annihilation

/euh nuy'euh lay"sheuhn/, n.

1. the act or an instance of annihilating.

2. the state of being annihilated; extinction; destruction.

3. Physics.

a. Also called pair annihilation. the process in which a particle and antiparticle unite, annihilate each other, and produce one or more photons. Cf. positronium.

b. the conversion of rest mass into energy in the form of electromagnetic radiation.

[1630-40; ( < F) < LL annihilation- (s. of annihilatio). See ANNIHILATE, -ION]

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In physics, a reaction in which a particle and its antiparticle (see antimatter) collide and disappear.

The annihilation releases energy equal to the original mass m multiplied by the square of the speed of light c, or E = mc², in accordance with Albert Einstein's special theory of relativity. The energy can appear directly as gamma rays or can convert back to particles and antiparticles (see pair production).

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▪ physics

      in physics, reaction in which a particle (subatomic particle) and its antiparticle collide and disappear, releasing energy. The most common annihilation on Earth occurs between an electron and its antiparticle, a positron. A positron, which may originate in radioactive decay (radioactivity) or, more commonly, in the interactions of cosmic rays (cosmic ray) in matter, usually combines briefly with an electron to form a quasi-atom called positronium. The quasi-atom is composed of the two particles spinning around each other before they annihilate. After the annihilation, two or three gamma rays (gamma ray) radiate from the point of collision.

      The amount of energy (E) produced by annihilation is equal to the mass (m) that disappears multiplied by the square of the speed of light in a vacuum (c)—i.e., E = mc². Thus, annihilation is an example of the equivalence of mass and energy and a confirmation of the theory of special relativity, which predicts this equivalence. (See Einstein's mass-energy relation.)

      At the higher energies characteristic of particle-antiparticle collisions taking place in colliding-beam storage ring particle accelerators or in the big-bang model (Cosmos) of the early universe, the annihilation energy is sufficient to create heavier particles and their antiparticles, such as muons (muon) and antimuons or quarks (quark) and antiquarks. Combinations of these latter particles and antiparticles, in turn, form mesons (meson)—including pi-mesons and K-mesons—which are classified within the hadron (subatomic particle) group of subatomic particles. Other annihilation reactions also occur. Nucleons (nucleon) (protons and neutrons), for example, annihilate antinucleons (antiprotons and antineutrons), and the energy is also carried away in the form of particles such as pi-mesons and K-mesons and their corresponding antiparticles.

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