Photon splitting above the pair creation threshold in a strong magnetic   field
  The process of photon splitting $\gamma \to \gamma \gamma$ in a strong magnetic field is investigated both below and above the pair creation threshold. Contrary to the statement by Baier et al., the ``allowed'' channel $\alw$ is shown not to be a comprehensive description of splitting in the strong field because the ``forbidden'' channel $\frb$ is also essential. The partial amplitudes and the splitting probabilities are calculated taking account of the photon dispersion and large radiative corrections near the resonance. 
M.V. Chistyakov, A.V. Kuznetsov and N.V. Mikheev