Dalitz decay

A Dalitz decay replaces a radiated real photon with a virtual photon converting to a charged-lepton pair. The neutral-pion example is ${\pi }^0\to \gamma e^{+}{e}^{-}$. Double Dalitz decay converts both virtual photons in the two-photon mode.

Double Dalitz decay converts both virtual photons from a two-photon hadronic transition into charged-lepton pairs. The neutral-pion four-electron final state avoids the specific two-lepton helicity suppression of ${\pi }^0\to e^{+}{e}^{-}$, despite its more restricted phase space.