Return of the Lepton Number: Sterile Neutrino Dark Matter Production and the Revival of the Shi-Fuller Mechanism

Abstract: 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 $L_\alpha \gtrsim 1$ at temperatures $T > 20\rm\,MeV$ are consistent with Big Bang Nucleosynthesis (BBN). Using a quasi-classical Boltzmann transport calculation of the dark matter production, we compute the non-thermal phase space distributions of sterile neutrinos across a broad range of particle mass $m_s$ and mixing angle $\sin^2{(2\theta)}$ 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 $m_{\mathrm{th}}$. This allows us to reinterpret existing structure formation limits and Lyman-$\alpha$ forest preferences in the context of SF production. We find that lepton asymmetries $L \gtrsim 0.5$ at high temperatures open significant viable parameter space in the $m_s \gtrsim 10\,\mathrm{keV}$ and $\sin^2 (2\theta) \lesssim 10^{-14}$ regime, compatible with both X-ray constraints from NuSTAR and INTEGRAL/SPI and recent Lyman-$\alpha$ inferences of $m_{\mathrm{th}} \approx 4.1\,\mathrm{keV}$. 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 $m_{\mathrm{th}}$ fitting function, and power-law approximations for $L$ across the parameter space. Our results motivate future X-ray observations targeting the $\sim 20$ keV photon regime and testing of the $m_\mathrm{th} \gtrsim 10\,\mathrm{keV}$ WDM region.