ANNIHILATION REACTION

Electron–positron annihilation occurs when an electron and a positron (the electron's anti-particle) collide. The result of the collision is the conversion of the electron and positron and 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. However, in quantum field theory this process is allowed.
Conservation of angular momentum.

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. 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.
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.

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.
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
The reverse reaction is a form of matter creation governed by two-photon physics.