Singlet-doublet fermionic dark matter and gravitational waves in a two-Higgs-doublet extension of the Standard Model

We present a study of singlet-doublet vectorlike leptonic dark matter (DM) in the framework of the two-Higgs-doublet model (2HDM), where the dark sector is comprised of one doublet and one singlet vectorlike fermion (VLF). The DM, that arises as an admixture of the neutral components of the VLFs, is stabilized by an imposed discrete symmetry Z2′. We test the viability of the DM candidate in the light of observations from Planck and recent limits on spin-independent direct detection experiments and search for its possible collider signals. In addition, we also look for the stochastic gravitational wave (GW) signatures resulting from strong first-order phase transition due to the presence of the second Higgs doublet. The model thus offers a viable parameter space for a stable DM candidate that can be probed from direct search, collider, and GW experiments.