Cosmic Structure Strikes Back: The Elimination of Vector-Mediated Nonstandard Interaction Models as a Mechanism for Sterile Neutrino Dark Matter Production

We revisit sterile-neutrino production enabled by nonstandard interactions (NSI) among active neutrinos mediated by new bosons. We focus on vector mediators, including neutrinophilic, gauged L_μ\!-\!L_τ, and $B\!-\!L$ realizations that modify in-medium dispersion and scattering, thereby altering the active--sterile conversion history. Building on a novel production framework with NSI thermal potentials and collision integrals, we compute nonthermal phase-space distributions across sterile neutrino mixing and NSI parameters and map each point to an equivalent thermal warm dark matter particle mass mₜₕ via linear-theory transfer function fitting with cosmological structure-formation Boltzmann solver. This enables a direct reinterpretation of state-of-the-art structure-formation limits from Milky Way satellites, strong lensing, and the Lyman-$α$ forest. These limits, in conjunction with X-ray decay searches, as well as results from a wide variety of particle physics experiments allow for a more complete examination of these models. We find that these vector-mediated models are ruled out when the full combination of current constraints, listed above, are taken into account. NSI scalar-mediated models and models with low-reheating temperatures remain viable.