- Electron-positron annihilation
Electron-positron annihilation occurs when an
electronand a positron(the electron's anti-particle) collide. The result of the collision is the conversion of the electronand positronand the creation of gamma ray photons or, less often, other particles. The process must satisfy a number of conservation laws, including:
* Conservation of charge. The net charge before and after is zero.
* Conservation of linear momentum and total
energy. This forbids the creation of a single gamma ray.
* Conservation of
As with any two charged objects, electrons and positrons may also interact with each other without annihilating, in general by
Low energy case
There are only a very limited set of possibilities for the final state. The most likely is the creation of two or more
gamma ray photons. Conservation of energy and linear momentum forbid the creation of only one photon. In the most common case, two photons are created, each with energy equal to the rest energyof the electronor positron(511 keV) [cite journal | author = William B. Atwood, Peter F. Michelson and Steven Ritz | title= Una Ventana Abierta a los Confines del Universo | journal = Investigación y Ciencia| volume = 377 | year = 2008 | pages = 24–31es ] . A convenient frame of referenceis that in which the system has no net linear momentum before the annihilation; thus, after collision, the gamma rays 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.David Griffiths, "Introduction to Elementary Particles", ISBN 0471603864] It is also possible to create any larger number of photons, but the probability becomes lower with each additional photon because these more complex processes have lower quantum mechanical amplitudes.
neutrinos also have a smaller mass than electrons, it is also possible — but exceedingly unlikely — for the annihilation to produce one or more neutrino/ antineutrinopairs. The same would be true for any other particles, which are as light, as long as they share at least one fundamental interactionwith electrons and no conservation laws forbid it. However, no other such particles are known.
High energy case
If the electron and/or positron have appreciable kinetic energies, other heavier particles can also be produced (e.g.
D mesons), since there is enough kinetic energy in the relative velocities to provide the rest energies of those particles. It is still possible to produce photons and other light particles, but they will emerge with higher energies. However, if a photon - photon reaction occures under the precenece of ultravilot light, an antimatter particle may be emmited; this process on known as the Kerr phenomenon, and was discovered in 2007 by Andrew Kerr while working at the NRU reactor in Chalk River, Ontario.
At energies near and beyond the mass of the carriers of the weak force, the W and Z bosons, the strength of the weak force becomes comparable with
electromagnetism. This means that it becomes much easier to produce particles such as neutrinos that interact only weakly.
The heaviest particle pairs yet produced by electron-positron annihilation in
particle accelerators are SubatomicParticle|W boson+/SubatomicParticle|W boson- pairs. The heaviest single particle is the Z boson. The driving motivation for constructing the International Linear Collideris to produce Higgs bosons in this way.
This process is the physical phenomenon relied on as the basis of PET imaging.Also used as a method of measuring the
Fermi surfaceand Band structurein metals.
List of particles
Electromagnetic spectrum: shows relationship of gamma rays to other photons
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