroweak, and gravita- tional forces. Five are known, called up, down, strange, charm, and bottom. A sixth, called top, is expected to exist. Relativistic. The term that describes particles moving with velocities close to the velocity of light. Scattering. When two particles collide, they are said to scatter off each other during the collision. Scintillation counter. A panicle detector in which the passage of a charged particle produces a flash of light called scintillation light. That light, when detected, records the time at which the particle passed through the counter. SLAC. Stanford Linear Accelerator Center in Stanford, California. Also refers to the electron linear accelerator there that is being rebuilt to have a total energy of 50 GeV. SLC. Stanford Linear Collider, a linear electron-positron collider with an initial total energy of about 100 GeV being constructed at SLAC. SPEAR. A circular electron-positron collider with a total energy of about 8 GeV at SLAC.

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r 220 GLOSSAR Y SppS. A circular proton-antiproton collider at CERN that uses the SPS accelerator there and has a total energy of about 600 GeV. SPS. A circular proton accelerator with a maximum beam energy of about 400 GeV at CERN. Switzerland. SSC. See S``perc o'~d``cting Sniper Collider. Standard model. A collection of established experimental knowledge and theories in particle physics that summarizes our present picture of that field. It includes the three generations of quarks and leptons. the electroweak theory of the weak and electromagnetic forces. and the quantum chromodynamic theory of the strong force. It does not include answers to some basic questions such as how to unify the electroweak forces with the strong or gravitational forces. Storc'~,~e ritzy,>. An acceieratorlike machine composed of magnets ar- ranged in a ring used to store circulating particles or to act as a collider. Sometimes a synonym for a collider. Strangeness. The distinguishing characteristic of the third type of quark' also called the s quark. Each quark is characterized by a number of properties. including familiar ones like mass and electric charge and less familiar ones that were arbitrarily given names like charm and strangeness. Strange particle. The name given to panicles thought to contain just one s quark. The remaining quarks in strange particles are either u or d quarks. Strong force or interac lion. . The short-range force and interaction between quarks that Is carried by the gluon. The strong force also dominates the behavior of interacting mesons and baryons and accounts for the strong binding among nucleons. Superconducting magnet. See Superconductivity. Superconducting S`'per Collider (SSC'. A design for a circular pro- ton-proton collider with a total energy that could be as high as 40 TeV being developed in the United States. Superconductivity. A property of some metals that when they are cooled to a temperature close to absolute zero, their electrical resistance becomes exactly zero. Magnets with superconducting coils can produce large magnetic fields while keeping size and power costs small. Supersymmetrv. A proposed theory of elementary particles in which a property of particles called spin is used. In most theories particles that differ in spin in some ways cannot be related. In this theory such particles can be related through a new proposed symmetry principle called supersymmetry. Symmetry. A general property of many objects and physical systems

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ClOSSARY 221 whereby the object or system appears unchanged when looked at from different reference frames or coordinate systems. For example, a tennis ball has spherical symmetry because it always looks the same to us no matter how we move around it. Synchrotro'`. A type of circular particle accelerator in which the frequency of acceleration is synchronized with the particle as it makes successive orbits. Synchrotron radiation. intense light or x rays emitted when electrons move in a circular orbit at relativistic speeds. Target. The material. often liquid hydrogen. that is struck by the beam of high-energy particles in some types of elementary-particle physics experiments. Tail. An elementary particle in the lepton family with a mass 3500 times that of the electron but with similar properties. There are positive and negative tau particles. Tec hail olor. A proposed theory for explaining the masses of particles that postulates the existence of a new force. TeV. (Tera electron volt) A unit of energy equal to one thousand billion ~ 10'') electron volts. Tevatron. A complex of accelerator facilities and beam lines at Fermilab. The main facility is a circular proton accelerator with superconducting magnets (the first large accelerator to use such magnets) with a maximum energy of 1 TeV. An addition is being constructed so that this accelerator can be used as an antiprotoo- proton collider with a total energy of 2 TeV. On completion, this will be the highest-energy collider in the world. Top. The distinguishing characteristic of the expected sixth type of quark, also called the truth quark or t quark. Each quark is characterized by a number of properties, including familiar ones like mass and electric charge and less familiar ones that were arbitrarily given names like bottom and top. TPC. (Time projection chamber) A particle detector in which the position of the track of ionized gas left by a charged particle is detected by the time it takes for the electrons in the gas to move to the ends of the chamber. TRISTAN. A circular electron-positron collider with a total energy of 60 to 70 GeV under construction at the KKK laboratory in Japan. Unified theories. Theories of forces in which the behavior of different kinds of forces is described by a unified or single set of equations and has a common origin. For example. the electric and magnetic forces are unified in the theory of electromagnetism.

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222 GLOSSAR Y UNK. A complex of high-energy circular proton accelerators and colliders under construction at Serpukhov, USSR. Up. The distinguishing property of one of the two lightest quarks also called the u quark. The up and down quarks form the first quark generation. [epsilon. A meson made up of a b quark and an anti-lo-quark. It Is approximately ten times as massive as the proton. VEPP4. A circular electron-positron collider with a total energy of up to 14 GeV at Novosibirsk, USSR. W. The charged particle that carries the weak force, also called an ~ntermediate-vector boson. Its mass is about 90 times the proton mass. Weak force or interaction. The force and interaction that is much weaker than the strong force, but stronger than gravity. It causes the decay of many particles and nuclei. It is carried by the W and Z particles. X rays. Photons produced when atoms in states of high energy decay to states of lower energy. Z. The neutral particle that carries the weak force, also called an intermediate-vector boson. It is slightly heavier than the W particle, with a mass about 100 times the proton mass. . .