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 Z 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 Z 2M χ . We broaden the experimental reach into the complementary region, where the Z mediator is much lighter than the dark matter. In this scenario the Z mediates an effective longrange 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 Z /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 Z benchmark beyond the region probed by mono-X searches. Assuming a baryonic Z 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. * This is different from fixed energy colliders where the η D and Υ D channels are comparably important [24].
We introduce a light dark photon A 0 μ to the minimal Higgs portal model, by coupling the Higgs boson to "dark QED" containing fermionic dark matter, which gives rise to rich and interesting collider phenomenology. There are two prominent features in such a simple extension-the Higgs boson could have decays into the long-lived dark photon through the "mono-A 0 channel," or into multiple collimated leptons via "darkonium," depending on the mixing parameter of the A 0 μ with the visible photon. We initiate a study on the possibility of probing the parameter space of the model in both the energy and the lifetime frontiers at the Large Hadron Collider.
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