Relativistic three-body recombination with the QED vacuum
  Electron-positron pair annihilation into a single photon is studied when a second free electron is present. Focussing on the relativistic regime, we show that the photon emitted in the three-lepton interaction may exhibit distinct angular distributions and polarization properties. Moreover, the process can dominate over two-photon annihilation in relativistic electron-positron plasmas of few-MeV temperature. An analogy with three-body recombination of electrons with ions is drawn. 
Huayu Hu and Carsten M\"uller
hep-ph physics.atom-ph physics.plasm-ph