Sterile neutrinos findings

Question

There is a relatively new paper https://arxiv.org/abs/1805.12028 of MiniBooNE on measurements of neutrino oscillations that can only be explained by the postulate of a sterile neutrino. I also read that results from other experiments like IceCube that don't confirm the findings.

Nevertheless I wonder if such a sterile neutrino could not be a good candidate for dark matter. However, on wikipedia: https://en.wikipedia.org/wiki/MiniBooNE it is said that cosmological data give a lower bound of a sterile neutrino to 0.26eV. Is there any possibility that such a particle could turn out as a dark matter candidate ? It was noted that the cosmological bound of the sterile neutrino mass would be quite model-dependent. Which are the models that are responsible for the low cosmological bound of the sterile neutrino mass ?

Answer 1

One has to clearly distinguish sterile neutrinos and their relevance to different areas of research like cosmology or oscillation experiments on earth by their masses. If you want a sterile neutrino to explain the anomalies in oscillation experiments, it should have a mass in the eV range or slightly below, which is the kind of sterile neutrino you are referring too in your question.

If you however want a sterile neutrino to be a dark matter candidate, it has turned out that you need at least a ~keV mass in order to avoid the so-called Tremaine-Gunn bound. It basically tells you that you cannot fit a fermionic dark matter candidate into some galaxies (like dwarf galaxies) at arbitrarily high densities due to Pauli blocking. There exist also many model-dependent mass bounds which might be much more restrictive, such as Lyman-$\alpha$ and Milky Way satellite counts.

I am however not sure how cosmological data give mass bounds to sterile neutrinos that the are subject of these short-baseline experiments like MiniBooNe, i.e. sterile neutrinos with masses in the eV-range. Very interesting though.