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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.
An elementary particle with mass equal to that of the electron, and positive charge equal in magnitude to the electron's negative charge. The positron is thus the antiparticle (charge-conjugate particle) to the electron. The positron has the same spin and statistics as the electron. Positrons, like electrons, appear as decay products of many heavier particles; electron-positron pairs are produced by high-energy photons in matter. See Antimatter, Electron, Electron-positron pair production, Elementary particle
A positron is, in itself, stable, but cannot exist indefinitely in the presence of matter, for it will ultimately collide with an electron. The two particles will be annihilated as a result of this collision, and photons will be created. However, a positron can first become bound to an electron to form a short-lived “atom” termed positronium. See Positronium
Quantum field theory predicts the occurrence of a fundamental positron creation process in the presence of strong, static electric fields. For a bare nucleus with atomic number Z > 173, it becomes energetically favorable to transform the electron binding energy of larger than 2m0c2, where m0 is the electron rest mass and c is the speed of light, into simultaneously creating an electron bound to the nucleus and a positron that escapes from the nucleus. This process of spontaneous positron emission has not been observed since atoms with Z > 173 are not available in nature. However, with the introduction of heavy-ion accelerators, it has become possible to simulate such an atom for a short period in a high-energy collision between two stable heavy atoms such as uranium. Experiments have utilized a variety of such collision systems with total Z ranging from 180 to 188 to search for spontaneous positron emission. A number of these experiments reproduce the salient features expected for this process. However, some inconsistencies with the predictions of the theory have yet to be resolved before spontaneous positron emission is established experimentally. See Nuclear molecule, Quasiatom, Supercritical fields
positron(poz -ă-tron) The antiparticle of an electron.
(e+), an elementary particle with a positive electric charge. It is the antiparticle of the electron. The masses Me and spins J of the positron and electron are equal. The electric charges e and magnetic moments μe of the two particles are equal in absolute value but opposite in sign: me = 9.10956 × 10-28 g; J = ½, in units of Planck’s constant ℏ; e = 4.80325 × 10-10 statcoulomb; and μe = 1.00116 Bohr magnetons. The existence of a positively charged twin of the electron follows from the Dirac equation; this possibility was pointed out by P. Dirac in 1931. C. D. Anderson detected the particle in cosmic rays in 1932 and called it a positron. The discovery of the positron was of fundamental importance. Unlike the other particles known by mid-1932—the electron, proton, and neutron—the positron was not found in the composition of “ordinary” matter on earth. There arose the concepts of antiparticle and antimatter. The processes of the annihilation and production of positron-electron pairs were predicted by Dirac. When experimentally observed in 1933, the processes became the first convincing evidence of the interconvertibility of elementary particles.
The positron takes part in the electromagnetic, weak, and gravitational interactions and belongs to the class of leptons. In terms of statistical properties, the positron is a fermion.
The positron is stable, but it exists in matter for only a short time because it undergoes annihilation with electrons. In lead, for example, positrons are annihilated on the average in 5 × 10-11 sec. Under certain conditions, the positron and electron can, before annihilation, form a bound system that is analogous to a hydrogen atom and called positronium. The lifetime of such a system is of the order of 10–7 sec in the case of orthopositronium, where the total spin of the electron and positron is equal to 1. Parapositronium, for which the total spin is equal to zero, has a lifetime of the order of 10–10 sec.
Positrons are formed during interconversion of free elementary particles—such as muon decays and the production of positron-electron pairs by γ-quanta in the electrostatic field of an atomic nucleus—and during the beta decay of certain radioactive isotopes. Positrons obtained from beta decay and pair production are used for research purposes. Thus, the study of the slowing down and subsequent annihilation of positrons in a substance yields a variety of data on the physical and chemical properties of the substance—for example, the velocity distribution of conduction electrons, defects in the crystal lattice, and the kinetics of certain types of chemical reactions. One method of investigation of elementary particles at very high energies is based on colliding beams of accelerated positrons and electrons.
REFERENCESDirac, P. A. M. Printsipy kvantovoi mekhaniki. Moscow, 1960. (Translated from English.)
Novozhilov, Iu. V. Elementarnye chastitsy, 3rd ed. Moscow, 1974.
Gol’danskii, V. I. Fizicheskaia khimiia pozitrona i pozitroniia. Moscow, 1968.
E. A. TAGIROV