Broadening dark matter searches at the LHC: Mono-X versus darkonium channels

Current searches for dark matter at the LHC focus on mono-X signatures: the production of dark matter in association with a Standard Model (SM) particle. The simplest benchmark introduces a massive spin-1 mediator, the Z⁰ boson, between the dark matter χ and the SM. Limits derived from mono-X channels are most effective when the mediator can decay into two on-shell dark matter particles: M_Z0 & 2M_χ. We broaden the experimental reach into the complementary region, where the Z⁰ mediator is much lighter than the dark matter. In this scenario the Z⁰ mediates an effective long-range force between the dark matter, thereby facilitating the formation of darkonium bound states, as is common in many dark sector models. The darkonium becomes active when M_χ > M_Z0/α_eff, where α_eff is the effective fine-structure constant in the dark sector. Moreover, the darkonium could decay back into SM quarks, without producing missing transverse momentum in the detector. Considering multijet final states, we reinterpret existing searches to constrain the simple Z⁰ benchmark beyond the region probed by mono-X searches. Assuming a baryonic Z⁰ mediator and a Dirac dark matter, direct detection bounds can be loosened by giving a small Majorana mass to the dark matter. We also consider the interplay between mono-X and darkonium channels at future high energy colliders, which is at the frontier of probing the model parameter space.