Collider Searches for Near-Continuum Dark Matter

TL;DR

The paper investigates collider constraints on near-continuum dark matter with a Z-portal coupling, modeled by a minimal spectral-density framework parameterized by the gap $ extmu_0$, peak-to-gap ratio $r= extmu_p/ extmu_0$, and normalization $ ho_0$. A dedicated Monte-Carlo tool handles near-continuous spectra to predict production and cascade decays through $Z^{(*)}$, yielding high jet multiplicities and missing energy; CMS Run-2 multijet+$H_T^{ m miss}$ data is recast to bound $ ho_0$ as a function of $ extmu_0$ and $r$, with HL-LHC projections extending the reach. The study also projects sensitivities for future lepton colliders at $ ts = 365$ GeV and 500 GeV, finding substantially improved reach over HL-LHC due to cleaner environments and greater spectral access. Overall, current LHC sensitivity is approaching the theoretically motivated region ($ ho_0 \nobreakackslash ext{O}(1)$ for relevant $ extmu_0$), while future $e^+e^-$ colliders can comprehensively test the near-continuum DM scenario, highlighting a promising collider window into densely spaced dark-sector states.

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.

Figures (13)

• Figure 1: Mixing angle $\sin^{2}\alpha$ required to reproduce the observed DM relic abundance, as a function of the gap scale $\mu_{0}$. From Ref. Csaki:2021gfm.
• Figure 2: 5D setup of the near-continuum dark matter model. From Ref. Ferrante2023.
• Figure 3: Dashed lines: numerical results for spectral densities $\rho_{5D}$ from the explicit 5D theory described in section \ref{['sec:5Dcont']}, with $\mu_{0} = 100$ GeV and SM brane located at $R^{-1} = 80$ GeV (blue) and 30 GeV (red). Solid lines: The parameterized spectral densities $\tilde{\rho}$ from Eq. (\ref{['eq:ToyRhoDef']}), with $\mu_{0} = 100$ GeV and values of $r$ and $\rho_0$ chosen to match each $\rho_{5D}$.
• Figure 4: Total cross section for pair production of near-continuum DM states in 13 TeV $pp$ collisions as a function of the gap scale $\mu_{0}$ and the ratio $r\equiv \mu_{p}/\mu_{0}$. The overall coefficients $\rho_{0}^{2}$ and $\text{sin}^{4}\alpha$ are set to 1.
• Figure 5: Number of final state leptons and jets of the signal for different values of gap scale $\mu_{0}$ and ratio $r\equiv \mu_{p}/\mu_{0}$ after showering and detector simulation. The number of jets and leptons shown here is before the selections for the analysis are applied, but after selecting jets with $p_{T} >$ 30 GeV and $\Delta R < 0.4$, and leptons with $p_{T} > 0.5$ GeV and $\eta < 2.4 \, (2.5)$ for muons (electrons).