The amplitude for Higgs boson decay into two photons in a homogeneous and
time-independent magnetic field is investigated by proper-time regularization
in a gauge invariant manner and is found to be singular at large field values.
The singularity is caused by the component of the charged vector boson field
that is tachyonic in a strong magnetic field. Also tools for the computation of
the amplitude in a more general electromagnetic background are developed.Comment: 31 page
