  Neutrino masses are likely to be a manifestation of the right-handed, or
sterile neutrinos. The number of sterile neutrinos and the scales of their
Majorana masses are unknown. We explore theoretical arguments in favor of the
high and low scale seesaw mechanisms, review the existing experimental results,
and discuss the astrophysical hints regarding sterile neutrinos.

High Energy Physics - Phenomenology;Astrophysics