Coscattering Dark Matter in Scotogenic Models

TL;DR

This work addresses the challenge of realizing dark matter in the Scotogenic inverse model by leveraging the coscattering mechanism in a framework with nearly degenerate dark scalars. It analyzes two portals—Higgs and Yukawa—showing that relic density can be achieved via coscattering $\phi_2\text{SM}\to\phi_1\text{SM}$ and related coannihilation channels, while respecting LFV, direct detection, BBN, and CMB constraints. The study uses Boltzmann equations and micrOMEGAs to map out viable regions, revealing that small mass splittings with sizable portal couplings favor coscattering, and that a long-lived dark partner $\phi_2$ yields distinctive displaced-vertex collider signatures. Distinguishing the two portals hinges on collider signals: hadronic decays in the Higgs portal versus leptonic decays in the Yukawa portal, enabling experimental discrimination alongside future direct-detection and collider probes.

Abstract

The Scotogenic mechanism is an appealing pathway to naturally explain the common origin of dark matter and tiny neutrino mass. However, the conventional scotogenic dark matter usually suffers stringent constraints from the non-observation of lepton flavor violation and direct detection. To generate the non-zero neutrino masses, at least two generations of dark particles are required. For example, two real scalar singlets $φ_1$ and $φ_2$ are involved in the scotogenic inverse model, which are odd under the $Z_2$ symmetry. In this paper, we consider the masses of dark scalars are nearly degenerate $m_{φ_1}\lesssim m_{φ_2}$, which opens new viable pathway for the generation of dark matter $φ_1$, such as the coscattering process $φ_1\text{SM}\to φ_2 \text{SM}$ and coannihilation processes $φ_1 φ_2 \to \text{SM SM}$ via the Higgs portal or Yukawa portal interactions. We explore the parameter space to produce the correct relic density through coscattering, as well as the contrastive coannihilation channel. We then comprehensively study the constraints of dark matter from Higgs decay, direct detection, and indirect detection. For the heavier dark scalar, the three-body decay $φ_2\toφ_1 f\bar{f}$ not only alerts the predictions of big bang nucleosynthesis and cosmic microwave background, but also leads to the observable displaced vertex signature at colliders.

Figures (12)

• Figure 1: The evolutions of various abundances $Y_i$ of coscattering (a) and coannihilation (b) benchmarks in the Higgs portal scenario. In panels (a) and (b), the solid red and green lines represent the abundance evolution of $\phi_1$ and $\phi_2$, while the corresponding dashed lines indicate their respective thermal equilibrium. Purple dot-dashed line stands for the observation of DM Planck:2018vyg. Panels (c) and (d) describe the thermal rates of relevant interactions in panels (a) and (b), respectively. Additionally, the conversion channels $\phi_2\phi_2\to\phi_1\phi_1$, $\phi_1\phi_2\to\phi_1\phi_1$ and $\phi_2\phi_2\to\phi_1\phi_2$ are added together as $\phi_2\phi_i\to\phi_1\phi_j$. The black vertical dashed line corresponds to the thermal decoupling temperature when $Y_{\phi_1}/Y_{\phi_1}^{\rm eq}=2.5$. The horizontal black line is $\Gamma_i=\mathcal{H}$.
• Figure 2: Freeze-out phase diagrams in the parameter spaces of $\Delta{m_\phi}-m_{\phi_1}$ in panel (a) and $\Delta{m_\phi}-\lambda_{2}$ in panel (b). The blue, red, and green regions correspond to the phases of coscattering, coannihilation, and conventional WIMP, respectively.
• Figure 3: Direct detection and ATLAS constraints on the $\lambda_{1}-m_{\phi_1}$ parameter space. Panels (a), (b), (c), and (d) correspond to distinct combinations of $\lambda_{2}$ and $\Delta{m_\phi}$, respectively. The present LZ LZ:2024zvo and future DARWIN DARWIN:2016hyl limits are denoted as the purple solid and dashed line. The gray solid curve is the bound of current ATLAS ATLAS:2023tktATLAS:2022vkf searches of Higgs decays. Samples marked as $\bullet$ are already excluded by current experimental searches. For the allowed samples, the coannihilation and coscattering regimes are denoted by the symbols of $+$ and $\blacktriangle$, respectively. The remaining prominent black star samples $\bigstar$ have been excluded by the BBN constraint Kawasaki:2017bqm, which is elaborated upon in subsequent Section \ref{['SEC:HP2']}.
• Figure 4: Constraints from the indirect detection experiments in the Higgs portal scenario. Panels (a)-(d) represent the four scenarios with combinations of $\lambda_{2}$ and $\Delta{m_\phi}$. The current direct detection constraint from LZ and the Higgs decay limit from ATLAS exclude the purple samples. The remaining samples with different colors and shapes share the same representation as those in Figure \ref{['FIG:fig3']}. The gray and black solid lines stand for the Fermi-LAT observed limit on the $b\bar{b}$ final state Fermi-LAT:2015att and H.E.S.S. observed limit on the $W^+W^-$ final state HESS:2016mib. The gray dashed line represents the predicted results of CTA in the $W^+W^-$ final state CTA:2020qlo.
• Figure 5: CMB and BBN constraints on the dark partner $\phi_2$ in the Higgs portal scenario. The horizontal axis $\tau_{\phi_2}$ is the lifespan of $\phi_2$, while the vertical axis represents the product of relic density $\Omega_{\phi_2}h^2$, hadronic branching ratio $f_h$, and the energy transfer factor $\epsilon$. Panels (a)-(d) and internal samples have the same definition as those in Figure \ref{['FIG:fig4']}. Here, we use the black curves to represent the current BBN constraint of hadronic final states Kawasaki:2017bqm. The gray dashed lines stand for the upcoming CMB results of purely electromagnetic decay Lucca:2019rxf.