antiparticle

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antiparticle,

elementary particleelementary particles,
the most basic physical constituents of the universe. Basic Constituents of Matter

Molecules are built up from the atom, which is the basic unit of any chemical element. The atom in turn is made from the proton, neutron, and electron.
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 corresponding to an ordinary particle such as the protonproton,
elementary particle having a single positive electrical charge and constituting the nucleus of the ordinary hydrogen atom. The positive charge of the nucleus of any atom is due to its protons.
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, neutronneutron,
uncharged elementary particle of slightly greater mass than the proton. It was discovered by James Chadwick in 1932. The stable isotopes of all elements except hydrogen and helium contain a number of neutrons equal to or greater than the number of protons.
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, or electronelectron,
elementary particle carrying a unit charge of negative electricity. Ordinary electric current is the flow of electrons through a wire conductor (see electricity). The electron is one of the basic constituents of matter.
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, 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 photonphoton
, the particle composing light and other forms of electromagnetic radiation, sometimes called light quantum. The photon has no charge and no mass. About the beginning of the 20th cent.
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 and the neutral pionpion
or pi meson,
lightest of the meson family of elementary particles. The existence of the pion was predicted in 1935 by Hideki Yukawa, who theorized that it was responsible for the force of the strong interactions holding the atomic nucleus together.
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, 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. DiracDirac, Paul Adrien Maurice
, 1902–84, English physicist. He was educated at the Univ. of Bristol and St. John's College, Cambridge, and became professor of mathematics at Cambridge in 1932.
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's relativistic quantum theoryquantum theory,
modern physical theory concerned with the emission and absorption of energy by matter and with the motion of material particles; the quantum theory and the theory of relativity together form the theoretical basis of modern physics.
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 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. AndersonAnderson, Carl David
, 1905–91, American physicist, b. New York City, grad. California Institute of Technology (B.S., 1927; Ph.D., 1930). Associated with the institute's physics department from 1930, he became professor in 1939.
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, while studying cosmic rayscosmic rays,
charged particles moving at nearly the speed of light reaching the earth from outer space. Primary cosmic rays consist mostly of protons (nuclei of hydrogen atoms), some alpha particles (helium nuclei), and lesser amounts of nuclei of carbon, nitrogen, oxygen, and
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, 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 ChamberlainChamberlain, Owen,
1920–2006, American physicist, b. San Francisco, Calif., Ph.D. Univ. of Chicago, 1948. He was on the faculty at the Univ. of California, Berkeley, from 1949 until his retirement in 1989, when he was named professor emeritus.
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 and Emilio SegrèSegrè, Emilio,
1905–89, Italian-American physicist, Ph.D. Univ. of Rome, 1928. Segrè was a professor at the Univ. of Rome (1932–36), a researcher at the Univ.
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 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 deuteriumdeuterium
, isotope of hydrogen with mass no. 2. The deuterium nucleus, called a deuteron, contains one proton and one neutron. Deuterium is also called heavy hydrogen, and water in which the hydrogen atoms are deuterium is called heavy water (deuterium oxide, D2O).
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). 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. FitchFitch, Val Logsdon,
1923–2015, American nuclear physicist, b. Merriman, Neb., Ph.D. Columbia, 1954. During World War II Fitch was drafted into the army and worked on the detonator for the atomic bomb at Los Alamos.
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 and James W. CroninCronin, James Watson,
1931–, American nuclear physicist, b. Chicago, Ph.D. Univ. of Chicago, 1955. Cronin and co-researcher Val Logsdon Fitch were awarded the 1980 Nobel Prize in Physics for a 1964 experiment that proved that certain subatomic reactions do not adhere to
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 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.

antiparticle

[′an·tē¦pärd·ə·kəl]
(particle physics)
A counterpart to a particle having mass, lifetime, and spin identical to the particle but with charge and magnetic moment reversed in sign.

antiparticle

any of a group of elementary particles that have the same mass and spin as their corresponding particle but have opposite values for all other nonzero quantum numbers. When a particle collides with its antiparticle, mutual annihilation occurs FORMULA