Annihilation is defined as "total destruction" or "complete obliteration" of an object;[1] having its root in the Latin nihil (nothing). A literal translation is "to make into nothing".
In physics, the word is used to denote the process that occurs when a subatomic particle collides with its respective antiparticle.[2] Since energy and momentum must be conserved, the particles are not actually made into nothing, but rather into new particles. Antiparticles have exactly opposite additive quantum numbers from particles, so the sums of all quantum numbers of the 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 and conservation of momentum are obeyed. When a particle and its antiparticle collide, their energy is converted into a force carrier particle, such as a gluon, W/Z force carrier particle, or a photon. These particles are afterwards transformed into other particles.[3]
During a low-energy annihilation, photon production is favored, since these particles have no mass. However, high-energy particle colliders produce annihilations where a wide variety of exotic heavy particles are created.
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This is an example of renormalization in quantum field theory— the field theory being necessary because the number of particles changes from one to two and back again.
When a low-energy electron annihilates a low-energy positron (antielectron), they can only produce two or more gamma ray photons, since the electron and positron do not carry enough mass-energy to produce heavier particles and conservation of energy and linear momentum forbid the creation of only one photon.When an electron and a positron collide to annihilate and create gamma rays, energy is given off. Both particles have a rest energy of 0.511 MeV or million electron volts. When the mass of the two particles are converted entirely into energy, this rest energy is what is given off. The energy is given off in the form of the aforementioned gamma rays. Each of the gamma rays have an energy of .511 MeV. Since the positron and electron are both briefly at rest during this annihilation, the system has no momentum during that moment. This is the reason that two gamma rays are created. Conservation of momentum would not be achieved if only one photon was created in this particular reaction. Momentum and energy are both conserved with 1.022 MeV of gamma rays (accounting for the rest energy of the particles) moving in opposite directions (accounting for the total zero momentum of the system).[4] However, if one or both particles carry a larger amount of kinetic energy, various other particle pairs can be produced. The annihilation (or decay) of an electron-positron pair into a single photon, cannot occur in free space because momentum would not be conserved in this process. The reverse reaction is also impossible for this reason, except in the presence of another particle that can carry away the excess momentum. However, in quantum field theory this process is allowed as an intermediate quantum state. Some authors justify this by saying that the photon exists for a time which is short enough that the violation of conservation of momentum can be accommodated by the uncertainty principle. Others choose to assign the intermediate photon a non-zero mass. (The mathematics of the theory are unaffected by which view is taken.) This opens the way for virtual pair production or annihilation in which a one-particle quantum state may fluctuate into a two-particle state and back again (coherent superposition). These processes are important in the vacuum state and renormalization of a quantum field theory. It also allows neutral particle mixing through processes such as the one pictured here.
This form of annihilation occurs when a quark from a proton, and an anti-quark from an anti-proton collide with each other. The two particles collide and annihilate, which results in the creation of virtual gluons. A top and antitop quark emerge from the gluon cloud, and these quarks begin moving apart and stretching the gluon field between them. As the particles move away from each other, the top quark and antiquark decay into a bottom and antibottom quark, and emit W force carrier particles(a W- boson and a W+ boson). An electron and neutrino emerge from the virtual W- boson and an up quark and down antiquark emerge from the virtual W+ boson. At the end of this process the bottom quark and bottom antiquark, electron, neutrino, up quark, and down antiquark all move away from each other.[3]