Possible capture of keV sterile neutrino dark matter on radioactive β-decaying nuclei

There exists an observed “desert” spanning six orders of magnitude between O(0.5) eV and O(0.5) MeV in the fermion mass spectrum. We argue that it might accommodate one or more keV sterile neutrinos as a natural candidate for warm dark matter. To illustrate this point of view, we simply assume that there is one keV sterile neutrino ν4 and its flavor eigenstate νs weakly mixes with three active neutrinos. We clarify different active-sterile neutrino mixing factors for the radiative decay of ν4 and β decays in a self-consistent parametrization. A direct detection of this keV sterile neutrino dark matter in the laboratory is in principle possible since the ν4 component of νe can leave a distinct imprint on the electron energy spectrum when it is captured on radioactive β-decaying nuclei. We carry out an analysis of its signatures in the capture reactions νe+H3→He3+e− and νe+Ru106→Rh106+e− against the β-decay backgrounds, and conclude that this experimental approach might not be hopeless in the long run.