Enhanced Electromagnetic Transition Dipole Moments and Radiative Decays
  of Massive Neutrinos due to the Seesaw-induced Non-unitary Effects
  In a simple extension of the standard electroweak theory where the phenomenon
of lepton flavor mixing is described by a 3x3 unitary matrix V, the electric
and magnetic dipole moments of three active neutrinos are suppressed not only
by their tiny masses but also by the Glashow-Iliopoulos-Maiani (GIM) mechanism.
We show that it is possible to lift the GIM suppression if the canonical seesaw
mechanism of neutrino mass generation, which allows V to be slightly
non-unitary, is taken into account. In view of current experimental constraints
on the non-unitarity of V, we find that the effective electromagnetic
transition dipole moments of three light Majorana neutrinos and the rates of
their radiative decays can be maximally enhanced by a factor of O(10^2) and a
factor of O(10^4), respectively. This important observation reveals an
intrinsic and presumably significant correlation between the electromagnetic
properties of massive neutrinos and the origin of their small masses.
