Charged Higgs Boson Phenomenology in the Dark Z mediated Fermionic Dark Matter Model

TL;DR

This work studies charged Higgs phenomenology in a hidden‑sector model where a new $U(1)_X$ gauge symmetry yields a light dark $Z'$ mediator connecting a fermionic dark matter candidate to the SM. The model, effectively a type‑I 2HDM with a messenger doublet, predicts a tightly constrained parameter space: a light $Z'$ with $m_{Z'} \sim 9.3$–$11.3$ GeV and a light charged Higgs with $m_{H^0} \sim 110$–$170$ GeV, both subject to electroweak and Higgs‑sector constraints. At the LHC, the charged Higgs is mainly produced in top decays and predominantly decays bosonically via $H^0 \to W^+ h$ and $H^0 \to W^+ Z'$, with direct production in association with $Z'$ or $h$ offering distinctive signatures, including lepton jets from boosted $Z'$. The dark matter sector requires resonant or near‑threshold annihilation to satisfy relic density and direct‑detection bounds, yielding narrow viable $m_X$ windows; overall, the scenario yields rich, testable collider and DM phenomenology compatible with current data and accessible to upcoming LHC runs.

Abstract

We present the phenomenology of the charged Higgs boson $H^\pm$ appearing in a fermionic dark matter model mediated by an additional scalar doublet. In order to couple the dark matter fermion to the scalar doublet, we introduce a U(1)$_X$ gauge symmetry, which is spontaneously broken at electroweak symmetry breaking, resulting in a massive $Z'$ gauge boson. Since $Z'$ is generically light, the model is subject to strong constraints from electroweak precision observables. As a result, the charged Higgs boson mass allowed by current experimental bounds is typically light in this model, 110 GeV $<m_{H^\pm}<$ 170 GeV. Such a light charged Higgs boson will be produced mainly through top-quark decays at the LHC. Additionally, depending on the mass of the additional neutral Higgs boson $h$ and the dark gauge boson $Z'$, the direct production channels $pp \to H^\pm Z'$ and $pp \to H^\pm h$ can become sizable. We investigate the corresponding signal processes at the LHC to assess the discovery potential for $H^\pm$. Current ATLAS and CMS searches for light charged Higgs bosons already impose further constraints on the model. We also discuss the implications of dark matter in relation to the charged Higgs boson phenomenology.

Figures (6)

• Figure 1: Parameter regions allowed by the electroweak constraints, HiggsBounds, and HiggsSignals. Shown are the viable ranges in the planes of $(m_{Z'}, \sin \theta_X)$, $(m_{Z'}, \tan \beta)$, $(m_{Z'}, \sin \alpha)$, $(m_{Z'}, m_h)$, $(m_{H^\pm}, \tan \beta)$. $(m_h, \sin \alpha)$.
• Figure 2: Branching ratios of the charged Higgs boson as functions of $m_{H^\pm}$. Left: $m_{Z'} = 9.5~\text{GeV}$, $m_h = 50~\text{GeV}$, and $\sin\alpha = 10^{-4}$. Right: $m_{Z'} = 9.5~\text{GeV}$, $m_h = 120~\text{GeV}$, and $\sin\alpha = 0.4$. The decay modes $H^\pm \to WZ'$, $Wh$, $c\bar{s}$, $c\bar{b}$, and $\tau\nu$ are shown.
• Figure 3: Model predictions for $\text{Br}(t \to H^+ b)\,\text{Br}(H^+ \to c\bar{s})$ compared with the experimental limits from the ATLAS and CMS collaborations at the LHC CMS:2020osdATLAS:2024oqu (left), and those for $\text{Br}(t \to H^+ b)\,\text{Br}(H^+ \to \tau\bar{\nu})$ compared with the ATLAS results ATLAS:2024hya (right).
• Figure 4: Parameter regions in the $(m_h, \, \tan\beta)$ plane with and without the limits ${\cal B}_{sig} < 5\times10^{-6}$ are shown, which are obtained by the CMS trilepton final states searches CMS:2019idx. The charged Higgs mass is fixed, $m_{H^\pm} = 160~\text{GeV}$
• Figure 5: Production cross sections of the charged Higgs boson in association with the dark $Z$, the neutral Higgs $h$, and the top quark. The results are shown as functions of $m_{H^\pm}$ for representative parameter sets: $m_{Z'} = 9.5~\text{GeV}$ with $g_X = 1$ and $0.4$, and $m_h = 50$, 70, 120 GeV with $\sin\alpha = 10^{-4}$ and $0.4$. The value of $\tan\beta$ is fixed at 10.