Coscattering Dark Matter in Scotogenic Models

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 $\phi_1$ and $\phi_2$ 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 $m_{\phi_1}\lesssim m_{\phi_2}$, which opens new viable pathway for the generation of dark matter $\phi_1$, such as the coscattering process $\phi_1\text{SM}\to \phi_2 \text{SM}$ and coannihilation processes $\phi_1 \phi_2 \to \text{SM SM}$ 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 $\phi_2\to\phi_1 f\bar{f}$ not only alerts the predictions of big bang nucleosynthesis and cosmic microwave background, but also leads to the observable displaced vertex signature at colliders.