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Sterile neutrinos: the dark side of the light fermions

The discovery of neutrino masses suggests the likely existence of gauge singlet fermions that participate in the neutrino mass generation via the seesaw mechanism. The masses of the corresponding degrees of freedom can range from well below the electroweak scale to the Planck scale. If some of the singlet fermions are light, the sterile neutrinos appear in the low-energy effective theory. They can play an important role in astrophysics and cosmology. In particular, sterile neutrinos with masses of several keV can account for cosmological dark matter, which can be relatively warm or cold, depending on the production mechanism. The same particles can explain the observed velocities of pulsars because of the anisotropy in their emission from a cooling neutron star born in a supernova explosion. Decays of the relic sterile neutrinos can produce a flux of X-rays that can affect the formation of the first stars. Existing and future X-ray telescopes can be used to search for the relic sterile neutrinos.