Lorentz-violating neutral-pion decays in isotropic modified Maxwell   theory
  We consider an extension of the Standard Model with isotropic nonbirefringent Lorentz violation in the photon sector and restrict the discussion to the case of a "fast" photon with a phase velocity larger than the maximum attainable velocity of the quarks and leptons. With our conventions, this case corresponds to a negative Lorentz-violating parameter $\kappa$ in the action. The decay rate of a neutral pion into two nonstandard photons is calculated as a function of the 3-momentum of the initial pion and the negative Lorentz-violating parameter $\kappa$ of the final photons. 
F.R. Klinkhamer
hep-ph