Dark matter in the Kim-Nilles mechanism

The Kim-Nilles mechanism relates the μ term with the axion scale fₐ, leading to the axino-Higgsino-Higgs Yukawa coupling of order μ/fₐ. This can bring a dangerous thermal production of axinos. If the axino is stable, its mass has to be as small as O(0.1 keV), or the reheat temperature should be lower than O(10 GeV) taking the lower axion scale 10¹⁰ GeV in order not to overclose the Universe. If the axino decays to a neutralino, the overproduced neutralinos can reannihilate appropriately to saturate the observed dark matter density if the annihilation rate is of order 10^-8 GeV^-2 for the axion scale larger than about 10¹¹ GeV. Thus, a light Higgsino-like lightest supersymmetric particle with a sizable bino mixture becomes a good dark matter candidate whose nucleonic cross section is larger than about 10^-45 cm².