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Glossary ACRONYMS AND ABBREVIATIONS ATLAS CEBAF CERN DDHF DOE eV fm Alternating Gradient Synchrotron, Brookhaven National Laboratory Argonne Tandem Linear Accelerator System, Argonne National Laboratory Continuous Electron Beam Accelerator Facility, proposed for construction at Newport News, Virginia. (Formerly called the National Electron Accelerator Laboratory, NEAL) CEN Saclay Centre d' etudes Nucleaires (Center for Nuclear Studies) de Saclay, Gif-sur-Yvette, France Centre Europeenne pour la Recherche Nucleaire (European Organization for Nuclear Research; also called the European Laboratory for Particle Physics), Geneva, Switzerland density-dependent Hartree-Fock (method) Department of Energy electron volt fermi (10-~5 m) Grand Accelerateur National d'Ions Lourds (National Large Heavy-Ion Accelerator), Caen, France giga-electron volt (109 eV) 203

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204 GLOSSAR Y GSI Gesellschaft fur Schwerionenforschung (Laboratory for Heavy-Ion Research), Darmstadt, West Germany HHIRF Holifield Heavy Ion Research Facility, Oak Ridge National Laboratory IUCF Indiana University Cyclotron Facility JACEE Japanese-American Cooperative Emulsion Experiment JINR Joint Institute for Nuclear Research, Dubna, USSR KKK Kokuritsu Ko-Enerugii Butsurigaku Kenkyusho (National High-Energy Physics Laboratory), Tsukuba, Japan keV kilo-electron volt (103 eV) km kilometer LAMPF Los Alamos Meson Physics Facility, Los Alamos National Laboratory LEAR Low-Energy Antiproton Ring at CERN MeV mega-electron volt (106 eV) msec millisecond NSAC Nuclear Science Advisory Committee of the Department of Energy and the National Science Foundation NSCL National Superconducting Cyclotron Laboratory, Michigan State University NSF National Science Foundation (United States) QCD quantum chromodynamics QED quantum electrodynamics QHD quantum hadrodynamics RNC relativistic nuclear collider SIN Swiss Institute of Nuclear Research, Villigen, Switzerland SLAC Stanford Linear Accelerator Center SURA Southeastern Universities Research Association TeV tera-electron volt (10'2 eV) TRIUMF Tri-University Meson Facility, Vancouver, British Columbia, Canada TUNL Triangle Universities Nuclear Laboratory, Duke University V volt

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G f OSSAR Y 205 TECHNICAL TERMS Accelerator. A machine designed to accelerate charged particles to some energy suitable for bombarding a target and studying the resulting nuclear reactions. The four major kinds of accelerators are Van de Graaff electrostatic accelerators, linear accelerators, cyclo- trons, and synchrotrons. Allowed process. Any physical process that is allowed by a given theory; it may or may not have been observed to occur. See also Forbidden process. Alpha particle. The nucleus of the helium-4 atom, consisting of two protons and two neutrons. It is also a product of radioactive decay. See also Beta particle. Antimatter. Matter that consists of antiparticles (e.g., positrons and antinucleons) instead of ordinary particles. Antiparticle. A particle that is identical to an ordinary particle in every respect except for having certain opposite elementary proper- ties, such as electric charge. For every particle, there is an antipar- ticle; some particles are their own antiparticles. Asymptotic freedom. . ~. , A phenomenon in which the strength of the color force between quarks approaches zero when the quarks come very close together and increases when they move apart. See also Quark confinement. Atom. The smallest unit of a chemical element, consisting of a central nucleus surrounded by orbital electrons. It is held together by the electromagnetic force. Atomic number, Z. The number of protons in an atomic nucleus. Bag model. The model of hadron structure that views the hadron as an impenetrable bag from which its constituent quarks cannot escape under any ordinary conditions. See also Quark confinement. Baryon. One of the two classes of hadrons, consisting of three quarks or three antiquarks confined in a bag. All baryons are fermions; the three principal kinds are nucleons, hyperons, and baryon reso- nances. See also Meson. Baryon resonance. An excited state of a baryon, having a greater mass and an extremely short lifetime. The most common baryon resonances are the nucleon resonances and delta resonances. Beta particle. A synonym for an electron or a positron when it is emitted in the process of beta radioactivity, or beta decay. See also Alpha particle. Binding energy. A measure of the strength with which a given

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206 G. f OSSAR Y physical system is bound; it is the amount of energy needed to break the bond in question and separate the particles. Boson. Any particle or group of particles (such as a nucleus) having an integral value of spin. Among the bosons, in addition to the elementary vector bosons, are the mesons. The Pauli exclusion principle does not apply to bosons. Central collision. A head-on collision of two particles, with near- maximum overlap of their cross-sectional areas; the impact param- eter is near zero. Collective model. Any model of nuclear structure in which the nucleons are viewed as moving in concert under the influence of some force. See also Liquid-drop model. Colliding-beam accelerator. An accelerator in which the projectile particles in two counterdirectional beams collide in flight. Color. The name for a property ascribed to quarks and gluons, somewhat analogous to electric charge. There are three such colors. Color force. The force through which quarks and gluons interact, by the exchange of gluons. It is the basis for quantum chromodynamics. See also Strong force. Compound nucleus. A heavy nucleus formed by the collision of two lighter nuclei. See also Fusion. Conservation law. A law stating that in every conceivable interaction the total amount of a certain quantity (e.g., electric charge or mass-energy) cannot change, i.e., the quantity is conserved. Coulomb barrier. The repulsive Coulomb force between a positively charged target nucleus and any positively charged projectile, inhib- iting their close contact. Coulomb force. The force of electrical attraction or repulsion be- tween particles of unlike charge or like charge, respectively. Cross section. A measure of the probability that an interaction of a given kind will occur; it is expressed in units of area and is one of the most commonly measured quantities in nuclear physics. Current. See Exchange current. Cyclotron. A circular accelerator in which the charged particles spiral outward from the center of the machine as they are given repeated energy boosts from an alternating electric field in a fixed magnetic field. Decay. Any process in which a radioactive nuclide or an unstable particle or system changes to another, lower-energy form by emitting one or more particles or gamma rays. Deep-inelastic scattering. A noncentral collision in which a great deal of the collision energy is converted to internal energy of the nuclei.

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GLOSSAR Y 207 Delta resonance. A baryon resonance; delta resonances differ in isospin from the nucleon resonances. Detector. Any device that can detect the presence of a particle or nuclear fragment produced in a nuclear reaction and measure one or more of its physical properties. Deuteron. The nucleus of deuterium (hydrogen-2), consisting of one proton and one neutron. Electromagnetic force. A component of the unified electroweak force, responsible for holding atoms together and for many other phenomena. It is experienced by all particles with an electric charge or magnetic moment, through the exchange of photons. See also Weak force. Electron. A light, negatively charged lepton with a mass of 0.511 MeV, about 1/1840 that of a nucleon. See also Beta particle, Positron, Muon, and Tauon. Electron volt (eV). The amount of energy acquired by any particle with unit electric charge when it is accelerated through a potential difference of 1 volt. In various multiples, such as keV, MeV, or GeV, it is used as a measure of beam energy, of rest mass, and of temperature. Electrostatic accelerator. See Van de Grauff electrostatic accelera- tor. Electrostatic force. See Coulomb force. Electroweak force. One of the three fundamental forces, comprising the actions of both the electromagnetic and weak forces, whose unification revealed them to be two very different aspects of one underlying force. See also Gravitation and Strong force. Elementary particle. A particle that, as far as is known, has no internal structure. The elementary particles are the leptons, quarks, and elementary vector bosons. Hadrons are not elementary parti- cles. Elementary vector boson. One of the three classes of elementary particles, consisting of photons, gluons, and the intermediate vector bosons; these particles are the carriers of the fundamental forces. See also Lepton and Quark. Equation of state. A mathematical equation that describes the behav- ior of a physical system over a wide range of conditions, on the basis of a few measurable quantities called state variables. Exchange current. The current, either charged or neutral, arising from the exchange of charged or neutral virtual particles as carriers of a force between two particles.

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208 GLOSSAR Y Exchange particle. Any virtual particle that acts as the carrier of a force between two particles. Excited state. Any energy level of a bound system of particles, such as a nucleus, above the ground state. Exclusion principle. See Pauli exclusion principle. Fermi. The common name for the femtometer (10-~5 meter), the characteristic dimension of nuclear and particle physics. The diam- eter of a nucleon is about 1 fermi. Fermion. Any particle or group of particles (such as a nucleus) having a half-integral value of spin. All leptons, quarks, and baryons are fermions. The Pauli exclusion principle applies only to fermions. Fission. The process-either spontaneous or induced in which a nucleus of a heavy element, such as uranium, splits into two lighter nuclei, with the release of energy. See also Fusion. Flavor. The name for the property that distinguishes the six basic kinds of quarks: up, down, strange, charm, bottom, and top. Each flavor can have any of the three different quark colors. Forbidden process. Any physical process that is forbidden by a given theory and that typically has never been observed to occur. If it is observed, the theory is compromised. See also Allowed process. Fusion. The process in which two nuclei of light elements, such as hydrogen or helium, fuse to form one heavier nucleus, with the release of energy. Also, the process in which two heavier nuclei fuse to form a compound nucleus, which may or may not quickly split apart. See also Fission. Gamma ray. An extremely energetic photon, emitted in many nu- clear reactions and in the decay of many radioactive nuclides and unstable particles. Gluon. Any of eight massless, colored particles that are the carriers of the color force. They are elementary vector bosons and are confined within hadron bags. Grand Unified Theory. A mathematical formalism that seeks to unite the strong and electroweak forces into a single underlying force at a deeper level, in the same way that electromagnetism and the weak force were unified into the electroweak force. Gravitation. One of the three fundamental forces, responsible for the large-scale structure of the universe. It is experienced by all particles but is so extremely weak that its effect on any but macroscopic objects is negligible. See also Electroweak force and Strong force. Ground state. The lowest (normal) energy level of a bound system of particles, such as a nucleus. See also Excited state.

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GLOSSAR Y 209 Hadron. Any particle that experiences the strong force. The two classes of hadrons are baryons and mesons. Hadronic matter. A state of nuclear matter encompassing normal nuclei as well as baryon resonances and other nonnucleonic baryons. Half-life. The time it takes for half of all the nuclei in a radioactive sample to decay to some other form; each type of radionuclide has a characteristic half-life. Heavy ion. Any ion with a mass number greater than about 40; this definition is arbitrary but convenient. Heisenberg uncertainty principle. A fundamental quantum- mechanical law, stating that it is impossible to measure simulta- neously both the position and momentum of a particle with arbi- trarily great precision; the structure of quantum mechanics leads to an analogous statement for energy and time. It plays an important role in nuclear processes. High energy. For the purposes of this report, a projectile energy (somewhat arbitrarily) of 100 MeV per nucleon to 1 GeV per nucleon. See also Relativistic energy. Hypernucleus. Any nucleus in which a nucleon has been replaced by a hyperon. Hyperon. Any baryon containing one or more strange quarks; the most common such baryon is the lambda hyperon. Impact parameter. A measure of the degree of overlap of the cross-sectional areas of two particles in a collision; it is zero in an idealized, perfectly central collision and significantly greater than zero in peripheral collisions. Independent-particle model. Any model of nuclear structure in which the motion of a single nucleon is viewed in terms of an average force field produced by all the other nucleons. See also Shell model. Intermediate vector boson. One of three massive, charged or neutral particles that are the carriers of the weak force. Designated as W+, W-, and Z, they are elementary vector bosons, as are photons and gluons. Ion. In general, any atom that has lost or gained one or more electrons. In nuclear physics, especially in connection with acceler- ators, the term is used as a synonym for nucleus, because frequently ions with some electrons still bound are accelerated; bare nuclei, however, are also referred to as ions. Isospin. A quantum number ascribed to hadrons that permits them to be grouped in simpler ways, such as a generalized nucleon that in different isospin states is either a proton or a neutron. Isotope. Any specific nucleus of a given chemical element. The

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210 GO OSSARY isotopes of an element (which is defined by its proton number) differ from one another in their neutron number. See also Nuclide. Kaon. A strange meson, i.e., one that contains a strange quark. Like pions, kaons can be positive, negative, or neutral. Lepton. One of the three classes of elementary particles, consisting of electrons, muons, tauons, their associated neutrinos, and the six corresponding antiparticles. All 12 leptons are fermions; they inter- act via the weak force but not the strong force. See also Elementary vector boson and Quark. Light ion. Any hydrogen ion or helium ion. Lithium ions are some- times also included in this category. Linear accelerator. A type of accelerator in which the charged particles follow a straight path as they are given repeated energy boosts from a series of electric fields. Liquid-drop model. A collective model in which the properties of the nucleus are viewed in terms analogous to those of an ordinary drop of liquid. Low energy. For the purposes of this report, a projectile energy (somewhat arbitrarily) of less than about 10 MeV per nucleon. Many-body problem. The mathematical problem of describing the dynamic behavior of any system of three or more mutually interact- ing particles (such as most nuclei). Mass-energy equivalence. The principle that mass and energy are equivalent, interconvertible quantities. In nuclear physics, masses are customarily expressed in terms of an equivalent energy, usually in units of MeV. Mass number, A. The number of protons plus neutrons (A = Z + N) in an atomic nucleus. Nuclei of different elements can have the same mass number. Medium energy. For the purposes of this report, a projectile energy (somewhat arbitrarily) of 10 to 100 MeV per nucleon. Medium ion. Any ion from lithium up to a mass number of about 40; this definition is arbitrary but convenient. Meson. One of the two classes of hadrons, consising of a quark- antiquark pair confined in a bag. All mesons are bosons; among the more common ones are pions and kaons. Mesons are the principal carriers of the strong force between hadrons. See also Baryon. Meson-exchange model. A model of nuclear interactions that takes into account the effects of the exchange of virtual mesons between nucleons, rather than considering the nuclei to be composed only of nucleons. Muon. A moderately massive, negatively charged lepton that appears ~. .

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GLOSSARY 21 1 to be identical to the electron in every respect except for its greater mass. See also Tauon. Neutrino. Any of three kinds of neutral, presumably massless leptons that are emitted in weak-interaction processes, such as beta decay. Neutrino oscillation. The postulated phenomenon whereby neutrinos change periodically from one form (electron neutrino, muon neu- trino, or tauon neutrino) to another during their flight through space. Such behavior has not been observed. Neutron. An uncharged (neutral) baryon with a mass almost identical to that of the proton. Neutron number, N. The number of neutrons in an atomic nucleus. Nuclear matter. Matter that consists primarily of nucleons whether in atomic nuclei or in an extended state, as in neutron stars. Nuclear reaction. Any change brought about in the states of two nuclei as a result of their collision with each other. Nuclear spectroscopy. The study of the detailed structure of nuclei their spectrum of energy levels, associated physical properties, decay modes, and other properties. Nucleon. A proton or a neutron; nucleons are the least massive, most stable baryons. Nucleon resonance. A baryon resonance that is an excited state of a nucleon; nucleon resonances differ is isospin from the delta reso nances. Nucleus. The small, dense, positively charged core of the atom, consisting primarily of nucleons (protons and neutrons). It is held together by the strong force, through the exchange of mesons between the nucleons. See also Ion. Nuclide. Any specific nucleus, as defined by a unique combination of proton number and neutron number. See also Isotope. Parity. A fundamental symmetry principle governing the nature of physical laws when the spatial coordinates of the system are totally reflected. The parity principle is obeyed (i.e., nature exhibits no spatial preference) in the strong and electromagnetic interactions, but it appears always to be violated in weak interactions, such as beta decay. Pauli exclusion principle. A fundamental quantum-mechanical law, obeyed by fermions but not by bosons, stating that in any system of particles, such as a nucleus, no two fermions are allowed to coexist in the identical quantum state. It plays a dominant role in determin- ing nuclear structures. Phase transition. A change in the physical state of a system from one form to a different form (e.g., ice to water).

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212 GLOSSARY Photon. A massless, neutral particle that is the quantum of electro- magnetic radiation and the carrier of the electromagnetic force. It is one of the elementary vector bosons. Pion. The most commonly observed meson, existing in any of three charge states: positive, negative, and neutral. Virtual plans exist in nuclei and are important for an understanding of nuclear structure. Positron. The positively charged antiparticle of the electron. Proton. A positively charged baryon with a mass of 938 MeV, about 1840 times greater than that of the electron. Proton number, Z. The number of protons in an atomic nucleus. Quantum. The smallest possible unit of energy associated with any change in a physical system. The best-known example of a quantum of energy is the photon. Quantum chromodynamics (QCD). The quantum field theory of the color interaction between quarks and gluons. It is also loosely referred to as the quantum field theory of the strong interaction which derives from the color interaction. Quantum electrodynamics (QED'. The quantum field theory of the electromagnetic interaction between any particles with electric or magnetic properties. Quantumfield theory. A mathematical formalism, based on relativity and quantum mechanics, that describes one of the fundamental interactions. The two most important such theories are quantum electrodynamics and quantum chromodynamics. Quantum hadrodynamics (QHD). A model quantum field theory that attempts to account for the actions of the strong force in terms of the hadrons themselves rather than of their constituent quarks and gluons. (quantum mechanics. The physical theory that underlies all phenom- ena at the level of molecules, atoms, nuclei, and elementary parti- cles. Quark. One of the three classes of elementary particles. There are six basic kinds of quarks (quark flavors) and six corresponding antipar- ticles. All 12 quarks are fermions; they interact via the color force as well as the weak force. All have a fractional electric charge and are confined within hadron bags. See also Elementary vector boson and Lepton. Quark confinement. The observation that it is apparently impossible, under any ordinary conditions, for quarks to escape from their hadron bags and exist as free particles. See also Asymptotic free- dom. Quark-gluon plasma. An extreme state of matter in which quarks and

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GLOSSARY 213 gluons are Reconfined and are free to move about in a much larger volume than that of a single hadron bag. It has never been observed on earth. Radioactivity. Any of several kinds of processes in which a nuclide changes to another nuclide by the emission of one or more particles. Relativistic energy. A projectile energy greater than about 1 GeV per nucleon, i.e., an energy comparable with or greater than the particle's rest mass. Relativity. The theory of space and time (special relativity) that describes the nature of physical laws in terms of postulates regarding the speed of light and the observation of motion made from moving frames of reference. Resonance. A large increase in the amplitude of oscillation of a physical system when it is acted on by an external driving force that oscillates at or near a particular frequency, the resonant frequency of the system. Also, an extremely unstable (short-lived) particle state. See also Baryon resonance. Rest mass. The mass of a particle when it is not moving with respect to some frame of reference (such as the laboratory). The mass of a moving particle is greater than its rest mass. See also Relativistic energy. Shell model. An independent-particle model in which the nucleons are viewed as occupying a series of shells analogous to those of the electrons in the theory of atomic structure. Spin. An intrinsic property of all particles and nuclei, analogous to rotation about an axis. Spin, however, occurs only in multiples of a basic quantum mechanical unit of measure. Particles having an integral value of spin are bosons; particles having a half-integral value are fermions. Spontaneousfission. See Fission. Standard Model. The combined (but not yet unified) theories of the electroweak interaction and quantum chromodynamics, with which all known facts of nuclear physics and elementary particle physics are consistent. State variable. One of a minimum set of measurable quantities whose values are sufficient to define the state of a given physical system and predict its behavior over a wide range of conditions. See also Equation of state. Strangeness. The property associated with the strange quark or anv , . . _A _ = ~^ ~ ~ particle containing a strange quark. Strong force. One of the three fundamental forces, responsible for holding nuclei together. It is experienced by all the hadrons through

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214 GLOSSARY the exchange of mesons and is actually a vestige of the much stronger color force between quarks and gluons. See also Electroweak force and Gravitation. Sum rule. A rule that sets an upper limit on the magnitude of some quantity within the framework of a given model. Symmetry principle. A fundamental principle governing the nature of physical laws under the effect of a symmetry transformation of some kind. Two of the most important symmetry principles in nuclear and particle physics are parity and time-reversal invariance. Synchrotron. A ring-shaped accelerator in which the charged parti- cles follow a fixed circular path as they are given repeated energy boosts from a radio-frequency field in a time-varying magnetic field. Tauon. A very massive, negatively charged lepton that appears to be identical to the electron in every respect except for its much greater mass. See also Muon. Time-reversal invariance. A fundamental symmetry principle gov- erning the nature of physical laws when the direction of the flow of time is considered to be reversed. Uncertainty principle. See Heisenberg uncertainty principle. Van de GraaJf electrostatic accelerator. A type of accelerator in which the charged particles are given a single energy boost by passing through a very large electrostatic potential drop. Vector boson. Any spin-1 boson that acts as the carrier of a force between two particles. See also Virtual particle. Virtual particle. A particle, typically a boson, whose ephemeral existence serves to carry a force between two material particles. The virtual particle appears spontaneously near one of the two particles and disappears near the other one. Under certain conditions, a virtual particle can become a material particle. Weakforce. A component of the unified electroweak force, respon- sible for the decay of many radioactive nuclides and unstable particles and for all neutrino interactions. It is experienced by all leptons, quarks, and hadrons, through the exchange of intermediate vector bosons. See also Electromagnetic force.