  We consider in detail the calculation of the decay rate of high-energy
superluminal neutrinos against (charged) lepton pair Cerenkov radiation (LPCR),
and neutrino pair Cerenkov radiation (NPCR), i.e., against the decay channels
nu -> nu e+ e- and nu -> nu nubar nu. Under the hypothesis of a tachyonic
nature of neutrinos, these decay channels put constraints on the lifetime of
high-energy neutrinos for terrestrial experiments as well as on cosmic scales.
For the oncoming neutrino, we use the Lorentz-covariant tachyonic relation E_nu
= (p^2 - m_nu^2)^(1/2), where m_nu is the tachyonic mass parameter. We derive
both threshold conditions as well as decay and energy loss rates, using the
plane-wave fundamental bispinor solutions of the tachyonic Dirac equation.
Various intricacies of rest frame versus lab frame calculations are
highlighted. The results are compared to the observations of high-energy
IceCube neutrinos of cosmological origin.
