Dark Matter on a Slide
We present a scenario for GeV-scale thermal dark matter that can only be tested with accelerator experiments. Dark matter is composed of dark pions arising from a confining strong interaction in the dark sector. The thermal relic density is obtained through the interplay of up-scatterings of dark pions to heavier dark mesons (the dark counterparts of the kaons and $\eta$), and decays of the unstable dark $\eta$ to Standard Model particles. This mechanism is analogous to a playground slide, where one climbs up first and then slides down with a release of energy. We illustrate the scenario with a minimal model based on the SU(3)/SO(3) coset, where dark matter is stabilized by a U(1) flavor symmetry. The correct relic density is obtained with dark meson mass splittings of 10% to 50% and a dark-$\eta$ lifetime shorter than $10^3\,\mathrm{m}/c$. Direct and indirect dark matter searches are mostly ineffective, as a consequence of the charge conjugation symmetry of the stabilizing U(1). The most striking signals arise at the LHC, from the production of dark showers containing long-lived dark $\eta$'s that decay to visible final states. These signatures crucially depend on the portal interaction connecting the dark sector to the Standard Model. We show that several well-known portals can complete the scenario above the weak scale, and outline the expected signals in each case.