When a proton encounters its antiparticle (and more generally, if any species of baryon encounters the corresponding antibaryon), the reaction is not as simple as electron–positron annihilation. The inverse process, pair production by a single real photon, is also possible in the electromagnetic field of a third particle. 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. If one or both charged particles carry a larger amount of kinetic energy, various other particles can be produced. 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). Each of the photons then has an energy of about 0.511 MeV. If their kinetic energies are relatively negligible, this total rest energy appears as the photon energy of the photons produced. Both the annihilating electron and positron particles have a rest energy of about 0.511 million electron-volts (MeV). When a low-energy electron annihilates a low-energy positron (antielectron), the most probable result is the creation of two or more photons, since the only other final-state Standard Model particles that electrons and positrons carry enough mass–energy to produce are neutrinos, 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. Main article: Electron–positron annihilation If the energy is large enough, aĮxamples Electron–positron annihilation Electron/positron annihilation at various energies An example is the annihilation of an electron with a positron to produce a virtual photon, which converts into a muon and anti-muon. Otherwise, the process is understood as the initial creation of a boson that is virtual, which immediately converts into a real particle + antiparticle pair. ), then that created particle will continue to exist until it decays according to its lifetime. If the total energy in the center-of-momentum frame is equal to the rest mass of a real boson (which is impossible for a massless boson such as the If the initial two particles are elementary (not composite), then they may combine to produce only a single elementary boson, such as a photon ( If the annihilating particles are composite, such as mesons or baryons, then several different particles are typically produced in the final state. An example is the "annihilation" of a high-energy electron antineutrino with an electron to produce a W − boson. Some quantum numbers may then not sum to zero in the initial state, but conserve with the same totals in the final state. The word "annihilation" takes use informally for the interaction of two particles that are not mutual antiparticles – not charge conjugate. High-energy particle colliders produce annihilations where a wide variety of exotic heavy particles are created. ĭuring a low-energy annihilation, photon production is favored, since these particles have no mass. 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. Antiparticles have exactly opposite additive quantum numbers from particles, so the sums of all quantum numbers of such an original pair are zero. 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. 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 photons.
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