Possible Capture of keV Sterile Neutrino Dark Matter on Radioactive
  beta-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 nu_4 and its flavor eigenstate nu_s weakly mixes with
three active neutrinos. We clarify different active-sterile neutrino mixing
factors for the radiative decay of nu_4 and beta 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 nu_4 component of nu_e can
leave a distinct imprint on the electron energy spectrum when it is captured on
radioactive beta-decaying nuclei. We carry out an analysis of its signatures in
the capture reactions nu_e + ^{3}H \to ^{3}He + e^- and nu_e + ^{106}Ru \to
^{106}Rh + e^- against the beta-decay backgrounds, and conclude that this
experimental approach might not be hopeless in the long run.
