Electron–positron annihilation occurs when an electron and a positron collide. At low energies, the result of the collision is the annihilation of the electron and positron, and the creation of energetic 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:
Conservation of total lepton number, which is the number of leptons minus the number of antileptons ; this can be described as a conservation of matter law.
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 probable is the creation of two or more 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 energyequal to the rest energy of the electron or positron. A convenient frame of reference is that in which the system has no net linear momentum before the annihilation; thus, after collision, the photons 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. 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 probability for such process is on the order of 10000 times less likely than the annihilation into photons. 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 either the electron or positron, or both, have appreciable kinetic energies, other heavier particles can also be produced, since there is enough kinetic energy in the relative velocities to provide the rest energies of those particles. Alternatively, it is possible to produce photons and other light particles, but they will emerge with higher kinetic 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 to the electromagnetic force. As a result, it becomes much easier to produce particles such as neutrinos that interact only weakly with other matter. The heaviest particle pairs yet produced by electron–positron annihilation in particle accelerators are – pairs. The heaviest single-charged particle is the Z boson. The driving motivation for constructing the International Linear Collider is to produce the Higgs bosons in this way.
