Double Dalitz decay (${\pi }^0\to {\gamma }^{\ast }{\gamma }^{\ast }\to e^{+}{e}^{-}{e}^{+}{e}^{-}$)

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.