Annihilating to the Darker: Thermal Relic Dark Matter with an Ultraweak Portal to the Standard Model

Abstract

Thermal relic dark matter has been severely constrained in recent years by direct and indirect dark matter searches, as well as multi-messenger probes of dark sectors. At the current level of experimental precision, it has become difficult for many thermal dark matter models to deplete their abundance sufficiently through freeze-out to reproduce the observed relic density. We study the possibility that thermal dark matter couples only ultraweakly to the Standard Model (SM), and therefore remains effectively undetectable in current experiments, while interacting much more strongly with a darker sector that controls its freeze-out history. Hence, the dominant annihilation channels of a thermal relic may proceed primarily into the darker sector rather than into SM particles. We first summarize the general classes of portal interactions that may connect the SM, a hidden sector, and a darker concealed sector, together with the corresponding experimental constraints. We then illustrate the mechanism in two representative realizations. The first is a prototype $U(1)_x \times U(1)_c$ setup with kinetic and mass mixing between the hidden and concealed gauge sectors. The second is a more motivated $U(1)_{B-L}\times U(1)_c$ construction, in which the $U(1)_{B-L}$ gauge interaction is strongly constrained and the hidden--concealed connection is mediated primarily by a real scalar. In both frameworks, we identify two qualitatively distinct scenarios: assisted depletion and darker conversion. By solving the full set of coupled Boltzmann equations and presenting benchmark models for dark matter masses in the 1--200~GeV range, we show that electroweak scale thermal relic dark matter may remain viable even when its direct portal to the SM is ultraweak, provided that sufficiently strong hidden--concealed interactions govern the cosmological evolution.