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 $m_\mathrm{th}$ 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.

💡 Research Summary

This paper presents a comprehensive study of sterile‑neutrino dark‑matter production mediated by vector‑type non‑standard neutrino interactions (NSI). The authors focus on three concrete vector‑mediated models: (i) a neutrinophilic vector that couples only to active neutrinos, (ii) a gauged U(1) Lµ‑Lτ boson that interacts with second‑ and third‑generation leptons, and (iii) a gauged U(1) B‑L boson that couples universally to all fermions. In each case the new vector boson V modifies both the in‑medium thermal potential V_T and the scattering rate Γ of active neutrinos, thereby reshaping the effective mixing angle θ_eff that governs sterile‑neutrino production via oscillations in the early Universe.

The authors develop a robust numerical framework that solves the Boltzmann‑like evolution equation for the sterile‑neutrino phase‑space distribution f_s(p,T). They pre‑compute tables of V_T and Γ over a four‑dimensional grid (E/T, T, m_V/T, g_V) and interpolate during the integration, ensuring accurate treatment of finite‑temperature screening, decoupling of non‑relativistic species, and opening/closing of scattering channels. The production dynamics are explored across three regimes determined by the mediator mass relative to the plasma temperature: heavy‑off‑shell exchange, on‑shell vector decay, and light‑mediator scattering. For any given sterile‑neutrino mass m_s and mixing sin²2θ, only specific combinations of the vector coupling g_V and mass m_V yield the observed dark‑matter relic density; these viable points trace an “S‑curve” in the (g_V, m_V) plane, which the authors locate with a root‑finding algorithm.

Each resulting non‑thermal phase‑space distribution is fed into the CLASS Boltzmann solver to compute the linear matter power spectrum. The authors then construct the transfer function T(k)=P_model(k)/P_ΛCDM(k) and fit it to the calibrated thermal‑warm‑dark‑matter (WDM) template of Ref.