The 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 T_R < 10^3 - 10^6 GeV for an axion decay constant of f_a = 10^9 - 10^12 GeV. Motivated by this result, we explore the cosmology of gravitino LSP and axino NLSP at low T_R and present three realistic scenarios for dark matter.