The Kim-Nilles mechanism relates the $\mu$ term with the axion scale $f_a$, leading to the axino-Higgsino-Higgs Yukawa coupling of order $\mu/f_a$. 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$^2$.
Categories: hep-ph astro-ph.CO