have been proposed within the MSSM ( e.g. [49]) – we will ignore this type of fine-tuning in our arguments, focusing instead on the model independent tuning explicit in Eq. (26). A thermal history is not the only way to achieve the correct DM relic abundance [10]. It has even been argued [50] that there is a “non-thermal WIMP miracle” when there exist TeV scale states which decay to the DM via Planck suppressed operators. For example, a heavy gravitino (or string-theory moduli fields) can live long enough to dominate the energy density of the universe. Then when these states decay, they will produce superpartners which will decay down to the lightest neutralino, resulting in a neutralino relic density. This relaxes the relationship between the mass/composition and relic density of a neutralino. A variety of other options have been proposed. Models where the energy density of the universe at the epoch of DM freeze-out was dominated by something other than radiation were studied in [51]. Alternately, if the DM interacts so feebly that it never achieves thermal equilibrium, one can achieve the correct value of the relic density via “freeze-in” production [52]. Since the total energy density of DM is close to that of the baryons, one can construct models where the DM relic density is set by an asymmetry which is determined by the baryon asymmetry [53]. In [54], it was shown that by varying the reheat temperature and allowing for non-thermal sources, any neutralino composition can result in the correct relic density. In [55], a low temperature phase transition in the early universe changes the DM properties after freeze-out. All of these options involve either non-trivial cosmological histories or other model building challenges. We will focus on the thermal – and hence well-tempered – case, with discussions of the deviations that arise when the thermal assumption is relaxed. In the MSSM, the neutralino mass mixing can often be approximately understood in terms of a two state system: a Dirac Higgsino mixing with either a Bino or a Wino. Thus, to understand the physics of SD scattering via Z 0 exchange, it is useful to consider the simple “Singlet-Doublet Model” (SDM) for DM, where the singlet has the same quantum numbers as either a Bino or a Wino, and the doublets have the same quantum numbers as the Higgsinos: L$_{SDM}$ ∋ µ$_{D}$ D ¯ D + λ h S D + λ $^{′}$h$^{∗}$S ¯ D + µ$_{S}$ 2 S $^{2}$. (27) V. SPIN DEPENDENT CROSS SECTIONS FOR MIXED DARK MATTER B. Non-thermal options 12