Editing Antiparticle (section)

== Particle–antiparticle annihilation ==
{{main|Annihilation}}
[[Image:kkbar had.svg|frame|alt=Feynman diagram of a kaon oscillation. A straight red line suddenly turns purple, showing a kaon changing into an antikaon. A medallion is show zooming in on the region where the line changes color. The medallion shows that the line is not straight, but rather that at the place the kaon changes into an antikaon, the red line breaks into two curved lines, corresponding the production of virtual pions, which rejoin into the violet line, corresponding to the annihilation of the virtual pions. |An example of a virtual [[pion]] pair that influences the propagation of a [[kaon]], causing a neutral kaon to ''mix'' with the antikaon. This is an example of [[renormalization]] in [[quantum field theory]] – the field theory being necessary because of the change in particle number.]]

If a particle and antiparticle are in the appropriate quantum states, then they can annihilate each other and produce other particles. Reactions such as {{Subatomic particle|Electron}}&nbsp;+&nbsp;{{Subatomic particle|Positron}}&nbsp;→&nbsp;{{Subatomic particle|Photon}}{{Subatomic particle|Photon}} (the two-photon annihilation of an electron-positron pair) are an example. The single-photon annihilation of an electron-positron pair, {{Subatomic particle|Electron}}&nbsp;+&nbsp;{{Subatomic particle|Positron}}&nbsp;→&nbsp;{{Subatomic particle|Photon}}, cannot occur in free space because it is impossible to conserve energy and [[momentum]] together in this process. However, in the Coulomb field of a nucleus the [[translational invariance]] is broken and single-photon annihilation may occur.<ref>
{{cite journal
 | last=Sodickson | first=L.
 |author2=W. Bowman |author3=J. Stephenson
  | date = 1961
 | title = Single-Quantum Annihilation of Positrons
 | journal = [[Physical Review]]
 | volume = 124  | issue = 6 | pages = 1851–1861
 | bibcode = 1961PhRv..124.1851S
 | doi = 10.1103/PhysRev.124.1851
}}</ref> The reverse reaction (in free space, without an atomic nucleus) is also impossible for this reason. In quantum field theory, this process is allowed only 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 quantum state may ''fluctuate'' into a two particle state and back. 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 here, which is a complicated example of [[mass renormalization]].