Electron–positron annihilation

A process in which an electron and a positron disappear together, converting their rest energy and kinetic energy into photons or other particles while conserving quantum numbers.

Two-photon annihilation: Annihilation of an electron and positron into two gamma-ray photons. This is the usual outcome for a low-energy pair in free space.
Electron: A stable, negatively charged elementary particle in the lepton family. It is one member of the annihilating pair.
Positron emission tomography: A medical imaging technique that maps radioactive tracer distributions using detected annihilation photons. It reconstructs tracer locations from pairs of photons produced after positron emission.
Positronium decay: The decay of positronium through annihilation of its electron and positron constituents. The bound state's singlet and triplet forms favor different photon counts.
Three-photon annihilation: Annihilation of an electron and positron into three photons. It is allowed when the pair's spin state forbids two-photon decay.
Positron: The electron's positively charged antiparticle, with the same mass and spin. Its opposite electric charge permits annihilation with an electron.
Positron annihilation spectroscopy: A family of methods that uses positron annihilation to probe material defects and electron structure. Changes in annihilation signals reveal vacancies and other defects in solids.
Electron–positron annihilation at rest: Annihilation of an electron and positron with negligible kinetic energy in their center-of-mass frame. Two photons emerge back-to-back, each carrying approximately 511 keV.
Conservation of four-momentum: The requirement that total energy and momentum remain unchanged in physical interactions. It explains why a free pair at rest cannot produce just one photon.
Antimatter: Matter composed of antiparticles, which have the same masses as corresponding particles and opposite additive charges. Electron–positron annihilation is a familiar example of matter meeting antimatter.

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