Interactions of keV sterile neutrinos with matter

A sterile neutrino with mass of several keV is a well-motivated dark-matter candidate, and it can also explain the observed velocities of pulsars via anisotropic emission of sterile neutrinos from a cooling neutron star. We discuss the interactions of such relic particles with matter and comment on the prospects of future direct detection experiments. A relic sterile neutrino can interact, via sterile-active mixing, with matter fermions by means of electroweak currents, with the final state containing a relativistic active neutrino. The recoil momentum impacted onto a matter fermion is determined by the sterile neutrino mass and is enough to ionize atoms and flip the spins of nuclei. While this suggests a possibility of direct experimental detection, we calculate the rates and show that building a realistic detector of the required size would be a daunting challenge.Comment: 5 pages, 1 figur

Thermal evolution of the primordial clouds in warm dark matter models with keV sterile neutrinos

We analyze the processes relevant for star formation in a model with dark matter in the form of sterile neutrinos. Sterile neutrino decays produce an X-ray background radiation that has a two-fold effect on the collapsing clouds of hydrogen. First, the X-rays ionize the gas and cause an increase in the fraction of molecular hydrogen, which makes it easier for the gas to cool and to form stars. Second, the same X-rays deposit a certain amount of heat, which could, in principle, thwart the cooling of gas. We find that, in all the cases we have examined, the overall effect of sterile dark matter is to facilitate the cooling of gas. Hence, we conclude that dark matter in the form of sterile neutrinos can help the early collapse of gas clouds and the subsequent star formation.Comment: aastex, 31 pages, 4 figures; one figure and some references added, minor changes in the text; to appear in Astrophysical Journa

A lower limit on the dark particle mass from dSphs

We use dwarf spheroidal galaxies as a tool to attempt to put precise lower limits on the mass of the dark matter particle, assuming it is a sterile neutrino. We begin by making cored dark halo fits to the line of sight velocity dispersions as a function of projected radius (taken from Walker et al. 2007) for six of the Milky Way's dwarf spheroidal galaxies. We test Osipkov-Merritt velocity anisotropy profiles, but find that no benefit is gained over constant velocity anisotropy. In contrast to previous attempts, we do not assume any relation between the stellar velocity dispersions and the dark matter ones, but instead we solve directly for the sterile neutrino velocity dispersion at all radii by using the equation of state for a partially degenerate neutrino gas (which ensures hydrostatic equilibrium of the sterile neutrino halo). This yields a 1:1 relation between the sterile neutrino density and velocity dispersion, and therefore gives us an accurate estimate of the Tremaine-Gunn limit at all radii. By varying the sterile neutrino particle mass, we locate the minimum mass for all six dwarf spheroidals such that the Tremaine-Gunn limit is not exceeded at any radius (in particular at the centre). We find sizeable differences between the ranges of feasible sterile neutrino particle mass for each dwarf, but interestingly there exists a small range 270-280eV which is consistent with all dSphs at the 1-$\sigma$ level.Comment: 13 pages, 2 figures, 1 tabl