Hara's theorem and W-exchange in hyperon weak radiative decays

We reconsider Hara's theorem and relate it to the well-known properties of beta-decay. All assumptions necessary for the theorem to be true are explicitly formulated. Further, we study the W-exchange contribution to weak radiative decays and show that it does not violate Hara's theorem. However, this contribution reveals the essential role of particle mixing in symmetry considerations and some peculiar features of gauge-invariant amplitudes under perturbative expansion. Together they explain an effect, which was treated as contradicting Hara's theorem, without any violation. The properties of W-exchange we describe here may have more general importance and should be taken into account in further detailed calculations of weak processes.