On the spin-dependent sensitivity of XENON100
  The latest XENON100 data severely constrains dark matter elastic scattering
off nuclei, leading to impressive upper limits on the spin-independent
cross-section. The main goal of this paper is to stress that the same data set
has also an excellent \emph{spin-dependent} sensitivity, which is of utmost
importance in probing dark matter models. We show in particular that the
constraints set by XENON100 on the spin-dependent neutron cross-section are by
far the best at present, whereas the corresponding spin-dependent proton limits
lag behind other direct detection results. The effect of nuclear uncertainties
on the structure functions of xenon isotopes is analysed in detail and found to
lessen the robustness of the constraints, especially for spin-dependent proton
couplings. Notwithstanding, the spin-dependent neutron prospects for XENON1T
and DARWIN are very encouraging. We apply our constraints to well-motivated
dark matter models and demonstrate that in both mass-degenerate scenarios and
the minimal supersymmetric standard model the spin-dependent neutron limits can
actually override the spin-independent limits. This opens the possibility of
probing additional unexplored regions of the dark matter parameter space with
the next generation of ton-scale direct detection experiments.
