= Electron-positron annihilation
= Positron annihilation
{synonym}
Electron-positron <annihilation> converts an <electron> and a <Positron> into other <particles> while conserving total <energy>, <momentum> and <electric charge>. A common low-energy channel produces two <photons>. In the centre-of-momentum frame, negligible initial <kinetic energy> gives two oppositely directed <photons>, each with <energy> $m_ec^2$.
The back-to-back <momentum> balance makes paired <photon> detections a useful signature of <annihilation>. Motion of the initial pair changes the laboratory <photon> directions and <energies>. At larger collision <energy>, other final states become accessible: an intermediate virtual <photon> can produce, for example, a <muon>-<antimuon> pair. Thus <annihilation> converts the initial pair into allowed final excitations rather than destroying conserved <energy>.
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