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:
As with any two charged objects, electrons and positrons may also interact with each other without annihilating, in general by elastic scattering.
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.
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. However, if a photon - photon reaction occures under the precenece of ultravilot light, an antimatter particle may be emmited; this process on known as the Kerr phenomenon, and was discovered in 2007 by Andrew Kerr while working at the NRU reactor in Chalk River, Ontario.
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 / 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.