ElectronPositron

Electron-positron refers to the pair consisting of the electron and its antiparticle, the positron. The electron is a negatively charged lepton with mass m_e ≈ 9.109×10^-31 kg, charge −e, and spin 1/2. The positron has the same mass and spin but opposite charge (+e) and is the electron’s antiparticle. As members of the same lepton family, they interact electromagnetically and can annihilate each other when they meet.

Two central processes involve electron and positron pairs: annihilation and production. Pair production occurs when a high-energy photon interacts with a nucleus or field, allowing creation of an electron-positron pair if the photon energy exceeds 1.022 MeV (twice the electron rest energy). Annihilation typically yields two gamma photons of 511 keV each in the center-of-mass frame, emitted back-to-back; if the system is moving, the photons may have different energies or additional particles can be produced at higher energies.

Positronium is a short-lived bound state of an electron and a positron, formed in vacuum or in materials. Para-positronium (singlet state) decays into two gamma photons with a lifetime of about 125 picoseconds, while ortho-positronium (triplet state) decays into three photons with a lifetime near 142 nanoseconds in vacuum.

Occurrences and applications span laboratory experiments, astrophysical environments, and medicine. Electron-positron annihilation is central to quantum electrodynamics and is exploited in positron emission tomography (PET), where positron-emitting radionuclides generate coincident 511 keV gamma rays detected to image metabolic processes. The positron was predicted by Dirac and discovered experimentally by Anderson in 1932, confirming the existence of antiparticles and underpinning many aspects of modern particle physics.