Width of photon decay in magnetic field: elementary semiclassical derivation and sensitivity to Lorentz violation

We present an elementary derivation of the width of photon decay in a weak magnetic field using the semiclassical method of worldline instantons. The calculation is generalized to a model of quantum electrodynamics with broken Lorentz symmetry. Implications for the search of deviations from Lorentz invariance in the cosmic ray experiments are discussed.