Cosmological axino problem

We revisit the cosmology of the supersymmetric QCD axion, highlighting the existence of a serious cosmological axino problem that is fully analogous to the gravitino problem of overclosure via thermal production. A general analysis implies that the QCD axino has a mass greater than or equal to that of the gravitino in the absence of unnatural fine-tuning or sequestering. As a consequence, bounds from thermal gravitino and QCD axino production are complementary in parameter space, and together provide a quite stringent limit on the reheating temperature after inflation given by Tₑ<10^3-10^6 GeV for an axion decay constant of f₀=10^9-10^12 GeV. Motivated by this result, we explore the cosmology of gravitino lightest supersymmetric particle and axino next to lightest supersymmetric particle at low Tₑ and present three realistic scenarios for dark matter.