Maximal Parameter Space of Sterile Neutrino Dark Matter with Lepton Asymmetries

We delineate the maximal parameter space of sterile neutrino dark matter in the presence of lepton-flavor asymmetries. We focus on large flavor asymmetries with vanishing total lepton asymmetry, which are washed out by neutrino oscillations at MeV temperatures and hence are consistent with big bang nucleosynthesis and cosmic microwave background constraints. We derive a semiclassical Boltzmann equation for sterile neutrinos applicable in this regime and validate it against quantum kinetic equations. For sterile neutrino masses up to 60 keV, the viable range of mixing angles extends by up to 2 orders of magnitude, with broad prospects for tests in forthcoming x-ray, cosmic microwave background, and structure formation observations. We also release a public framework to compute the production of sterile neutrinos, and in particular their momentum distribution, enabling dedicated structure formation analyses.