Mixed WIMP-FIMP scenario in a two-component dark matter model

TL;DR

The work analyzes a two-component dark matter model stabilized by a $Z_2\times Z_4$ symmetry, introducing a singlet scalar $S$ and a Majorana fermion $\chi$ with an additional singlet $S_0$ that acquires a vev to generate $\chi$ mass after symmetry breaking. It investigates two mixed WIMP-FIMP scenarios under a decoupling limit: Case I where $\chi$ is the WIMP and $S$ the FIMP, and Case II with $S$ as the WIMP and $\chi$ as the FIMP, solving the coupled Boltzmann equations and applying relic-density and direct-detection constraints. Case I yields viable parameter space across broad mass ranges, requiring $y_{sf}$ to exceed unity and allowing $S$ to dominate the DM when $\lambda_{ds}$ or $\lambda_{dh}$ are above roughly $10^{-11}$–$10^{-12}$. Case II is more constrained by direct detection, yet reveals two allowed regions around $m_S \approx m_1/2$ and $m_S \gtrsim 400$ GeV, with the latter enabling a light scalar DM mass down to a few hundred GeV in some contexts. Overall, the mixed WIMP-FIMP framework demonstrates how multi-component DM can reconcile relic-density requirements with stringent direct-detection bounds, opening viable parameter spaces not available to single-component WIMP models.

Abstract

We consider the mixed WIMP-FIMP scenario in a two-component dark matter model with $Z_2 \times Z_4$ symmetry, where a singlet scalar $S$ and a Majarano fermion $χ$ are introduced as dark matter candidates. We also introduce another singlet scalar $S_0$ with a non-zero vacuum expectation value to the SM so that the fermion dark matter can obtain mass after spontaneous symmetry breaking. Either $S$ or $χ$ relic density can be generated via the "Freeze-out" mechanism. In contrast, the other DM candidate relic density is obtained by the "Freeze-in" mechanism, and we therefore have two different cases. In the case of $χ$ as WIMP and $S$ as FIMP, we perform random scans to estimate the allowed parameter space consistent with the dark matter constraint. The results show that this case is viable over a wide range of dark matter masses with the Yukawa coupling of $S_0$ and $χ$ should be larger than 1. Instead, for the case of $S$ as WIMP and $χ$ as FIMP, the viable parameter space is more constrained by the direct detection experiements, and we have two regions with $m_S \approx 62.5$ GeV and $m_S>400$ GeV under the constraints, which is consistent with the singlet scalar DM result but the scalar DM mass can be as low as a few hundred GeV for the heavy mass region in the model.

Figures (9)

• Figure 1: Evolutions of $\Omega_{\chi}h^2$ (left) and $\Omega_S h^2$ (right) with $m_{\chi}$. We fix $m_2=1000$ GeV, $m_S=50$ GeV, $\lambda_{ds}=10^{-8}$,$\lambda_{dh}=10^{-11}$,$y_{sf}=1$ as benchmark values corresponding to the yellow lines , and other colored lines represent one of the above parameters varying.
• Figure 2: Viable parameter space of $m_{\chi}-y_{sf}$ (a), $m_S-\lambda_{dh}$ (b) and $\lambda_{ds}-\lambda_{dh}$ (c) under dark matter relic density constraint in the case of $m_S<m_1/2$.
• Figure 3: Viable parameter space of $m_{\chi}-y_{sf}$ (a), $m_S-\lambda_{dh}$ (b) and $\lambda_{ds}-\lambda_{dh}$ (c) under dark matter relic density constraint in the case of $m_1/2<m_S<m_2/2$.
• Figure 4: Viable parameter space of $m_{\chi}-y_{sf}$ (a), $m_S-\lambda_{dh}$ (b) and $\lambda_{ds}-\lambda_{dh}$ (c) under dark matter relic density constraint in the case of $m_S>m_2/2$.
• Figure 5: Evolutions of $\Omega_Sh^2$ (left) and $\Omega_{\chi} h^2$ (right) with $m_S$. We fix $m_2=1000$ GeV, $m_{\chi}= 550$ GeV, $\lambda_{ds}=10^{-8}$,$\lambda_{dh}=1$,$y_{sf}=10^{-11}$ as benchmark values corresponding to the yellow lines , and other colored lines represent one of the above parameters varying.