Bosonic super-WIMPs as keV-scale dark matter

We consider models of light superweakly interacting cold dark matter, with $\mathcal{O}(10–100)\mathrm{keV}$ mass, focusing on bosonic candidates such as pseudoscalars and vectors. We analyze the cosmological abundance, the $\gamma$ background created by particle decays, the impact on stellar processes due to cooling, and the direct-detection capabilities in order to identify classes of models that pass all the constraints. In certain models, variants of photoelectric (or axioelectric) absorption of dark matter in direct-detection experiments can provide a sensitivity to the superweak couplings to the standard model which is superior to all existing indirect constraints. In all models studied, the annual modulation of the direct-detection signal is at the currently unobservable level of $O({10}^{-5})$.