Gauge Coupling Unification and Non-Equilibrium Thermal Dark Matter

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We study a new mechanism for the production of dark matter in the universe which does not rely on thermal equilibrium. Dark matter is populated from the thermal bath subsequent to inflationary reheating via a massive mediator whose mass is above the reheating scale, Tᵣ. To this end, we consider models with an extra U(1) gauge symmetry broken at some intermediate scale M, of the order of 10¹0 -- 10¹2 GeV. We show that not only does the model allow for gauge coupling unification (at a higher scale associated with grand unification) but can naturally provide a dark matter candidate which is a Standard Model singlet but charged under the extra U(1). The intermediate scale gauge boson(s) which are predicted in several E6/SO(10) constructions can be a natural mediator between dark matter and the thermal bath. We show that the dark matter abundance, while never having achieved thermal equilibrium, is fixed shortly after the reheating epoch by the relation Tᵣ³/M⁴. As a consequence, we show that the unification of gauge couplings which determines M also fixes the reheating temperature Tᵣ, which can be as high as 10¹1 GeV.