Editing Electron–positron annihilation (section)

==Low-energy case==
There are only a very limited set of possibilities for the final state. The most probable is the creation of two or more gamma photons. Conservation of energy and linear momentum forbid the creation of only one photon. (An exception to this rule can occur for tightly bound atomic electrons.<ref>
{{cite journal
 |author1=L. Sodickson |author2=W. Bowman |author3=J. Stephenson |author4=R. Weinstein |year=1961
 |title=Single-Quantum Annihilation of Positrons
 |journal=[[Physical Review]]
 |volume=124 |issue=6 |pages=1851–1861
 |doi=10.1103/PhysRev.124.1851
|bibcode=1961PhRv..124.1851S}}</ref>) In the most common case, two gamma photons are created, each with [[photon energy|energy]] equal to the [[rest energy]] of the electron or positron ({{val|.511|ul=MeV}}).<ref>
{{cite journal
 |author=W.B. Atwood, P.F. Michelson, S.Ritz
 |year=2008
 |title=Una Ventana Abierta a los Confines del Universo
 |journal=[[Investigación y Ciencia]]
 |volume=377 |pages=24–31
|language=es}}</ref> A convenient [[frame of reference]] is that in which the system has [[center of mass frame|no net linear momentum]] before the annihilation; thus, after collision, the gamma photons are emitted in opposite directions. It is also common for three to be created, since in some angular momentum states, this is necessary to conserve [[C parity|charge parity]].<ref name="griffiths">
{{cite book
 |author=D.J. Griffiths
 |year=1987
 |title=Introduction to Elementary Particles
 |publisher=[[John Wiley & Sons|Wiley]]
 |isbn=0-471-60386-4
}}</ref> It is also possible to create any larger number of photons, but the probability becomes lower with each additional gamma photon because these more complex processes have lower [[probability amplitude]]s.

Since [[neutrino]]s also have a smaller mass than electrons, it is also possible – but exceedingly unlikely – for the annihilation to produce one or more neutrino–[[antineutrino]] pairs. The probability for such process is on the order of 10000 times less likely than the annihilation into photons. The same would be true for any other particles, which are as light, as long as they share at least one [[fundamental interaction]] with electrons and no conservation laws forbid it. However, no other such particles are known.