An EFT origin of Secluded Dark Matter

TL;DR

The paper develops an EFT-based secluded dark-matter scenario where a DM candidate χ and a lighter mediator a (ξ) arise from an extended type-X 2HDM and are populated non-thermally via dim-6 four-Fermi operators generated by integrating out heavy Higgs states. It analyzes the coupled Boltzmann evolution of χ and ξ, including DM–DM and DM–ξ conversions inside the dark sector and χ production from the visible sector, allowing relic abundance to be set by either freeze-in or freeze-out depending on DS couplings. Through detailed numerical and analytic treatment, the study identifies viable regions of parameter space that satisfy perturbativity, BBN, and indirect-detection constraints, with m_χ ≳ 20 GeV and portal and DS couplings constrained to be very small. The framework yields testable implications for future gamma-ray searches and collider LFUV observables, while remaining extendable to other UV completions beyond the specific 2HDM realization. Overall, the work demonstrates how EFT methods can capture secluded dark-sector dynamics and produce a consistent cosmological history for non-thermally produced DM.

Abstract

The present study aims to unveil a scenario with a non-minimal secluded dark sector (DS) in an effective field theory (EFT) framework. To explore this, we have examined a suitable extension of the type-X Two Higgs Doublet Model (2HDM) as a potential origin for the secluded DS. The DS comprises a dark matter (DM) candidate and a mediator particle `$a$' and possesses some non-minimal characteristics. It becomes non-thermally populated through diverse dim-6 four-Fermi operators, effectively generated by integrating out the heavier Higgs particles. The analysis further focuses on the consequences of the collision processes $\textit{DM}+ a \leftrightarrow a + a$ and $\textit{DM}+ \textit{DM} \leftrightarrow a + a$ occurring within the DS. We have investigated the significance of employing an EFT approach in tracking the temperature evolution of the DS. Within the present framework, the observed relic abundance of the DM can be realized through both dark freeze-out and freeze-in mechanisms. Further, we have delineated the permissible ranges of the relevant parameters, viz., the DM mass ($m_χ\gtrsim 20 \, \text{GeV}$), the portal coupling ($C_τ\lesssim 10^{-14}\, \text{GeV}^{-2}$), and the DS coupling ($λ\lesssim 10^{-6} \,\text{GeV}^{-2}$) by taking into account the perturbativity of the involved couplings while reproducing the observed DM relic and complying with the bounds from a successful Big Bang Nucleosynthesis (BBN) and $γ$-ray searches.

Figures (8)

• Figure 1: A cartoon illustrating the dynamics of DS phenomenology in the current setup. Both the initial population of the DS, as well as the conversion processes within the DS, are driven by dimension-6 four-Fermi operators, as indicated. Here, '$f$' represents the SM leptons.
• Figure 2: Situation with a possible thermal equilibrium between the VS and the DS presented in the $m_\xi-C_\tau$ plane with $T=\Lambda \, (= \, 100 \, m_\chi)$. Also shown is the region where $C_\tau$ ceases to be perturbative. See text for details.
• Figure 3: Variations of $\Omega_i/\Omega_{\text{obs}}$ with respect to $x = m_\chi/T$ for the scenario BP-1 under Case-I. The value of $\lambda$ for which the observed DM relic abundance is reproduced is indicated. Here, the solid (dashed) lines represent evolutions of $\chi$ ($\xi$). See text for details.
• Figure 4: Variations of $\frac{\mu_D}{T_D}$ (left) and $T_D$ and $T$ (right; in black and red, respectively) as functions of $x (=m_\chi/T)$ for the scenario BP-1 under Case-II. The appropriate value of $\lambda_2$ that results in the correct relic abundance of DM is also indicated. See text for details.
• Figure 5: Evolution of $\frac{\Omega}{\Omega_{\text{obs}}}$ as functions of $x (=m_\chi/T)$ for $\chi$ (solid) and $\xi$ (dashed) for scenarios BP-1 (left) and BP-2 (right) under Case-II. Results for both freeze-out (red) and freeze-in (blue) scenarios are shown. The value of $\lambda_2$ that results in the observed relic abundance of the DM for the freeze-out (freeze-in) scenario is indicated in red (blue) color.