The Double-Dark Portal

TL;DR

This paper introduces the dark mediator Dark matter (dmDM) model where dark matter and its force carrier are charged under a new symmetry, leading to unique detection signals and astrophysical constraints.

Contribution

It proposes the dmDM model with light scalars coupling to the SM, systematically surveys experimental constraints, and explores detection prospects and astrophysical implications.

Findings

Neutron star cooling excludes minimal single-mediator models.

Two-mediator scenarios remain viable with strong detection signals.

dmDM can mimic different WIMP masses in experiments.

Abstract

In most models of the dark sector, dark matter is charged under some new symmetry to make it stable. We explore the possibility that not just dark matter, but also the force carrier connecting it to the visible sector is charged under this symmetry. This dark mediator then acts as a Double-Dark Portal. We realize this setup in the \emph{dark mediator Dark matter} model (dmDM), featuring a fermionic DM candidate $\chi$ with Yukawa couplings to light scalars $\phi_i$. The scalars couple to SM quarks via the operator $\bar q q \phi_i^* \phi_j/\Lambda_{ij}$. This can lead to large direct detection signals via the $2\rightarrow3$ process $\chi N \rightarrow \chi N \phi$ if one of the scalars has mass $ \lesssim 10$ keV. For dark matter Yukawa couplings $y_\chi \sim 10^{-3} - 10^{-2}$, dmDM features a thermal relic dark matter candidate while also implementing the SIDM scenario for ameliorating inconsistencies between dwarf galaxy simulations and observations. We undertake the first systematic survey of constraints on light scalars coupled to the SM via the above operator. The strongest constraints are derived from a detailed examination of the light mediator's effects on stellar astrophysics. LHC experiments and cosmological considerations also yield important bounds. Observations of neutron star cooling exclude the minimal model with one dark mediator, but a scenario with two dark mediators remains viable and can give strong direct detection signals. We explore the direct detection consequences of this scenario and find that a heavy $\mathcal{O}(100)$ GeV dmDM candidate fakes different $\mathcal{O}(10)$ GeV WIMPs at different experiments. Large regions of dmDM parameter space are accessible above the irreducible neutrino background.