  Low energy effects induced by heavy extra degrees of freedom are suppressed
by powers of the large mass scale, thus preserving, if sufficiently heavy, the
successes of the Standard Model in describing low energy phenomena. However, as
is well known, heavy right-handed neutrinos may play an important role in low
energy phenomenology as an explanation of the smallness of neutrino masses. We
consider in this paper an extension of the Standard Model by heavy right-handed
neutrinos and heavy Higgs doublets and we show, using a renormalization group
approach, that this model can simultaneously provide an explanation for the
small neutrino masses and for the mild hierarchy observed between the
atmospheric and the solar mass splittings, even when the extra degrees of
freedom are very heavy. We analyze the necessary conditions to reproduce the
oscillation parameters and we discuss possible experimental signatures of this
model.
