Two-Component Dark Matter in the Type-I 2HDM

Abstract

We investigate a two-component dark matter scenario in the type-I two-Higgs-doublet model. The dark sector contains a real scalar $s$ and a Dirac fermion $χ$, whose stability is ensured by a $Z_4$ symmetry together with kinematic conditions. The scalar interacts with the visible sector through Higgs-portal couplings, while the fermion interacts with the scalar via Yukawa interactions. In this framework, we analyze the thermal freeze-out production of both candidates, accounting for annihilation, conversion, and semi-annihilation processes. A comprehensive scan over the multidimensional parameter space is performed in terms of physical masses, mixing angles, and portal couplings, imposing theoretical requirements such as perturbativity and vacuum stability. We confront the model with current experimental constraints, including the observed relic abundance, invisible Higgs decays, direct detection limits on spin-independent scattering cross sections, and electroweak precision observables. We find that viable regions of parameter space can satisfy all dark matter constraints, but collider bounds strongly constrain the scalar sector, narrowing the allowed regions and creating tension with those favored by dark matter phenomenology, particularly in the sub-TeV mass regime.

Figures (8)

• Figure 1: Relevant diagrams for freeze-out. Here, $\varphi$ collectively denotes the CP-even scalars $h$ and $H$, while $X$ represents SM particles and additional 2HDM scalars ($h, H, h^\pm, A$). Diagrams (a) and (b) correspond to $s$- and $t$-channel semi-annihilation processes, respectively; (c) and (d) to DM conversion processes; and (e)--(h) to $s$ pair annihilation into $X$ final states.
• Figure 2: We plot $\Omega_{\rm DM}h^2 = (\Omega_\chi + \Omega_s)h^2$ as a function of the DM masses, $m_\chi$ or $m_s$. The horizontal gray band corresponds to the region compatible with PLANCK observations Planck:2018vyg. The colorbar indicates the fractional abundance of each component, $\xi_{\chi,s} = \Omega_{\chi,s}/\Omega_{\rm DM}$. The remaining parameters are fixed to the benchmark points defined in Tab. \ref{['tab:benchmarks']}, obtained from the full parameter-space scan.
• Figure 3: Feynman diagrams for DM--quark elastic scattering. (a) Tree-level process for scalar DM $s$. (b) One-loop process for fermionic DM $\chi$.
• Figure 4: Numerical scan satisfying $m_s < m_\chi$. Triangles denote points with $m_H = 125$ GeV (light partner regime), while squares denote points with $m_h = 125$ GeV (heavy partner regime). Left panel: projection onto the $m_s$ versus fermionic DM fraction plane. Right panel: projection onto the $m_s$ versus $m_\chi/m_s$ plane. The colorbar indicates the scalar semi-annihilation fraction $\zeta_s$, defined in \ref{['eq:zetas']}.
• Figure 5: Numerical scan satisfying $m_s > m_\chi$. Triangles denote points with $m_H = 125$ GeV (light partner regime), while squares denote points with $m_h = 125$ GeV (heavy partner regime). Left panel: projection onto the $m_\chi$ versus scalar DM fraction plane. Right panel: projection onto the $m_\chi$ versus $m_s/m_\chi$ plane. The colorbar indicates the fermionic semi-annihilation fraction $\zeta_\chi$, defined in \ref{['eq:zetachi']}.