Abstract
We explore simplified models of mixed dark matter(DM), defined here to be a stable relic composed of a singlet and anelectroweak charged state. Our setup describes a broad spectrum ofthermal DM candidates that can naturally accommodate the observed DMabundance but are subject to substantial constraints from current andupcoming direct detection experiments. We identify ``blind spots'' atwhich the DM-Higgs coupling is identically zero, thus nullifyingdirect detection constraints on spin independent scattering.Furthermore, we characterize the fine-tuning in mixing angles,i.e. well-tempering, required for thermal freeze-out toaccommodate the observed abundance. Present and projected limits fromLUX and XENON1T force many thermal relic models into blind spottuning, well-tempering, or both. This simplified model frameworkgeneralizes bino-Higgsino DM in the MSSM, singlino-Higgsino DM in theNMSSM, and scalar DM candidates that appear in models of extendedHiggs sectors.
Highlights
Simplified models are a powerful tool for studying the generic behavior of WIMP dark matter (DM)
Theories in which DM couples to the SM via the Higgs are of particular interest because Higgsmediated DM-nucleon scattering is just being probed by the current generation of direct detection experiments
Mixing between the singlet and non-singlet states is induced via DM-Higgs couplings, and is in general correlated with signals in direct detection
Summary
We explicitly define our simplified models. Throughout, we focus on the case of a singlet mixed with a non-singlet, which is a natural generalization of many models of theoretical interest. For real scalar DM the same formula applies except with the replacement Mχ(v) → Mχ2(v) In this case we define a dimensionful coupling ahχχ = ∂ Mχ2(v) /∂v which is proportional to the DM mass, though for ease of discussion we will sometimes use the effective dimensionless coupling, chχχ = ahχχ/Mχ instead. We will be interested in models which evade present and projected limits from direct detection while accommodating a thermal relic abundance consistent with observation. These theories may require tuning for either or both of these aspects. Blind spot tuning grows more severe as ξBS → 0 This effectively captures the tuning inherent in chχχ → 0 when individual Higgs couplings remain non-zero. In the limit that the entire DM multiplet is exactly degenerate, the mixing angle is very fine-tuned, M2 ∝ 1 and ξWT → 0
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