Broadening Dark Matter Searches at the LHC: Mono-X versus Darkonium Channels

TL;DR

This paper explores alternative dark matter detection channels at the LHC, focusing on darkonium formation mediated by a light $Z'$ boson, which can decay into SM particles and evade traditional mono-X search constraints.

Contribution

It introduces the concept of darkonium formation via a light $Z'$ mediator, expanding the experimental search strategies beyond mono-X signatures at the LHC.

Findings

Darkonium states can be produced when $M_{\chi}>M_{Z'}/\alpha_{\rm eff}$.

Darkonium can decay into SM quarks, mimicking multijet events without missing energy.

Existing searches can be reinterpreted to constrain darkonium production, extending beyond mono-X limits.

Abstract

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 $Z^\prime$ boson, between the dark matter $\chi$ and the SM. Limits derived from mono-X channels are most effective when the mediator can decay into two on-shell dark matter particles: $M_{Z'}\gtrsim 2M_\chi$. We broaden the experimental reach into the complementary region, where the $Z^\prime$ mediator is much lighter than the dark matter. In this scenario the $Z^\prime$ 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 $M_{\chi}>M_{Z'}/\alpha_{\rm eff}$, where $\alpha_{\rm eff}$ 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 $Z^\prime$ benchmark beyond the region probed by mono-X searches. Assuming a baryonic $Z^\prime$ 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.