Electron-positron annihilation
Electron–positron annihilation occurs when an electron (
e−
) and a positron (
e+
, the electron's antiparticle) collide. The result of the collision at low energies is the annihilation of the electron and positron, and the creation of gamma ray photons:
At high energies, other particles, such as B mesons or the W and Z bosons, can be created. All processes 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.
There are only a very limited set of possibilities for the final state. The most probable 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 (0.511 MeV). 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 charge 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.
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