3.5 keV X-rays as the "21 cm line" of dark atoms, and a link to light
  sterile neutrinos
  The recently discovered 3.5 keV X-ray line from extragalactic sources may be
evidence of dark matter scatterings or decays. We show that dark atoms can be
the source of the emission, through their hyperfine transitions, which would be
the analog of 21 cm radiation from a dark sector. We identify two families of
dark atom models that match the X-ray observations and are consistent with
other constraints. In the first, the hyperfine excited state is long-lived
compared to the age of the universe, and the dark atom mass is relatively
unconstrained; dark atoms could be strongly self-interacting in this case. In
the second, the excited state is short-lived and viable models are
parameterized by the value of the dark proton-to-electron mass ratio $R$: for
$R = 10^2-10^4$, the dark atom mass is predicted be in the range $350-1300$
GeV, with fine structure constant $\alpha'\cong 0.1-0.6$. In either class of
models, the dark photon must be massive with $m_{\gamma'} \gtrsim$ 1 MeV and
decay into $e^+ e^-$. Evidence for the model could come from direct detection
of the dark atoms. In a natural extension of this framework, the dark photon
could decay predominantly into invisible particles, for example $\sim 0.5$ eV
sterile neutrinos, explaining the extra radiation degree of freedom recently
suggested by data from BICEP2, while remaining compatible with BBN.
