Collider Searches for Near-Continuum Dark Matter

TL;DR

This paper investigates collider constraints on a near-continuum dark matter model with a tower of states, introducing a minimal parameterization and simulations to analyze current and future collider sensitivities.

Contribution

It presents a model-agnostic parameterization for near-continuum dark matter and provides collider constraints and projections for current and future experiments.

Findings

Current LHC bounds are approaching the theoretically motivated parameter space.

Future colliders will significantly improve sensitivity to the model.

A new Monte-Carlo tool was developed for simulating near-continuum spectra.

Abstract

We study collider constraints on the near-continuum dark matter model, in which the dark sector consists of a tower of closely spaced states with weak-scale masses coupled to the Standard Model through a $Z$-portal. To capture this structure in a model-agnostic way, we introduce a minimal parameterization that encodes the dominant geometric information with three parameters. Using a custom-built Monte-Carlo tool for near-continuous spectra, we simulate DM-pair production at $\sqrt{s}=13$ TeV and subsequent cascade decays via on/off-shell $Z$ bosons, which yield events with large missing transverse momentum and high jet multiplicity. Recasting the CMS multijet$+H_T^{\rm miss}$ analysis of Run-2 data (35.9 fb$^{-1}$), we derive bounds on the model parameter space. Extrapolating these bounds, we provide High-Luminosity LHC projections (3 ab$^{-1}$). We also project sensitivities for future electron-positron colliders at $\sqrt{s}=365$ GeV and $\sqrt{s}=500$ GeV, showing substantial improvements over the HL-LHC. The current LHC sensitivity is beginning to approach the theoretically motivated region of the parameter space, while future colliders will be able to comprehensively test this model.