Wednesday, August 5, 2009



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. However, in quantum field theory this process is allowed.
Conservation of angular momentum.


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. 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 quantum mechanical amplitudes.

High energy case
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 W+/W− pairs. The heaviest single particle is the Z boson. The driving motivation for constructing the International Linear Collider is to produce Higgs bosons in this way.

Practical uses
This process is the physical phenomenon relied on as the basis of PET imaging and Positron Annihilation Spectroscopy. Also used as a method of measuring the Fermi surface and Band structure in metals.

Reverse reaction

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