Sterile neutrinos with keV masses can constitute all or part of the cosmological dark matter. The electroweak-singlet fermions, which are usually introduced to explain the masses of active neutrinos, need not be heavier than the electroweak scale; if one of them has a keV-scale mass, it can be the dark-matter particle, and it can also explain the observed pulsar kicks. The relic sterile neutrinos could be produced by several different mechanisms. If they originate primarily from the Higgs decays at temperatures of the order of 100 GeV, the resulting dark matter is much “colder ” than the warm dark matter produced in neutrino oscillations. The signature of this form of dark matter is the spectral line from the two-body decay, which can be detected by the X-ray telescopes. The same X-rays can have other observable manifestations, in particular, though their effects on the formation of the first stars. 1. Neutrino masses and the emergence of sterile neutrinos Most discoveries in particle physics amount to either a measurement of some parameter related to a known particle, or a detection of some new degrees of freedom, new particles. The discovery of the neutrino mass 1 is both. Not only is it a measurement of the non-zero mass, but it also implies the existence of some additional, SU(2) singlet fermions, “right-handed” neutrinos. The corresponding particles can be made very heavy even for small masses of the active neutrinos (the seesaw mechanism 2), but they can also be light, in which case they are called sterile neutrinos. The name sterile neutrino was coined by Bruno Pontecorvo, who hypothesized the existence of the right-handed neutrinos in a seminal paper, 3 in which he also considered vacuum neutrino oscillations in the laboratory and in astrophysics, the lepton number violation, the neutrinoless double beta decay, some rare processes, such as µ → eγ, and several other questions that have dominated the neutrino physics for the next four decades. Most models of the neutrino masses introduce sterile (or right-handed) neutrinos to generat
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