Light Higgsino Dark Matter

We re–investigate the question whether a light higgsino–like neutralino is a viable Dark Matter candidate. To this end we compute the dominant one–loop corrections to the masses of the higgsino–like states in the minimal Supersymmetric Standard Model (MSSM), due to loops involving heavy quarks and their superpartners. We also calculate analogous corrections to the couplings of higgsino–like neutralinos to Z and Higgs bosons. In the region of parameter space leading to high higgsino purity of the lightest neutralino, these corrections can change the expected relic density by up to a factor of five in either direction. We conclude that for favorable choices of soft supersymmetry breaking parameters, a state with more than 99% higgsino purity could indeed form all cold Dark Matter in the Universe. In some cases these corrections can also increase the expected cross section for LSP scattering off spinless nuclei by up to two orders of magnitude, or reduce it to zero.