Dark matter in the Kim-Nilles mechanism

The Kim-Nilles mechanism relates the $μ$ term with the axion scale $f_a$, leading to the axino-Higgsino-Higgs Yukawa coupling of order $μ/f_a$. This can bring a dangerous thermal production of axinos. If the axino is stable, its mass has to be as small as ${\cal O}$(0.1keV), or the reheat temperature should be lower than ${\cal O}$(10GeV) taking the lower axion scale $10^{10}$ GeV in order not to overclose the Universe. If the axino decays to a neutralino, the overproduced neutralinos can re-annihilate 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 $10^{11}$ 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 of order $10^{-45}$cm$^2$.

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