Dark sterile neutrinos on a linear seesaw of neutrino masses

Abstract: Sterile neutrinos as source of mass and flavor mixing of active neutrinos as well as genesis of dark matter (DM) and matter-antimatter asymmetry have gained special interest. Here we study the case of the Standard Model (SM) extended with three right-handed (RH) neutrinos and a dark sector with two extra sterile neutrinos, odd under a discrete $Z_2$ symmetry. The RH neutrinos are responsible for producing baryon asymmetry via high-scale unflavored leptogenesis. They are superheavy and their abundance at the electroweak broken stage is vanishingly small, so that they have no impact on phenomenology at low energies. The two dark neutrinos generate the tiny mass of two active neutrinos through a mechanism similar to the minimal linear seesaw, and saturate the relic abundance as freeze-in DM coming mainly from decays of SM weak-gauge bosons via active-dark neutrino mixing. The absence of the dark Majorana mass terms in the dark linear seesaw is explained by invoking a hidden symmetry, the so-called presymmetry, and the DM candidate appears in the form of a quasi-Dirac neutrino. The $Z_2$ symmetry is broken in the dark neutrino sector, but exact in the realm of RH neutrinos. The required coupling weakness for the freeze-in DM neutrino is related to a very small breach of the unitarity of the active neutrino mixing matrix. We show how phenomenological constraints on the production and decay of the DM neutrino imply an upper bound around 1 MeV for its mass and unitarity up to $\mathcal{O}(10^{-7})$ for the mixing matrix.