It has been proposed that a sterile neutrino \nu_h with m_h \approx 50 MeV
and a dominant decay mode (\nu_h -> \nu\gamma) may be the origin of the
experimental anomaly observed at LSND. We define a particular model that could
also explain the MiniBooNE excess consistently with the data at other neutrino
experiments (radiative muon capture at TRIUMF, T2K, or single photon at NOMAD).
The key ingredients are (i) its long lifetime (\tau_h\approx 3-7x10^{-9} s),
which introduces a 1/E dependence with the event energy, and (ii) its Dirac
nature, which implies a photon preferably emitted opposite to the beam
direction and further reduces the event energy at MiniBooNE. We show that these
neutrinos are mostly produced through electromagnetic interactions with nuclei,
and that T2K observations force BR(\nu_h -> \nu_\tau\gamma) \le 0.01 \approx
BR(\nu_h -> \nu_\mu\gamma). The scenario implies then the presence of a second
sterile neutrino \nu_{h'} which is lighter, longer lived and less mixed with
the standard flavors than \nu_h. Since such particle would be copiously
produced in air showers through (\nu_h -> \nu_{h'}\gamma) decays, we comment on
the possible contamination that its photon-mediated elastic interactions with
matter could introduce in dark matter experiments.Comment: 18 pages, typo in Eq.(6) correcte
