Speaker
Description
We study the phenomenology of a feebly interacting sterile neutrino dark matter candidate in a low-scale Type-I seesaw framework extended by an additional scalar doublet. The second Higgs doublet couples exclusively to right-handed neutrinos and lepton doublets, generating the neutrino Dirac mass, while the Standard Model--like Higgs is responsible for the masses of the remaining particles.
In this setup, the lightest sterile neutrino acts as a feebly interacting massive particle (FIMP) and is produced via freeze-in from the decays of $W^\pm$, $Z$, and the additional scalar states. We show that, contrary to previous studies, the $W^\pm$ and $Z$ boson decay channels dominate the dark matter production, enhancing the relic abundance by up to $\mathcal{O}(10^{13})$ and effectively determining the final dark matter density.
Including these contributions leads to several novel phenomenological consequences, including a new non-thermal production condition that allows for significantly smaller Yukawa couplings. We also discuss how the sterile neutrino mass is controlled by the small vacuum expectation value of the second Higgs doublet, opening up new regions of parameter space. Taking into account the latest Lyman-$\alpha$ forest constraints, we show that this framework can consistently accommodate both warm and cold dark matter scenarios.