Electron–positron annihilation

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Naturally occurring electron-positron annihilation as a result of beta plus decay
Antimatter
A Feynman diagram showing the annihilation of an electron and a positron (antielectron), creating a photon that later decays into an new electron–positron pair.
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Electron–positron annihilation occurs when an electron (
e−
) and a positron (
e+
, the electron's antiparticle) collide. At low energies, the result of the collision is the annihilation of the electron and positron, and the creation of energetic photons:


e−
 + 
e+
 → 
γ
 + 
γ

At high energies, other particles, such as

conservation laws
, including:

As with any two charged objects, electrons and positrons may also interact with each other without annihilating, in general by elastic scattering.

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.

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 charge parity.[3] 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 amplitudes
.

Since

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.

High-energy case

If either the electron or positron, or both, have appreciable

rest energies
of those particles. Alternatively, it is possible to produce photons and other light particles, but they will emerge with higher kinetic energies.

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 to the electromagnetic force.[3] As a result, it becomes much easier to produce particles such as neutrinos that interact only weakly with other matter.

The heaviest particle pairs yet produced by electron–positron annihilation in

Z boson (mass 91.188 GeV/c2). The driving motivation for constructing the International Linear Collider is to produce the Higgs bosons (mass 125.09 GeV/c2) in this way.[citation needed
]

Electron/positron annihilation at various energies

Practical uses

The electron–positron annihilation process is the physical phenomenon relied on as the basis of

band structure in metals by a technique called Angular Correlation of Electron Positron Annihilation Radiation
. It is also used for nuclear transition. Positron annihilation spectroscopy is also used for the study of crystallographic defects in metals and semiconductors; it is considered the only direct probe for vacancy-type defects.[4]

Reverse reaction

The reverse reaction, electron–positron creation, is a form of pair production governed by two-photon physics.

See also

References

  1. ^ L. Sodickson; W. Bowman; J. Stephenson; R. Weinstein (1970). "Single-Quantum Annihilation of Positrons".
    doi:10.1103/PhysRev.124.1851
    .
  2. ^ W.B. Atwood, P.F. Michelson, S.Ritz (2008). "Una Ventana Abierta a los Confines del Universo".
    Investigación y Ciencia (in Spanish). 377: 24–31.{{cite journal}}: CS1 maint: multiple names: authors list (link
    )
  3. ^ a b D.J. Griffiths (1987). Introduction to Elementary Particles.
    ISBN 0-471-60386-4
    .
  4. ^ F. Tuomisto and I. Makkonen (2013). "Defect identification in semiconductors with positron annihilation: Experiment and theory".
    S2CID 41119818
    .
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