Annihilation 346133 221815854 2008-06-26T06:21:11Z Mike Rosoft 75230 Added [[Category:Antimatter]] {{otheruses1|destruction}} [[Image:Electron-positron-annihilation.svg|200px|thumb|right|A [[Feynman diagram]] of a [[positron]] and an [[electron]] annihilating into a photon which then [[Pair production|decays back]] into a positron and an electron.]] '''Annihilation''' is defined as "total destruction" or "complete obliteration" of an object;<ref> [http://dictionary.reference.com/search?r=2&q=Annihilation - Dictionary Definition] (2006) Dictionary.com.</ref> having its root in the Latin ''nihil'' (nothing). A literal translation is "to make into nothing". Annihilation is the opposite of exnihilation, which means "to create something out of nothing". In [[physics]], the word is used to denote the process that occurs when a [[subatomic particle]] collides with its respective [[antiparticle]]<ref>{{cite web | url=http://www.lbl.gov/abc/Antimatter.html | title=Antimatter | author=Nuclear Science Division ---- Lawrence Berkeley National Laboratory | accessdate=09-03-2008}}</ref>. 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 number]]s 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. During a low-energy annihilation, [[photon]] production is favored, since these particles have no mass. However, high-energy [[particle collider]]s produce annihilations where a wide variety of exotic heavy particles are created. == Examples of annihilation == This is an example of [[renormalization]] in [[quantum field theory]]&mdash; 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]] (anti-electron), they can only produce two or more [[gamma ray]] [[photon]]s, since the electron and positron do not carry enough [[mass-energy]] to produce heavier particles. However, if one or both particles carry a larger amount of kinetic energy, various other particle pairs can be produced. See [[electron-positron annihilation]]. The annihilation (or decay) of an electron-positron pair into a ''single'' photon, e<sup>+</sup>&nbsp;+&nbsp;e<sup>-</sup>&nbsp;→&nbsp;γ, cannot occur because energy and 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 energy and 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 energy|energy conservation]] 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). {{Fact|date=February 2007}} 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. == References == ===Notations=== * {{cite book | author=Kragh, Helge| title= Quantum Generations : A history of physics in the twentieth century | publisher= Princeton University Press | year=1999| id =ISBN 0-691-01206-7 }} ===Footnotes=== {{reflist}} ==See also== *[[Pair production]] [[Category:Particle physics]] [[Category:Destruction]] [[Category:Antimatter]] [[ca:Aniquilació de matèria amb antimatèria]] [[cs:Anihilace]] [[de:Annihilation]] [[fr:Annihilation électrons-positrons]] [[it:Annichilazione]] [[he:איון]] [[lv:Anihilācija]] [[lt:Anihiliacija]] [[nl:Annihilatie]] [[ja:対消滅]] [[pl:Anihilacja]] [[ro:Anihilare]] [[ru:Аннигиляция]] [[sk:Anihilácia]] [[sr:Анихилација]] [[sh:Anihilacija]] [[fi:Annihilaatio]] [[sv:Annihilation]] [[uk:Анігіляція]] [[zh:湮灭]]