Testing frozen-in pNGB dark matter with a long-lived dark Higgs

TL;DR

This work analyzes a Higgs-portal model with a complex singlet where the imaginary component $\chi$ is a stable pNGB DM produced via freeze-in at low reheating temperatures $T_\text{rh}$. The real part of the singlet yields a dark Higgs $h_2$ whose lifetime can be long, making LLP searches a key probe; the DM relic density is set by $T_\text{rh}$ and the Higgs-portal mixing angle $\theta$, with analytic yields $Y_0^d$ and $Y_0^s$ derived for decay- and scattering-dominated regimes. Collider studies show HL-LHC has limited sensitivity to these LLP signals, while a future FCC-hh at 100 TeV could access parts of the viable parameter space through displaced $h_2$ decays in tracker volumes, linking collider signatures to the cosmic reheating history. Overall, LLP searches emerge as a promising avenue to illuminate the dynamics of reheating and hidden-sector DM production in freeze-in scenarios.

Abstract

We consider a simple Higgs portal dark matter (DM) model, where the Standard Model is extended with a complex singlet scalar. The imaginary part of the scalar becomes a massive and stable pseudo-Nambu-Goldstone boson, serving as the DM candidate, while the real part gives rise to a second (dark) Higgs boson. We focus on the freeze-in production of the DM, paying particular attention to low-reheating temperature scenarios, where the dark Higgs can be a long-lived particle (LLP). We also explore the phenomenology of this dark Higgs at the LHC and the Future Circular Collider in hadron-hadron mode, discussing its discovery prospects in regions of parameter space consistent with current DM constraints. Our results emphasize the impact of the cosmic reheating dynamics on the DM freeze-in production and their critical role in interpreting collider signatures. Furthermore, our findings suggest that LLP searches may provide insights into the fundamental dynamics of reheating.

Figures (8)

• Figure 1: Main processes contributing to the DM production in the early Universe.
• Figure 2: Left panel: Relic abundance as a function of the reheating temperature $T_\text{rh}$, considering $m_{h_2} =$ 10 GeV, $m_\chi =$ 8 GeV and $m_\chi =$ 50 GeV, $\sin\theta = 10^{-4}$ and $v_s =$ 100 GeV. We have separated the freeze-in contributions from Higgs decay (dashed lines) and scattering (dotted lines) contributions obtained analytically in this section for the two DM masses shown in the legend. The blue and black lines are the corresponding micrOMEGAs result, which give the full relic abundance. Right panel: Required values of $\sin\theta$ to fit the observed relic abundance, as a function of $T_\text{rh}$, for different combination of the pair $(m_{h_2}, m_\chi)$, and $v_s = 100$ GeV.
• Figure 3: Parameter space fitting the observed DM relic abundance, assuming $m_{h_2} = 10$ GeV and $v_s = 100$ GeV, for different reheating temperatures (thick solid lines). The thin dashed lines take into account only the contribution from scatterings.
• Figure 4: Proper decay length of $h_2$ as a function of $m_{h_2}$, assuming $v_s = 10^2$ GeV and $m_\chi = 20$ GeV. The horizontal pink band ('D.V.') corresponds to a displacement between 0.1 cm and 10 cm. The red, blue and green points correspond to the benchmark points indicated in Table \ref{['tab:LHC_BM']}.
• Figure 5: Schematic overview of the FCC-hh reference detector: geometry and distances.