Return of the lepton number: Sterile neutrino dark matter production and the revival of the Shi-Fuller mechanism

We explore resonant production of sterile neutrino dark matter via the Shi-Fuller (SF) mechanism, revisiting its cosmological viability in light of recent results demonstrating that lepton-number asymmetries L α ≳ 1 at temperatures T > 20 MeV are consistent with big bang nucleosynthesis (BBN). Using a quasiclassical Boltzmann transport calculation of the dark matter production, we compute the nonthermal phase space distributions of sterile neutrinos across a broad range of particle mass m s and mixing angle sin 2 ( 2 θ ) parameter space. We then evolve the resulting distributions through linear structure formation using CLASS and fit the resulting matter power spectra to thermal warm dark matter (WDM) transfer functions, enabling a direct mapping between SF models and equivalent thermal WDM particle masses m th . This allows us to reinterpret existing structure formation limits and Lyman- α forest preferences in the context of SF production. We find that lepton asymmetries L ≳ 0.5 at high temperatures open significant viable parameter space in the m s ≳ 10 keV and sin 2 ( 2 θ ) ≲ 10 − 14 regime, compatible with both x-ray constraints from and and recent Lyman- α inferences of m th ≈ 4.1 keV . Following lepton number evolution below 20 MeV, we also specifically show that this lepton asymmetry parameter space is compatible with BBN and cosmic microwave background constraints. We present updated constraints, a refined m th fitting function, and power-law approximations for L across the parameter space. Our results motivate future x-ray observations targeting the ∼ 20 keV photon regime and testing of the m th ≳ 10 keV WDM region.