Electron–positron annihilation occurs when an electron (e−
) and a positron (e+
, the electron's antiparticle) collide. The result of the collision is the annihilation of the electron and positron, and the creation of gamma ray photons or, at higher energies, other particles:
The process must satisfy a number of 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 possible is the creation of two or more gamma ray 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.) In the most common case, two photons are created, each with energy equal to the rest energy of the electron or positron (511 keV). A convenient frame of reference is 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. 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 probability amplitudes.
Since neutrinos 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 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.
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