The New $\nu$MSM : Radiative Neutrino Masses, keV-Scale Dark Matter and Viable Leptogenesis with sub-TeV New Physics

Abstract

We study the scenario in which the Standard model is augmented by three generations of right-handed neutrinos and a scalar doublet. The newly introduced fields share an odd charge under a $\mathbb{Z}_2$ parity symmetry. This model, commonly known as "Scotogenic", was designed to provide a mechanism for active neutrino mass generation as well as a viable dark matter candidate. In this paper we consider a scenario in which the dark matter particle is at the keV-scale. Such particle is free from X-ray limits due to the unbroken parity symmetry that forbids the mixing between active and right-handed neutrinos. The active neutrino masses are radiatively generated from the new scalars and the two heavier right-handed states with $\sim \mathcal{O}(100)$ GeV masses. These heavy fermions can produce the observed baryon asymmetry of the Universe through the combination of Akhmedov-Rubakov-Smirnov mechanism and recently proposed scalar decays. To the best of our knowledge, this is the first time that these two mechanisms are shown to be successful in any radiative model. We identify the parameter space where the successful leptogenesis is compatible with the observed abundance of dark matter as well as the measurements from the neutrino oscillation experiments. Interestingly, combining dark matter production and successful leptogenesis gives rise to strict limits from big bang nucleosynthesis which do not allow the mass of dark matter to lie above $\sim 10$ keV, providing a phenomenological hint for considered low-scale dark matter. By featuring the keV-scale dark matter free from stringent X-ray limits, successful baryon asymmetry generation and non-zero active neutrino masses, the model is a direct analogue to the $\nu$MSM model proposed by Asaka, Blanchet and Shaposhnikov. Therefore we dub the presented framework as "The new $\nu$MSM" abbreviated as $\nu\nu$MSM.