Light mass window of inert doublet dark matter with lepton portal interaction

We study phenomenology of a light scalar dark matter (DM). In the model, there is an inert doublet scalar and a singlet Dirac fermion ψ, both charged under a global Z₂ symmetry. The mass of the lightest inert scalar H can be lighter than 10 GeV by imposing appropriate relations between three scalar quartic couplings. The lightest Z₂ odd particle is stable and DM. In this paper, focusing on the parameter space where H is lighter than ψ and is DM, we discuss DM physics related to relic density, direct detection, indirect detection, collider searches, and other cosmological observations. We clarify differences from the case where ψ is instead DM, which has been focused on in previous works.