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In particle physics, annihilation is the process that occurs when a subatomic particle collides with its respective antiparticle to produce other particles, such as an electron colliding with a positron to produce two . The total energy and momentum of the initial pair are conserved in the process and distributed among a set of other particles in the final state. Antiparticles have exactly opposite additive from particles, so the sums of all quantum numbers of such an original pair are zero. Hence, any set of particles may be produced whose total quantum numbers are also zero as long as conservation of energy, conservation of momentum, and conservation of spin are obeyed.
During a low-energy annihilation, photon production is favored, since these particles have no mass. High-energy particle colliders produce annihilations where a wide variety of exotic heavy particles are created.
The word "annihilation" takes its use informally for the interaction of two particles that are not mutual antiparticles not charge conjugate. Some quantum numbers may then not sum to zero in the initial state, but conserve with the same totals in the final state. An example is the "annihilation" of a high-energy electron antineutrino with an electron to produce a W boson.
If the annihilating particles are composite, such as or , then several different particles are typically produced in the final state.
The inverse of annihilation is pair production, the process in which a high-energy photon converts its energy into mass.
When a low-energy electron annihilates a low-energy positron (antielectron), the most probable result is the creation of two or more , since the only other final-state Standard Model particles that electrons and positrons carry enough mass–energy to produce are , which are approximately 10,000 times less likely to produce, and the creation of only one photon is forbidden by momentum conservation—a single photon would carry nonzero momentum in any frame, including the center-of-momentum frame where the total momentum vanishes. Both the annihilating electron and positron particles have a rest energy of about 0.511 million electron-volts (MeV). If their kinetic energies are relatively negligible, this total rest energy appears as the photon of the photons produced. Each of the photons then has an energy of about 0.511 MeV. Momentum and energy are both conserved, with 1.022 MeV of photon energy (accounting for the rest energy of the particles) moving in opposite directions (accounting for the total zero momentum of the system).
If one or both charged particles carry a larger amount of kinetic energy, various other particles can be produced. Furthermore, the annihilation (or decay) of an electron–positron pair into a single photon can occur in the presence of a third charged particle, to which the excess momentum can be transferred by a virtual photon from the electron or positron. The inverse process, pair production by a single real photon, is also possible in the electromagnetic field of a third particle.
Reactions in which proton–antiproton annihilation produces as many as 9 mesons have been observed, while production of 13 mesons is theoretically possible. The generated mesons leave the site of the annihilation at moderate fractions of the speed of light and decay with whatever lifetime is appropriate for their type of meson.
Similar reactions will occur when an antinucleon annihilates within a more complex atomic nucleus, save that the resulting mesons, being strongly interacting, have a significant probability of being absorbed by one of the remaining "spectator" nucleons rather than escaping. Since the absorbed energy can be as much as ~2 GeV, it can in principle exceed the binding energy of even the heaviest nuclei. Thus, when an antiproton annihilates inside a heavy nucleus such as uranium or plutonium, partial or complete disruption of the nucleus can occur, releasing large numbers of fast neutrons. Such reactions open the possibility for triggering a significant number of secondary Nuclear fission in a Critical mass and may potentially be useful for spacecraft propulsion.
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