Annihilation is defined as "total destruction" or "complete obliteration" of a particular object. Annihilation of an atomic or subatomic particle, occurs when such a particle collides with its respective antiparticle. If a particle and its respective antiparticle are both tuned to the appropriate quantum states, then they annihilate each other and their "destruction" yields other particles; usually into particles elementary of other particles. Nothing can ever be "annihilated" physically, as stipulated by its definition, as this would break the Law of Conservation of Matter.
Instead, "annihlation" transforms matter into energy of some form, such as a radiation of photons. The energy is carried by force carriers; particles that can decay into other particles. As no true elementary particle has been found, experimentally or theoretically, annihilation can yield almost any type of energy or more elementary particles, instead of just photons, which is more generally believed to be a product of annihilation.
Examples of Annihilation
- When an electron annihilates a positron (anti-electron), it yields gamma rays only, which are pure energy.
- When a proton annihilates an antiproton they produce gamma rays and a swarm of secondary particles, like pairs of top-anti-top quarks. The secondary particles will eventually decay into neutrinos and low-energy gamma rays. It is widely believed that neutrinos could have mass, implying that the p-p annihilation doesn't transform all the mass into energy.
Reactions such as e+ + e- → γ + γ (the two-photon annihilation of an electron-positron pair) is another example.
The single-photon annihilation of an electron-positron pair, e+ + e- → γ cannot occur because it is impossible to conserve energy and momentum together in this process. The reverse reaction is also impossible for this reason. However, in quantum field theory this process is allowed as an intermediate quantum state for times short enough that the violation of energy conservation can be accommodated by the uncertainty principle. This opens the way for virtual pair production or annihilation in which a one particle's quantum state may fluctuate into a two particle state and back again. These processes are important in the vacuum state and renormalization of a quantum field theory. It also opens the way for neutral particle mixing through processes such as the one pictured on the right: which is a complicated example of mass renormalization.