Sterile neutrino dark matter from a TeV Scale Seesaw

Abstract: In this article, we investigate the phenomenological aspects of a feebly interacting sterile neutrino dark matter in a low-scale seesaw setup. The Type-I seesaw framework is augmented by a second complex scalar doublet ($Φ_ν$), which couples exclusively with the heavy right-handed neutrinos and the lepton doublet, thereby forming the neutrino Dirac mass term, while the first doublet is responsible for the mass generation of the remaining Standard Model particles. The lightest sterile neutrino ($N_1$) serves as a feebly interacting massive particle (FIMP), produced primarily through W and Z boson decays -a previously overlooked dominant contribution that solely determines the relic abundance. Owing to the small vev ($v_ν\sim 10$ MeV) of the second Higgs doublet, an enhancement in the available parameter space of the sterile neutrino masses is observed, spanning from sub-keV to $0.2$ GeV. After incorporating the latest Lyman-$α$ forest observations it is found that the setup is able to accommodate both warm and cold dark matter options.