antiparticle

[an-tee-pahr-ti-kuhl, an-tahy-]
noun Physics.
a particle all of whose properties, as mass, spin, or charge, have the same magnitude as but, where appropriate, the opposite sign of a specific elementary particle; neutral pions, photons, and gravitons are considered to be their own antiparticles: The positron is the antiparticle of the electron. Compare antimatter, annihilation ( def 3 ).

Origin:
1930–35; anti- + particle

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World English Dictionary
antiparticle (ˈæntɪˌpɑːtɪkəl)
 
n
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

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American Heritage
Science Dictionary
antiparticle   (ān'tē-pär'tĭ-kəl, ān'tī-)  Pronunciation Key 
A subatomic particle, such as an antiproton, having the same mass as its corresponding particle, but opposite values of other properties such as charge, parity, spin, and direction of magnetic moment. For example, the antiparticle of the electron is the positron, which has a charge that is equal in magnitude to that of the electron but opposite in sign. Some particles, such as photons, are nondistinct from their antiparticles. When a particle and its antiparticle collide, they may annihilate one other and produce other particles.
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Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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American Heritage
Cultural Dictionary

antiparticle definition


In physics, a rare form of subatomic matter that is a mirror image of normal matter. The antiparticle corresponding to an elementary particle has the same mass as the particle but is opposite in all other properties. The antiparticle corresponding to an electron is a positron, which has the same mass as an electron but a positive charge. Antiprotons have the same mass as protons but a negative charge. When matter and antimatter come together, the two particles annihilate each other, converting their mass into energy or into other types of particles.

Note: As far as scientists can tell, there is almost no naturally occurring antimatter in the universe, although it is possible to make antimatter in particle accelerators.
The American Heritage® New Dictionary of Cultural Literacy, Third Edition
Copyright © 2005 by Houghton Mifflin Company.
Published by Houghton Mifflin Company. All rights reserved.
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Encyclopedia Britannica
Encyclopedia

antiparticle

subatomic particle having the same mass as one of the particles of ordinary matter but opposite electric charge and magnetic moment. Thus, the positron (positively charged electron) is the antiparticle of the negatively charged electron. The spinning antineutron, like the ordinary neutron, has a net electric charge of zero, but its magnetic polarity is opposite to that of a similarly spinning neutron. The neutrino, a massless uncharged particle that travels at the speed of light, spins counterclockwise as viewed from behind, whereas the antineutrino spins clockwise as viewed from behind. A particle and its antiparticle mutually react to produce energy by annihilation.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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Example sentences
When one antiparticle is attracted over the event horizon and its matching
  particle escapes, radiation seems to be emitted.
Pair production is the production of a particle of matter and its antiparticle.
His equations predicted the existence of an antiparticle, which was later
  confirmed by experiment.
It is now believed that every fundamental particle has a corresponding
  antiparticle.
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