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antiparticle, elementary particle corresponding to an ordinary particle such as the proton, neutron, or electron, but having the opposite electrical charge and magnetic moment. Every elementary particle has a corresponding antiparticle; the antiparticle of an antiparticle is the original particle. In a few cases, such as the photon and the neutral pion, the particle is its own antiparticle, but most antiparticles are distinct from their ordinary counterparts.

When a particle and its antiparticle collide, both can be annihilated and other particles such as photons or pions produced. In some cases this represents the total conversion of mass into energy. For example, the collision between an electron and its antiparticle, a positron, results in the conversion of their combined masses into the energy of two photons. The reverse process, pair production, is the simultaneous creation of a particle and its antiparticle from the particles that result from their mutual annihilation.

The existence of antiparticles for electrons was predicted in 1928 by P. A. M. Dirac's relativistic quantum theory of the electron. According to the theory both positive and negative values are possible for the total relativistic energy of a free electron. In 1932, Carl D. Anderson, while studying cosmic rays, discovered the predicted positron, the first known antiparticle. About 23 years passed before the discovery of the next antiparticles—the antiproton was discovered by Owen Chamberlain and Emilio Segrè in 1955 at the Univ. of California, and the antineutron was discovered the following year—but the existence of antiparticles for all known particles was by then firmly established in theory.

The existence of antiparticles makes possible the creation of antimatter, composed of atoms made up of antiprotons and antineutrons in a nucleus surrounded by positrons. A very simple type of “atom” incorporating antiparticles is positronium, a brief pairing of a positron and an electron that may occur before their annihilation; it was first created and identified in the laboratory in 1951. Di-positronium, a molecule consisting of two positronium atoms, was created in 2007. A few simple nuclei of antimatter have been created in the laboratory, such as the antideuteron (see deuterium). In 1995 nine atoms of antihydrogen (a single positively charged positron orbiting a single negatively charged antiproton) were created at CERN (near Geneva, Switzerland) by an Italian-German team headed by Walter Oelert.

Any antimatter in our part of the universe is necessarily very short-lived (the antihydrogen atoms, for example, survived for only 40 billionths of a second) because of the overwhelming preponderance of ordinary matter, by which the antimatter is quickly annihilated. Although scientists for a time considered the possibility that entire galaxies of antimatter could have evolved in a part of the universe far removed from our own, observations now indicate that this is not the case. The experimental work of Val L. Fitch and James W. Cronin in 1964 demonstrated an asymmetry in matter-antimatter reactions involving neutral K mesons (kaons) that may explain why the universe is composed mostly of matter. For their discovery, they shared the 1980 Nobel Prize in Physics. Later studies at Fermi National Accelerator Laboratory and CERN concerning the decay of other neutral mesons have found a matter-antimatter asymmetry in their decay.

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Matter which is made up of antiparticles. At the most fundamental level every type of elementary particle has its anticounterpart, its antiparticle. The existence of antiparticles was implied by the relativistic wave equation derived in 1928 by P. A. M. Dirac in his successful attempt to reconcile quantum mechanics and special relativity. The antiparticle of the electron (the positron) was first observed in cosmic rays by C. D. Anderson in 1932, while that of the proton (the antiproton) was produced in the laboratory and observed by E. Segré, O. Chamberlain, and their colleagues in 1955. See Electron, Elementary particle, Positron, Proton, Quantum mechanics, Relativity

The mass, intrinsic angular momentum (spin), and lifetime (in the case of unstable particles) of antiparticles and their particles are equal, while their electromagnetic properties, that is, charge and magnetic moment, are equal in magnitude but opposite in sign. Some neutrally charged particles such as the photon and &pgr;0 meson are their own antiparticles. Certain other abstract properties such as baryon number (protons and neutrons are baryons and have baryon number) and lepton number (electrons and muons are leptons and have lepton number) are reversed in sign between particles and antiparticles. See Angular momentum, Baryon, Lepton

The quantum-mechanical operation of turning particles into their corresponding antiparticles is termed charge conjugation (C), that of reversing the handedness of particles is parity conjugation (P), and that of reversing the direction of time is time reversal (T). A fundamental theorem, the CPT theorem, states that correct theories of particle physics must be invariant under the simultaneous operation of C, P, and T. Simply put, the description of physics in a universe of antiparticles with opposite handedness where time runs backward must be the same as the description of the universe. One consequence of the CPT theorem is that the above-mentioned properties of antiparticles (mass, intrinsic angular momentum, lifetime, and the magnitudes of charge and magnetic moment) must be identical to those properties of the corresponding particles. This has been experimentally verified to a high precision in many instances. See CPT theorem, Parity (quantum mechanics)

When a particle and its antiparticle are brought together, they can annihilate into electromagnetic energy or other particles and their antiparticles in such a way that all memory of the nature of the initial particle and antiparticle is lost. Only the total energy and total angular momentum remain. In the reverse process, antiparticles can be produced in particle collisions with matter if the colliding particles possess sufficient energy to create the required mass. For example, a photon with sufficient energy which interacts with a nucleus can produce an electron-positron pair. See Electron-positron pair production

Since mesons do not possess baryon or lepton number, only charge, energy, and angular momentum need be conserved in their production. Thus, a process such as a collision of a proton with a proton can produce a single neutral pi meson. Other quantum numbers, such as strangeness and charm, must be conserved if production of mesons possessing these quantum numbers is to proceed through strong or electromagnetic interactions. In these cases a particle with the negative values of the particular quantum number must also be produced. Such a process is termed associated production. See Charm, Quantum numbers

Isolated neutral particles, notably K0 and B0 mesons, can spontaneously transform into their antiparticles via the weak interaction. These quantum-mechanical phenomena are termed K– or mixing, respectively. Mixing can lead to particle-antiparticle oscillations wherein a K0 can become its antiparticle, a0, and later oscillate back to a K0. It was through this phenomenon that observation of CP violation first occurred. That observation, coupled to the CPT theorem, implies that physics is not exactly symmetric under time reversal, for example, that the probability of a K0 becoming a 0 is not exactly the same as that in the reverse process.

Experimental observations, both ground- and balloon-based, indicate that the number of cosmic ray antiprotons is less than 1/10,000 that of protons. This number is consistent with the antibaryon production that would be expected from collisions of cosmic protons with the Earth's atmosphere, and is consistent with the lack of appreciable antimatter in the Milky Way Galaxy. Attempts to find antimatter beyond the Milky Way involve searches for gamma radiation resulting from matter-antimatter annihilation in the intergalactic gas that exists between galactic clusters. The null results of these searches suggests that at least the local cluster of galaxies consists mostly of matter. If matter dominates everywhere in the universe, a question arises as to how this came to be. In the standard model of cosmology, the big bang model, the initial condition of the universe was that the baryon number was zero; that is, there was no preference of matter over antimatter. The current theory of how the matter-antimatter asymmetry evolved requires three ingredients: interactions in which baryon number is violated, time reversal (or CP) violation, and a lack of thermodynamic equilibrium. The last requirement was satisfied during the first few microseconds after the big bang. Time reversal violation has been observed in the laboratory in K0 decays, albeit perhaps not of sufficient size to explain the observed baryon-antibaryon asymmetry. But the first ingredient, baryon number violation, has not yet been observed in spite of sensitive searches. Thus, the origin of the dominance of matter over antimatter remains an outstanding mystery of particle and cosmological physics. See Thermodynamic processes

McGraw-Hill Concise Encyclopedia of Physics. © 2002 by The McGraw-Hill Companies, Inc.


Matter composed entirely of antiparticles. Ordinary matter and antimatter would annihilate on contact. Although individual antiparticles are produced in cosmic-ray showers and in high-energy particle accelerators, the search for antimatter in the Universe has so far proved unsuccessful. It is thought therefore that the Universe is not now symmetric between matter and antimatter, although initially equal amounts were created. An excess of matter over antimatter may have resulted from processes occurring very early in the evolution of the Universe while it was out of equilibrium.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



material which is constructed of antipar-ticles. The nuclei of the atoms of matter consist of protons and neutrons, and the shells of atoms are formed of electrons. In antimatter the nuclei consist of antiprotons and antineutrons, with positrons occupying the place of electrons in their shells.

According to a contemporary theory, the nuclear forces which determine the stability of atomic nuclei are identical for particles and antiparticles. The same can be said of electromagnetic and exchange forces, owing to which stable configurations of electrons in atoms and molecules take place; because the charges of all antiparticles are opposite to the charges of the corresponding particles, the negatively charged nuclei of antiatoms attract positrons in exactly the same way as nuclei attract electrons in atoms. Therefore the whole hierarchy of the construction of matter from particles must also be feasible for antimatter, which consists of antiparticles. In 1965 it was first demonstrated experimentally that aggregates of the same type as those made up of particles can be constructed from antiparticles. A group of American physicists under the direction of L. Lederman obtained on the accelerator and recorded the first anti-nucleus, an antideuteron (a cohesive aggregate of an antiproton and an antineutron). In 1969, in experiments on a Serpukhov proton accelerator of 70 giga electron volts, Soviet physicists (under Iu. D. Prokoshkin) recorded nuclei of antihelium-3, consisting of two antiprotons and one anti-neutron.

Inasmuch as the laws of physics are identical for particles and antiparticles, the question arises as to whether there is an equal amount of matter and antimatter in the whole universe. In the part of the universe observable to us, no significant accumulation of antimatter has been discovered. In particular, there are no antiprotons or antinuclei in cosmic rays. However, the question of the abundance of antimatter in the universe, important to astrophysics and cosmology, remains open.

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.


Material consisting of atoms which are composed of positrons, antiprotons, and antineutrons.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.


a form of matter composed of antiparticles, such as antihydrogen, consisting of antiprotons and positrons
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005