Bosonic super-WIMPs as keV-scale dark matter

We consider models of light superweakly interacting cold dark matter, with O (10--100) keV mass, focusing on bosonic candidates such as pseudoscalars and vectors. We analyze the cosmological abundance, the 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).