Anatomy of Coannihilation with a Scalar Top Partner

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We investigate a simplified model of dark matter where a Majorana fermion χ coannihilates with a colored scalar top partner t. We explore the cosmological history, with particular emphasis on the most relevant low-energy parameters: the mass splitting between the dark matter and the coannihilator, and the Yukawa coupling yχ that connects these fields to the Standard Model top quarks. We also allow a free quartic coupling λₕ between a pair of Higgs bosons and t pairs. We pay special attention to the case where the values take on those expected where t corresponds to the superpartner of the right-handed top, and χ is a bino. Direct detection, indirect detection, and colliders are complementary probes of this simple model.