Fine-Tuning Implications of Direct Dark Matter Searches in the MSSM
  We study theoretical implications of direct dark matter searches in the
minimal supersymmetric standard model (MSSM). We assume that no accidental
cancellations occur in the spin-independent elastic neutralino-quark scattering
cross section, but do not impose any relations among the weak-scale MSSM
parameters. We show that direct detection cross section below 10^-44 cm^2
requires the lightest supersymmetric particle (LSP) neutralino to be close to
either a pure gaugino or pure Higgsino limit, with smaller cross sections
correlated with smaller admixture of the subdominant components. The current
XENON100 bound rules out essentially all models in which the lightest
neutralino has the Higgsino fraction between 0.2 and 0.8. Furthermore, smaller
direct detection cross sections correlate with stronger fine-tuning in the
electroweak symmetry breaking sector. In the gaugino LSP scenario, the current
XENON100 bound already implies some fine-tuning: for example, at least 10%
tuning is required if the LSP mass is above 70 GeV. In both gaugino and
Higgsino LSP scenarios, the direct dark matter searches currently being
conducted and designed should lead to a discovery if no accidental
cancellations or fine-tuning at a level below 1% is present.
