  Using the modern Kaluza-Klein theory of gravity (or the Induced Matter
theory), we study the Dirac equation for massive neutrinos on a de Sitter
background metric from a 5D Riemann-flat (and hence Ricci-flat) extended de
Sitter metric, on which is defined the vacuum for test massless 1/2-neutral
fields minimally coupled to gravity and free of any other interactions. We
obtain that the effective 4D masses of the neutrinos can only take three
possible values, which are related to the (static) foliation of the fifth and
noncompact extra dimension.
