Sterile neutrinos: the dark side of the light fermions

Abstract

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.

Figures (13)

• Figure 1: Experimental and observational limits on sterile neutrinos that have a non-zero mixing with the electron neutrino only. This figure is based on the limits from Refs. Kusenko:2004qcSmirnov:2006buAtre:2009rg. The X-ray limits and Lyman-$\alpha$ limits shown here are based on the abundances of relic neutrinos produced by neutrino oscillations for zero lepton asymmetry; see discussion in the text and in Refs. Kusenko:2006rhPalazzo:2007gzBoyarsky:2008xjBoyarsky:2008mt
• Figure 2: Same as in Fig. \ref{['figure:limits_e']}, for the sterile neutrino mixing with $\nu_\mu$ only.
• Figure 3: Same as in Fig. \ref{['figure:limits_e']}, for the sterile neutrino mixing with $\nu_\tau$ only.
• Figure 4: For MSW resonance in the core, the sterile neutrino energy depends on the temperature around the resonance point.
• Figure 5: For MSW resonance outside the core, the neutrino passes between $r_-=r_0-\delta \cos \phi$ and $r_+=r_0+\delta \cos \phi$ as a sterile $\nu_s$ on one side of the star, while it still propagates as an active $\nu_a$ on the other side. The active neutrinos interact and deposit some extra momentum on the right-hand side, between $r_-$ and $r_+$. Since the neutron star is a gravitationally bound object, the momentum deposited asymmetrically in its outer layers gives the whole star a kick.