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Yang's theorem states that an initial J=1 state cannot decay into twophotons. Because of this result some reactions relating to elementary particlesor atomic transitions can be ruled out. The theorem is not valid in thepresence of background electric or magnetic fields. In this work we show thatthe decay of a J=1 particle into two photons is permitted by Bose symmetry androtational invariance when the background of the decay process is not purevacuum but contains an external classical magnetic/electric field. We alsodiscuss constraints on these amplitudes from {\bf CP} invariance.Comment: Tex fil

DECAY OF SPIN-ONE PARTICLE INTO TWO PHOTONS IN PRESENCE OF UNIFORM EXTERNAL MAGNETIC FIELD