antiparticleelementary 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, 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 that may explain why the universe is composed mostly of matter. For their discovery, they shared the 1980 Nobel Prize in Physics. In 2010 an eight-year study of B meson decay at the Fermi National Accelerator Laboratory found a tendency to produce roughly 1% more muon pairs than antimuon pairs.

Antiparticle

Antiparticles are subatomic particles that are the opposite of other subatomic particles in some way or another. In the case of the antielectron and the antiproton, this difference is a matter of charge. The electron is negatively charged, and the antielectron is positively charged; the proton is positively charged, and the antiproton is negatively charged. Since the neutron carries no electric charge, its antiparticle, the antineutron, is characterized has having a spin opposite to that of the neutron.

Dirac's hypothesis

The discovery of antiparticles is a rather remarkable scientific detective story. In the late 1920s, British physicist Paul Dirac (1902–1984) was working to improve the model of the atom then used by scientists. As he performed his mathematical calculations, he found that electrons should be expected in two energy states, one positive and one negative. However, the concept of negative energy was unknown to scientists at that time.

Dirac suggested that some electrons might carry a positive electrical charge, the opposite of that normally found in an electron. Scientists were skeptical about the idea. Electrons were well known, and the only form in which they had ever been observed was with a negative charge.

The dilemma was soon resolved, however. Only five years after Dirac proposed the concept of a positive electron, just such a particle was found by American physicist Carl Anderson (1905–1991). Anderson named the newly found particle a positron, for posi tive electron.

Other antiparticles and antimatter

Anderson's discovery raised an obvious question: If an antielectron exists, could there also be an antiproton, a proton with a negative charge? That question took much longer to answer than did Dirac's original problem. It was not until 1955 that Italian-American physicist Emilio Segrè (1905–1989) and American physicist Owen Chamberlain (1920– ) produced antiprotons by colliding normal protons with each other inside a powerful cyclotron (atom-smashing machine).

If antielectrons and antiprotons exist, is it possible that antimatter also exists? Antimatter would consist of antiatoms made of antiprotons and antielectrons. The idea may seem bizarre because we have no experience with antimatter in our everyday lives. Scientists now believe that antimatter is common in the universe, but we don't have any direct contact with it.

If antimatter does exist, locating it may be a problem. Scientists know that the collision of an antiparticle with its mirror image—an electron with a positron, for example—results in the annihilation of both, with the release of huge amounts of energy. Thus, any time matter comes into contact with antimatter, both are destroyed and converted into energy.

[See also Subatomic particles ]

antimatter Matter composed of antiparticles: subatomic particles that have identical rest mass to corresponding particles of ordinary matter but opposite charge, and are opposites in other fundamental properties. For example, the antiparticle of the electron is the positron, which has a positive charge equal to the electron's negative charge; the antiproton has a negative charge equal to the proton's positive charge. When matter and antimatter meet, they annihilate each other, releasing energy. The Universe seems to be almost entirely in the form of matter rather than antimatter; why this should be so is presumably related to events shortly after the Big Bang.

antimatter Matter made up of antiparticles, identical to ordinary particles in every way except the charge, spin and magnetic moment are reversed. Its existence is predicted by the quantum mechanics. When an antiparticle, such as a positron (anti-electron), antiproton, or antineutron meets its respective particle, both are annihilated. The possibility exists that there are stars or galaxies composed entirely of antimatter. See also subatomic particles

an·ti·mat·ter / ˈantēˌmatər; ˈantī-/ • n. Physics molecules formed by atoms consisting of antiprotons, antineutrons, and positrons. Stable antimatter does not appear to exist in our universe.

antimatter see antiparticle .