Photon decay in a CPT-violating extension of quantum electrodynamics
  We consider the process of photon decay in quantum electrodynamics with a CPT-violating Chern-Simons-like term added to the action. For a simplified model with only the quadratic Maxwell and Chern-Simons-like terms and the quartic Euler-Heisenberg term, we obtain a nonvanishing probability for the decay of a particular photon state into three others. 
C. Adam and F.R. Klinkhamer