A Dark Sink Enhances the Direct Detection of Freeze-in Dark Matter

We describe a simple dark sector structure which, if present, has implications for the direct detection of dark matter (DM): the Dark Sink. A Dark Sink transports energy density from the DM into light dark-sector states that do not appreciably contribute to the DM density. As an example, we consider a light, neutral fermion $\psi$ which interacts solely with DM $\chi$ via the exchange of a heavy scalar $\Phi$. We illustrate the impact of a Dark Sink by adding one to a DM freeze-in model in which $\chi$ couples to a light dark photon $\gamma '$ which kinetically mixes with the Standard Model (SM) photon. This freeze-in model (absent the sink) is itself a benchmark for ongoing experiments. In some cases, the literature for this benchmark has contained errors; we correct the predictions and provide them as a public code. We then analyze how the Dark Sink modifies this benchmark, solving coupled Boltzmann equations for the dark-sector energy density and DM yield. We check the contribution of the Dark Sink $\psi$'s to dark radiation; consistency with existing data limits the maximum attainable cross section. For DM with a mass between $\text{MeV} -\mathcal{O}(10\text{ GeV})$, adding the Dark Sink can increase predictions for the direct detection cross section all the way up to the current limits.