Dark radiation and small-scale structure problems with decaying particles

Although the standard model describes the cosmic microwave background radiation and the large-scale structure of the Universe with great success, it has some tensions with observations in the effective number of neutrino species (dark radiation) and the number of small-scale structures (overabundance problem). Here we propose a scenario which can relax these tensions by producing both dark matter and dark radiation by late decays of heavy particles. Thanks to the generation mechanism, dark matter is rather warm, so that the small-scale structure problem is resolved. This scenario can be naturally realized in the supersymmetric axion model, in which axions produced by saxion decays provide dark radiation, while axinos from saxion decays form warm dark matter. We identify a parameter region of the supersymmetric axion model satisfying all known cosmological constraints.