Two-photon decays of heavy neutrinos

The process ${\nu }_1\to {\nu }_2+\gamma +\gamma$ is studed in detail. Formulas for the decay rate are given in the context of the SU(2)×U(1) model with massive neutrinos and it is shown that this process can dominate over ${\nu }_1\to {\nu }_2+\gamma$ for some range of values of $m_{{\nu }_1}$, depending on the relevant mixing angles. A general, model-independent analysis is also given with particular emphasis on the difference between the cases of Dirac and Majorana neutrinos. In particular, we show how the invariant amplitudes in the Majorana case can be expressed in terms of the amplitudes calculated as if the neutrinos were Dirac particles.